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There has been much evidence showing that the central sympathetic nervous system may be involved in the control of blood pressure. In the present study, we investigated the role of the presynaptic alpha2-adrenergic receptors and the cyclic adenosine monophosphate-dependent protein kinase (protein kinase A) in the regulation of norepinephrine release in the central nervous system in hypertension. The alpha2-adrenergic receptor agonists UK 14, 304 and clonidine inhibited the stimulation-evoked [3H]norepinephrine release in a dose-dependent manner in the medulla oblongata of Sprague-Dawley rats. Pretreatment of pertussis toxin (a potent inhibitor of the Gi-protein) attenuated the suppression of NE release by UK 14, 304. The protein kinase A inhibitor H-8 also reduced the stimulation-evoked [3H]norepinephrine release in rat medulla oblongata. In spontaneously hypertensive rats, the inhibitory effect of UK 14, 304 on the stimulation-evoked norepinephrine release was significantly less than in age-matched normotensive Wistar-Kyoto rats. By contrast, the protein kinase A inhibitor H-8 reduced the stimulation-evoked norepinephrine release to a greater extent in hypertension than in normotensive controls. The results of the present study showed that the alteration in the presynaptic alpha2-receptor-protein kinase A system might actively participate in the regulation of norepinephrine release in the central nervous system in hypertension.

In the present study, in order to elucidate the role of dihydropyridine (DHP)-sensitive calcium (Ca) channels in the regulation of neurotransmitter release in hypertension, we examined the effects of the DHP-sensitive Ca channel blocker nicardipine on norepinephrine (NE) release in blood vessels of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. The stimulation-evoked pressor responses and NE release were significantly greater in the mesenteric arteries of SHR than in the mesenteric arteries of WKY rats. Nicardipine significantly inhibited the stimulation-evoked NE release as well as vasoconstrictor responses in the mesenteric arteries to a greater extent in SHR than in WKY rats. These results demonstrated that nicardipine markedly reduced the stimulation-evoked NE release in blood vessels of SHR, which might suggest that the DHP-sensitive Ca channels could be involved, at least in part, in the regulation of NE release in hypertension.

Recent studies have suggested an involvement of the endogenous opioid system in blood pressure control. The purpose of the present study was to determine the role of beta-endorphin in the regulation of sympathetic nervous activity in the central nervous system of hypertension. The effects of beta-endorphin on the electrically evoked release of [3H]norepinephrine (NE) were investigated in superfused slices of rat medulla oblongata. Beta-endorphin inhibited the stimulation-evoked NE release in a dose-dependent manner in rat medulla oblongata. In the medulla oblongata of spontaneously hypertensive rats (SHR), the inhibitory effect of beta-endorphin on the stimulation-evoked NE release was significantly smaller than in the medulla oblongata of Wistar-Kyoto rats. These results showed that beta-endorphin might reduce NE release in rat medulla oblongata. Furthermore, the lesser inhibitory effect of beta-endorphin on NE release in SHR might suggest that the opioid peptide could be involved in the regulation of central sympathetic nervous activity in hypertension.

1. In the present study, we examined the effects of Bay K 8644, a dihydropyridine (DHP)-sensitive Ca2+ channel agonist, and bradykinin on norepinephrine release in the hypothalamus of spontaneously hypertensive rats (SHR). 2. In the preliminary studies using Sprague-Dawley rats, Bay K 8644 by itself had no significant effects on the stimulation-evoked [3H]-norepinephrine release from hypothalamic slices. Bradykinin increased the stimulation-evoked [3H]-norepinephrine release in a dose-related fashion. The facilitatory effects of bradykinin on norepinephrine release were potentiated by Bay K 8644. 3. In SHR, Bay K 8644 significantly increased the stimulation-evoked norepinephrine release from hypothalamic slices. However, exposure of slices to Bay K 8644 caused no significant effects on norepinephrine release in Wistar-Kyoto (WKY) rats. The effects of Bay K 8644 in combination with bradykinin on the stimulation-evoked norepinephrine release were also greater in SHR than in WKY rats. 4. These results demonstrate that Bay K 8644 significantly potentiated the facilitatory effects of bradykinin on norepinephrine release in rat hypothalamus. The finding indicates a possible interaction of bradykinin with DHP-sensitive Ca2+ channels in the central nervous system. Furthermore, the pronounced effects of Bay K 8644 and bradykinin in SHR suggest that bradykinin-related Ca2+ channels might have a role in the regulation of norepinephrine release in the hypothalamus of SHR.

Neuropeptide Y (NPY) has a wide and specific distribution both in the central and peripheral nervous systems. In the present study, we have investigated the effects of NPY on norepinephrine release in rat hypothalamus, and further examined the interaction of NPY with alpha 2-adrenergic receptors, as well as the influence of sodium ions on the modulation of norepinephrine release. In an in vitro study, NPY significantly inhibited the stimulation-evoked norepinephrine release from hypothalamic slices in a dose-dependent manner. The alpha 2-adrenergic receptor agonist, UK 14,304, also reduced the stimulation-evoked norepinephrine release. A low concentration of NPY, which had no effects on its own, significantly potentiated the inhibitory effect of UK 14,304 on the stimulation-evoked [3H]norepinephrine release. The blockade of alpha 2-adrenergic receptors by RX 781094 diminished the inhibitory effects of NPY on norepinephrine release. Pretreatment of slices with pertussis toxin (a potent inhibitor of the Gi-proteins) significantly attenuated the suppressive effects of NPY and UK 14,304 on norepinephrine release. When the sodium concentration of the perfusion medium was increased, the inhibitory effects of NPY and UK 14,304 on norepinephrine release were significantly reduced. These results show that NPY might inhibit norepinephrine release that is partially mediated by alpha 2-adrenergic receptors and the pertussis toxin-sensitive Gi-proteins in rat hypothalamus. Moreover, less suppressive effects of NPY and UK 14,304 on norepinephrine release in the presence of excess sodium ions suggest that sodium ions might actively participate in regulating the NPY and alpha 2-adrenergic receptor mediated functions in the central nervous system.

1. The present study was performed to investigate the effects of captopril (an angiotensin converting enzyme inhibitor, ACE-I) on noradrenergic transmission in the rat central nervous system. 2. Slices of rat hypothalamus and medulla oblongata were prepared and prelabelled with [3H]-norepinephrine. Slices were continuously superfused with Krebs-Ringer solution, and electrical stimulation (1 Hz) was performed. 3. Captopril significantly inhibited the stimulation-evoked [3H]-norepinephrine release from rat hypothalamic slices in a dose-dependent manner (S2/S1 ratio: control 0.904 +/- 0.025, n = 6, captopril 1 x 10(-5) mol/L 0.617 +/- 0.043, n = 6, P < 0.05, captopril 5 x 10(-5) mol/L 0.547 +/- 0.037, n = 6, P < 0.05). However, the basal release of [3H]-norepinephrine was not affected by captopril. 4. Captopril also reduced the stimulation-evoked [3H]-norepinephrine release in the medulla oblongata (S2/S1 ratio: control 0.878 +/- 0.018, n = 6, captopril 3.3 x 10(-5) mol/L 0.624 +/- 0.046, n = 6, P < 0.05). 5. These results show that captopril might inhibit the stimulation-evoked norepinephrine release in rat hypothalamus and medulla oblongata. Although the precise mechanisms underlying the neurosuppressive effect of captopril are still uncertain, the finding suggests that the inhibition of noradrenergic transmission might be related to the central action of the ACE-I.

Objective: To assess in vitro the role of glutamate receptors in the regulation of noradrenaline release from the medulla oblongata of normotensive and hypertensive rats., Design and Methods: The effects of L-glutamate (an endogenous ligand for glutamate receptors), glycine (an allosteric agonist for the N-methyl-D-aspartate type of glutamate receptors) and MK-801 (an antagonist for N-methyl-D-aspartate receptors) on [3H]-noradrenaline release were examined in slices of rat medulla oblongata., Results: L-Glutamate elicited [3H]-noradrenaline release from slices of rat medulla oblongata in magnesium-free medium. Glycine also increased the release of noradrenaline. Moreover, the effect of L-glutamate on noradrenaline release was significantly potentiated by glycine. MK-801 inhibited the increase in noradrenaline release evoked by L-glutamate. In spontaneously hypertensive rats (SHR) the facilitatory effect of L-glutamate on noradrenaline release was significantly more pronounced than in Wistar-Kyoto (WKY) rats. Furthermore, glycine alone and in combination with L-glutamate increased the noradrenaline release to a greater extent in SHR than in WKY rats., Conclusion: The present results show that the excitatory amino acids might increase noradrenaline release from rat medulla oblongata, which was partially dependent on the N-methyl-D-aspartate type of glutamate receptors. The greater effect of L-glutamate and glycine in SHR suggests that these amino acids might be involved in the regulation of noradrenaline release in the medulla oblongata of hypertension.

1. We examined the regulatory actions of bradykinin on norepinephrine release in the hypothalamus of rats. 2. Bradykinin increased the stimulation-evoked [3H]-norepinephrine release from hypothalamic slices of Sprague-Dawley rats in a dose-dependent manner (1 Hz: S2/S1 ratio, mean +/- s.e.m., control 0.868 +/- 0.016, n = 6; bradykinin 1 x 10(-6) mol/L 1.039 +/- 0.018, n = 6, P < 0.05; bradykinin 3.3 x 10(-6) mol/L 1.130 +/- 0.064, n = 6, P < 0.05). The basal release of [3H]-norepinephrine was not affected by the peptide. 3. Bay K 8644, a dihydropyridine-sensitive calcium channel agonist, significantly potentiated the facilitatory effect of bradykinin on norepinephrine release, although Bay K 8644 by itself had no significant effect. By contrast, nicardipine, a dihydropyridine-sensitive calcium channel blocker, reversed the increase in norepinephrine release induced by bradykinin and Bay K 8644. 4. These results indicate that bradykinin may increase norepinephrine release in rat hypothalamus, partially mediated by interactions with dihydropyridine-sensitive calcium channels.

The present study was performed to investigate the effects of neurotensin on noradrenergic transmission in blood vessels of hypertension. In the perfused mesenteric vasculatures prepared from spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) rats, we examined the influences of neurotensin on vascular responsiveness and norepinephrine release from adrenergic neurons. In studies with male standard Wistar rats, endogenous norepinephrine release during periarterial nerve stimulation was inhibited by neurotensin in a dose-dependent manner. Vasoconstrictor responses to electrical nerve stimulation were also inhibited by neurotensin. In SHR, the stimulation-evoked pressor responses and norepinephrine release were enhanced compared with WKY rats. The suppression of these responses by neurotensin was significantly less in SHR than in WKY rats. These results show that neurotensin could have a modulatory effect on noradrenergic activity and cause a decrease in stimulation-evoked norepinephrine release from vascular adrenergic neurons. The lesser reduction of pressor responses and norepinephrine release by neurotensin in SHR might suggest an insufficient regulation of vascular adrenergic transmission by the peptide hormone in hypertension.

Galanin, a 29-amino acid peptide, is widely distributed in both the central and peripheral nervous systems and is colocalized with catecholamines, although its physiological significance remains to be elucidated. In the present study we investigated the regulatory mechanisms of galanin on norepinephrine release in rat medulla oblongata. In slices of medulla oblongata of Sprague-Dawley rats, galanin inhibited the stimulation-evoked [3H]norepinephrine release in a concentration-dependent manner (fractional release ratio during electrical stimulation: control 0.937 +/- 0.043, mean +/- SEM, n = 6; galanin 1 x 10(-7) M 0.501 +/- 0.037, n = 6, p less than 0.05; and galanin 1 x 10(-6) M 0.299 +/- 0.018 n = 6, p less than 0.05). Galanin potentiated inhibition of [3H]norepinephrine release by the alpha 2-agonists (UK 14,304 and clonidine). The blockade of alpha 2-adrenergic receptors by RX 781094 diminished the inhibition of norepinephrine release by galanin. Pretreatment of pertussis toxin, which interferes with the coupling of inhibitory guanosine triphosphate-binding proteins to adenylate cyclase, significantly attenuated the suppressive effects of galanin on norepinephrine release. In slices of medulla oblongata obtained from spontaneously hypertensive rats (SHR), the inhibitory effect of galanin on norepinephrine release was significantly less than in those from age-matched Wistar-Kyoto rats. These results show that galanin might inhibit the stimulation-evoked norepinephrine release in rat medulla oblongata, at least partially mediated by alpha 2-adrenergic receptors and the pertussis toxin-sensitive guanosine triphosphate-binding proteins. Moreover, less suppression of norepinephrine release by galanin in SHR suggests that galanin might be involved in the regulation of central sympathetic nervous activity in hypertension.

The present study was designed to investigate the presynaptic alpha 2-adrenoceptor function to inhibit norepinephrine (NE) release in blood vessels of reduced renal mass salt hypertensive rats (Na-loaded HT). Isolated perfused mesenteric vasculatures were prepared from Na-loaded HT and normotensive control rats (NT-control), and the NE release and vascular responsiveness were examined. Periarterial nerve stimulation caused a significantly greater release of NE and pressor responses in Na-loaded HT than in NT-control. Yohimbine, a potent alpha 2-adrenoceptor antagonist, demonstrated the facilitatory effects on NE release during nerve stimulation. The effects were significantly attenuated in Na-loaded HT compared with NT-control. These results demonstrate that vascular sympathetic nervous activity might be enhanced in Na-loaded HT. Furthermore, the increased NE release from vascular adrenergic neurons in Na-loaded HT could partially depend on impaired presynaptic alpha 2-adrenoceptor-mediated modulation, which might contribute to the pathogenesis and maintenance of this form of salt-dependent hypertension.

Neuropeptide Y (NPY) has a wide and specific distribution in the central nervous system, and is colocalized with catecholamines in specific neuronal systems. In this study, in order to investigate the regulatory mechanisms of NPY and presynaptic alpha 2-adrenergic receptors on central noradrenergic transmission in hypertension, we have examined the effects of NPY and the alpha 2-agonist, UK 14,304, on (3H)-noradrenaline (NA) release from hypothalamic slices of spontaneously hypertensive rats (SHR). Electrical stimulation (1 Hz)-evoked (3H)-NA release was significantly greater in the hypothalamic slices of SHR than in those of Wistar Kyoto rats (WKY). NPY and the alpha 2-agonist, UK 14,304, inhibited the stimulation-evoked (3H)-NA release in a dose-related manner. The inhibitory effects of NPY and UK 14,304 on NA release were significantly attenuated in SHR compared with WKY. These results suggest that NPY and alpha 2-adrenoceptors might be involved in the regulation of central sympathetic nervous activity in hypertension.

This study was performed to investigate the role of substance P in the vascular adrenergic transmission in hypertension. In perfused mesenteric vasculatures prepared from spontaneously hypertensive rats (SHR, 7 to 10 weeks old) and age-matched Wistar-Kyoto rats (WKY), we have examined the effects of substance P on vascular responsiveness as well as on norepinephrine release from the vascular adrenergic neurons. In preliminary studies with normotensive Wistar rats, pressor responses and endogenous norepinephrine release during electrical nerve stimulation were inhibited by substance P in a dose-dependent manner. However, vasoconstrictor responses to exogenous norepinephrine were not affected by the peptide. In SHR, the stimulation-evoked pressor responses and norepinephrine release were enhanced compared with WKY. Alternatively, the suppression of these responses by substance P was significantly less in SHR than in WKY. These results demonstrate that substance P could have a modulatory effect on noradrenergic activity and cause a decrease in stimulation-evoked norepinephrine release from the vascular adrenergic neurons. The attenuated reduction of pressor responses and norepinephrine release by substance P in SHR might suggest insufficient regulation of vascular adrenergic transmission by the peptide in hypertension.

We describe the effects of Bay K 8644, a dihydropyridine-sensitive Ca2+ channel agonist, on [3H]norepinephrine (NE) release from the hypothalamus of spontaneously hypertensive rats (SHR). The electrical stimulation-evoked [3H]NE release was greater in hypothalamic slice of SHR than in those of Wistar Kyoto (WKY) rats. Bay K 8644 (10(-6) M) significantly increased the stimulation-evoked [3H]NE release in SHR. However, the agonist showed no significant effects in normotensive WKY rats. The results suggest that the dihydropyridine-sensitive Ca2+ channels might participate actively in the regulation of the central sympathetic tone of hypertension.

The ability of a beta-blocker to inhibit vascular sympathetic nerve activity associated with hypertension was studied in DOCA-salt hypertension in rats. A seven week treatment of DOCA and salt resulted in a significant increase in the systolic blood pressure of the uninephrectomized rats. The administration of propranolol (40 mg/L and 80 mg/L in drinking water) had little effect on the development of hypertension. After a three week administration of propranolol, perfused mesenteric vasculatures were prepared in vitro, and endogenous norepinephrine release as well as vascular responsiveness were examined. Endogenous norepinephrine and pressor responses during periarterial nerve stimulation were greater in the untreated DOCA-salt hypertensive rats than in the normotensive rats. In the DOCA-salt hypertensive rats treated with propranolol, the stimulation-evoked norepinephrine release and pressor responses were significantly attenuated, at both doses, compared with the untreated DOCA-salt hypertensive rats. These results demonstrate that propranolol inhibited the vascular sympathetic nerve activity in DOCA-salt hypertensive rats. This occurrence suggests a possible role of presynaptic beta-adrenoceptors in the regulation of sympathetic tone in DOCA-salt hypertension.

We now describe the effects of calcitonin gene-related peptide (CGRP) on [3H]norepinephrine (NE) release in medulla oblongata of spontaneously hypertensive rats (SHR). CGRP inhibited stimulation-evoked [3H]NE release in a dose-dependent manner in Sprague-Dawley rats, although the basal release of [3H]NE was not affected by the peptide. In SHR, the inhibitory effect of CGRP on stimulation-evoked [3H]NE release was significantly attenuated compared with Wistar Kyoto rats. These results suggest that CGRP might be involved in the regulation of central sympathetic nervous activity in hypertension.

This study was designed to investigate the role of protein kinase C and calcium in vascular adrenergic transmission in hypertension. In perfused mesenteric vasculatures of spontaneously hypertensive rats (SHR, 7 to 10 weeks old) and age-matched Wistar-Kyoto rats (WKY), we have examined the effects of the protein kinase C inhibitor H-7 on endogenous norepinephrine release and vascular responsiveness during nerve stimulation. Endogenous norepinephrine release and pressor responses during periarterial nerve stimulation were significantly greater in SHR than in WKY. The protein kinase C inhibitor H-7 inhibited the stimulation-induced norepinephrine release and pressor responses in a dose-dependent manner. The magnitude of these suppressive responses were more pronounced in SHR than in WKY. Calcium removal from extracellular fluid also reduced the norepinephrine release more strongly in SHR than in WKY. These results demonstrate that the regulation of norepinephrine release might be more dependent on protein kinase C and calcium in the blood vessels of SHR, which could contribute, at least partially, to the pathogenesis of this form of hypertension.

The medulla oblongata is thought to play an important role in the central regulation of blood pressure. Neuropeptide Y (NPY) and galanin (Gal) coexist with norepinephrine (NE) and may have a functional interaction in this region. The aim of the present study was to investigate whether NPY and Gal could modulate the presynaptic neuronal mechanisms, especially the alpha-adrenoceptor function, inhibiting NE release in medulla oblongata. In slices of rat medulla oblongata, clonidine, an alpha-adrenoceptor agonist, inhibited the NE release elicited by electrical stimulation in a concentration-dependent manner. A combination of clonidine and low concentration of NPY (1 X 10(-9) mol/L) resulted in an increase in the inhibitory action of clonidine on stimulation-evoked NE release. The inhibitory action of clonidine was also potentiated by Gal (1 X 10(-8) mol/L). These results indicate the existence of the presynaptic NPY- and Gal-receptors on noradrenergic nerve terminals, which may enhance the presynaptic alpha-adrenoceptor function to inhibit NE release. This suggests a possible involvement of NPY and Gal in the regulation of sympathetic nerve activity in medulla oblongata.

This study investigated the effects of diltiazem (a Ca2(+)-entry blocker) on neuromuscular junctions of blood vessels in hypertension. In isolated perfused mesenteric vasculatures prepared from spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto rats (WKY), the effects of diltiazem on norepinephrine release from vascular adrenergic neurons and pressor responses were examined. The influences of extracellular Ca2(+)-reduction on these responses were also studied. Stimulation evoked pressor responses and norepinephrine release were significantly greater in the mesenteric vasculatures of SHR than in those of WKY. Diltiazem inhibited both pressor responses and norepinephrine release during electrical nerve stimulation in a dose-dependent manner. The suppression of these responses was more pronounced in SHR than in WKY. Reduction of extracellular Ca2(+)-concentration also decreased the responses in SHR and WKY, and the inhibitory degree was significantly greater in SHR than in WKY. These results demonstrate that diltiazem affected the presynaptic site of the mesenteric vasculatures and decreased the stimulation-evoked norepinephrine release from vascular adrenergic neurons with a concomitant reduction of pressor responses of the preparation. Furthermore, the marked inhibition of pressor responses and norepinephrine release by diltiazem or Ca2(+)-depletion in SHR may suggest the increased Ca2(+)-dependency in vascular neurotransmission in this model of hypertension.

We describe the effects of neuropeptide Y (NPY) on [3H]norepinephrine (NE) release from hypothalamic slices of spontaneously hypertensive rats (SHR). The electrical stimulation (1 Hz)-evoked [3H]NE release was significantly greater in hypothalamic slices of SHR than in those of Wistar Kyoto rats (WKY). NPY inhibited the stimulation-evoked [3H]NE release in a dose-dependent manner. The inhibitory effect of NPY was significantly attenuated in SHR compared with WKY. The results may indicate that the less inhibitory effect of NPY on NE release induces increased sympathetic nerve activity in the hypothalamus of SHR.

Neuropeptide Y is colocalized with norepinephrine in both central and peripheral noradrenergic neurons. In this study, we examined the regulatory mechanisms of neuropeptide Y on norepinephrine release in the medulla oblongata of rats. Neuropeptide Y inhibited the stimulation-evoked [3H]norepinephrine release in a dose-dependent manner in slices of medulla oblongata of Sprague-Dawley rats (1 Hz, S2/S1 ratio, control, 0.946 +/- 0.040 [+/- SEM], n = 6; neuropeptide Y 1 x 10(-8) M, 0.676 +/- 0.022, n = 6, p less than 0.05; neuropeptide Y 1 x 10(-7) M, 0.589 +/- 0.014, n = 6, p less than 0.05). Neuropeptide Y potentiated inhibition of [3H]norepinephrine release by the alpha 2-agonists UK 14,304 and clonidine. The blockade of alpha 2-adrenergic receptors by RX 781,094 diminished inhibitory effects of neuropeptide Y on norepinephrine release. Pretreatment of pertussis toxin (a toxin that interferes with the coupling of inhibitory receptors to adenylate cyclase) attenuated the suppression of norepinephrine release by neuropeptide Y. In spontaneously hypertensive rats, the inhibitory effect of UK 14,304 and neuropeptide Y on norepinephrine release from the medulla oblongata was significantly less than in age-matched Wistar-Kyoto rats. These results show that neuropeptide Y inhibits norepinephrine release partially mediated by alpha 2-adrenergic receptors and the pertussis toxin-sensitive guanosine triphosphate-binding proteins in rat medulla oblongata. Furthermore, less suppression of norepinephrine release by UK 14,304 and neuropeptide Y in spontaneously hypertensive rats suggests that alpha 2-adrenergic receptors and neuropeptide Y might be involved in the regulation of central sympathetic tone in hypertension.

This study was performed to investigate the effects of a specific protein kinase C inhibitor (H-7) on vascular adrenergic transmission in hypertension. In the isolated mesenteric vasculature of spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY), we have examined the effects of H-7 on norepinephrine (NE) release from vascular adrenergic neurons. Endogenous NE release during periarterial nerve stimulation was inhibited by H-7 in a dose-dependent manner with a concomitant reduction of pressor responses of the preparation. The inhibition of NE release was not affected by an uptake blocker of NE (desipramine). In SHR, the stimulation-evoked NE release and pressor responses were significantly greater than in age-matched WKY. The suppressive magnitude of stimulation-evoked NE release and pressor responses by H-7 were pronounced in SHR compared with WKY. These results demonstrate that endogenous NE release and pressor responses were increased in the mesenteric vasculature of SHR. Furthermore, the marked inhibition of NE release and pressor responses by H-7 in SHR may suggest the presence of enhanced protein kinase C-dependent regulation of vascular adrenergic transmission, which may contribute to the calcium-related abnormalities in this form of hypertension.

This study was performed to investigate the role of endogenous epinephrine in the regulation of vascular tone in hypertension. The release of endogenous epinephrine and norepinephrine from the vascular adrenergic neurons by periarterial nerve stimulation was examined in spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto rats (WKY). The isolated mesenteric vasculatures were prepared, and the epinephrine and norepinephrine release during electrical nerve stimulation was determined as the increase in epinephrine and norepinephrine contents in the vascular perfusate. Epinephrine and norepinephrine were measured by high performance liquid chromatography with an electrochemical detector. Vasoconstrictor responses and norepinephrine overflow during electrical nerve stimulation were significantly greater in SHR than in WKY. The amount of stimulation-evoked epinephrine overflow into the perfusate was also increased in SHR compared to that in WKY, especially at low frequency stimulation. These results suggest that epinephrine could be released from the vascular adrenergic neurons as a cotransmitter of norepinephrine and contribute to increased vascular tone in hypertension.

Noradrenergic neurons in the locus ceruleus contain neuropeptide Y and galanin, which project to the hypothalamic region. We have investigated the regulatory mechanisms of these peptides on norepinephrine release in rat hypothalamic slices in vitro. Neuropeptide Y and galanin significantly inhibited the stimulation-evoked [3H]norepinephrine release in a dose-dependent manner (1 Hz: S2/S1 ratio (mean +/- SEM), control 0.947 +/- 0.040, n = 11, neuropeptide Y 1 x 10(-8) M 0.509 +/- 0.013, n = 8, p less than 0.01, neuropeptide Y 1 x 10(-7) M 0.283 +/- 0.021, n = 8, p less than 0.01; galanin 1 x 10(-7) M 0.448 +/- 0.026, n = 8, p less than 0.01, galanin 1 x 10(-6) M 0.261 +/- 0.023, n = 8, p less than 0.01). The inhibition of norepinephrine release by the alpha-2 agonist UK 14,304 was potentiated by neuropeptide Y and galanin. The blockade of the alpha 2-adrenergic receptors by RX 781094 diminished the inhibitory effects of neuropeptide Y and galanin on norepinephrine release. Pretreatment of hypothalamic slices with islet activating protein (a toxin that interferes with the coupling of inhibitory receptors to adenylate cyclase) attenuated the suppression of norepinephrine release by UK 14,304, neuropeptide Y, and galanin. These results support the idea that neuropeptide Y and galanin are involved in the regulation of central adrenergic transmission partially mediated by alpha 2-adrenergic receptors and islet-activating protein-sensitive guanosine triphosphate-binding proteins in rat hypothalamus.

This study was performed to investigate prostaglandin (PG)-mediated regulation of norepinephrine release from the sympathetic nerve endings in spontaneously hypertensive rats (SHR). The effects of PGE2 on the pressor responses and norepinephrine overflow during the periarterial nerve stimulation were examined in the perfused mesenteric vascular beds from SHR and age-matched Wistar Kyoto rats (WKY). In preliminary studies using normotensive Wistar rats, PGE2 inhibited the overflow of norepinephrine in response to the electrical nerve stimulation. On the contrary, when indomethacin was infused into the preparations, the norepinephrine overflow induced by the nerve stimulation was increased. In SHR, the neurogenic vasoconstriction and norepinephrine overflow during the nerve stimulation were significantly greater than in WKY. PGE2 in the medium inhibited the pressor responses and norepinephrine overflow during the nerve stimulation dose-dependently in WKY, while the inhibitory effects of PGE2 in SHR were blunted. These results suggest that PGE2 is an important hormone in regulating sympathetic vascular tone and that presynaptic inhibition of vascular adrenergic transmission by PGE2 is attenuated in SHR.

The purpose of the present study was to analyze the influence of dopamine on norepinephrine release in resistance vessels in spontaneously hypertensive rats (SHR). Perfused mesenteric vasculature preparations from spontaneously hypertensive rats (7-10 weeks old) and age-matched normotensive Wistar Kyoto rats (WKY) were used to compare the effects of dopamine on both pressor responses and norepinephrine release. Both responses to electrical nerve stimulation were significantly greater in SHR than in WKY rats. Dopamine reduced these responses in a dose-dependent manner in WKY. However, this suppression of responses to electrical stimulation was attenuated in SHR. These results suggest that the enhanced adrenergic transmission in SHR may partly reflect impaired dopamine-mediated inhibition of nerve terminals, which would contribute to the pathogenesis of hypertension.

The present study was designed to evaluate norepinephrine release during sympathetic nerve stimulation and its inhibitory characteristics by presynaptic alpha 2-adrenoceptor in the perfused mesenteric arteries of young and old spontaneously hypertensive rats (SHR) compared with age-matched Wistar-Kyoto rats (WKY). Sympathetic nerve stimulation caused significantly greater overflow of endogenous norepinephrine from adrenergic nerve terminals in young SHR than age-matched WKY. Yohimbine, alpha 2-adrenoceptor antagonist, exerted a facilitatory effect on norepinephrine overflow from mesenteric vasculature. This effect was less in young SHR than in aged-matched WKY. On the other hand, in adult SHR the amount of norepinephrine overflow was somewhat reduced compared with age-matched WKY. The difference of the facilitatory effect on norepinephrine overflow by yohimbine between SHR and WKY was not so marked as that observed in young age. These results indicate that increased norepinephrine release from sympathetic nerve endings in young SHR could depend at least partly on impaired presynaptic alpha 2-mediated inhibition, while normal or reduced norepinephrine release in adult SHR might be an adaptive mechanism to prevent further increase in blood pressure in SHR.

The present study was carried out to elucidate the role of epinephrine as a neuromodulator in hypertension. The effects of epinephrine on norepinephrine release from the sympathetic nerve endings were examined in isolated perfused mesenteric arteries of spontaneously hypertensive rats (SHR) and age-matched Wistar Kyoto rats (WKY). Norepinephrine overflow during electrical nerve stimulation (5, 15 Hz) was significantly greater in SHR than in WKY. Low concentration of exogenous epinephrine (5.5 X 10(-9) M) potentiated norepinephrine overflow during nerve stimulation in SHR, and this (at 15 Hz stimulation) was antagonized by propranolol (5.0 x 10(-7) M), whereas, the overflow in WKY was reduced by the same concentration of epinephrine. A higher concentration of epinephrine (1.4 x 10(-8) M) decreased norepinephrine overflow in both SHR and WKY, and this change (at 15 Hz stimulation) was antagonized by yohimbine (1.0 x 10(-7) M). Further, magnitudes of the suppressions were smaller in SHR than in WKY. These results suggest that altered modulations of norepinephrine release by epinephrine through presynaptic beta- and alpha 2-adrenoceptors might induce increased sympathetic nerve activity in SHR.

The present study was designed to evaluate the role of the presynaptic alpha 2-adrenoceptor in the pathogenesis of hypertension. Norepinephrine overflow during sympathetic nerve stimulation and its changes by presynaptic alpha 2-adrenoceptor inhibition were examined in the perfused mesenteric vasculatures of young and adult spontaneously hypertensive rats (SHR) compared with age-matched Wistar Kyoto rats (WKY). Electrical sympathetic nerve stimulation caused significantly greater overflow of endogenous norepinephrine from the adrenergic nerve terminals in young SHR than in age-matched WKY. Yohimbine, an alpha 2-adrenoceptor blocking agent, facilitated norepinephrine overflow from the adrenergic nerve terminals. The effects of yohimbine on norepinephrine overflow and pressor responses to electrical nerve stimulation were less in young SHR than in age-matched WKY. Norepinephrine overflow in adult SHR was similar to that in adult WKY, and differences in the effect of yohimbine on norepinephrine overflow between SHR and WKY were not marked at this chronic stage. These results suggest that enhanced norepinephrine overflow in the mesenteric vasculatures can be observed only in young SHR; this may be due in part to an impaired negative feed-back mechanism on the nerve terminals by presynaptic alpha 2-adrenoceptors.

This study was designed to investigate the role of protein kinase C and calmodulin in adrenergic transmission in hypertension. In isolated mesenteric vasculature prepared from spontaneously hypertensive rats (SHR, Okamoto and Aoki strain) and age-matched Wistar-Kyoto rats (WKY), we examined the effects of protein kinase C inhibitor (H-7) and calmodulin antagonist (W-7) on pressor responses and noradrenaline release from the vascular adrenergic neurons. Endogenous noradrenaline release and vasoconstrictor responses evoked by periarterial nerve stimulation were significantly enhanced in SHR compared to those in age-matched WKY. Protein kinase C inhibitor H-7 and the calmodulin antagonist W-7 inhibited the stimulation-evoked noradrenaline release and pressor responses, respectively, in a dose-dependent manner. Further, these inhibitory effects of H-7 and W-7 were greater in SHR than in WKY. These results demonstrate that noradrenaline release and vascular responsiveness are increased in the mesenteric vasculatures of SHR. The marked reduction in noradrenaline release and pressor responses induced by H-7 and W-7 in SHR suggests the presence of enhanced protein kinase C-dependent and calmodulin-dependent regulation of adrenergic neurotransmission, which may contribute to the calcium abnormalities in this model of hypertension.

To investigate the role of norepinephrine release from the sympathetic nerve endings and vascular responsiveness in the pathogenesis of hypertension, the perfused mesenteric preparations were used in DOCA-salt hypertensive rats (acute phase: 10 days after operation, chronic phase: 7-8 weeks). In addition, the effects of a Ca-antagonist (verapamil) on the norepinephrine release and vascular responsiveness were also examined. Vasoconstrictor responses to the electrical nerve stimulation were significantly greater in DOCA-salt hypertension in the chronic phase than the age-matched normotensive controls. The pressor responses to exogenous norepinephrine were significantly enhanced in DOCA-salt hypertension both in acute and chronic phases. Endogenous norepinephrine overflow from the sympathetic nerve endings during the electrical nerve stimulation was enhanced in the chronic phase of DOCA-salt hypertension, but not in the acute phase, compared with the age-matched normotensive controls. After infusion of verapamil, the pressor responses and norepinephrine overflow by the electrical nerve stimulation were significantly inhibited, and the suppression was greater in chronic DOCA-salt hypertension than in the normotensive controls. These results demonstrate that the vascular responsiveness was increased in both acute and chronic phases of DOCA-salt hypertensive rats, while the norepinephrine overflow from the adrenergic nerve terminals was enhanced only in the chronic phase. More marked inhibition of the vasoconstrictor responses and norepinephrine overflow in the presence of a Ca-antagonist in chronic DOCA-salt hypertension might represent the higher Ca-dependency in the neurotransmission of the peripheral resistance vessels, especially in the mechanism of presynaptic norepinephrine release, than their normotensive controls, and it could partly contribute to the development and maintenance of DOCA-salt hypertension.

The purpose of the present study was to investigate the effects of vasoactive neuropeptides, such as substance P or neurotensin, on vascular adrenergic neurotransmission in hypertension. In perfused mesenteric vasculatures prepared from spontaneously hypertensive rats (SHR), we examined the effects of substance P and neurotensin on vascular responsiveness and noradrenaline release from the sympathetic nerve endings. Stimulation-evoked noradrenaline release and pressor responses were inhibited by substance P and neurotensin. In SHR, the noradrenaline release and pressor responses during electrical nerve stimulation were significantly greater than in Wistar-Kyoto rats (WKY). The inhibitory effects of these responses by substance P and neurotensin were attenuated in SHR compared with WKY. These results showed that substance P and neurotensin could affect the presynaptic site of the blood vessels and cause a decrease in electrically stimulated noradrenaline release from the vascular adrenergic neurons. The reduction of noradrenaline release and pressor responses by substance P and neurotensin in SHR might suggest insufficient regulation of the vascular adrenergic transmission by these vasoactive peptides in hypertension.

The present study was carried out to investigate the role of calmodulin in the adrenergic transmission of deoxycorticosterone acetate (DOCA)-salt hypertension. In perfused mesenteric vasculatures prepared from DOCA-salt hypertensive rats (seven to eight weeks after operation) and age-matched normotensive control rats, the effects of a specific calmodulin antagonist (W-7: N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide) on the norepinephrine release and vascular responsiveness were examined. Endogenous norepinephrine overflow during electrical nerve stimulation was significantly greater in the mesenteric vasculatures of DOCA-salt hypertensive rats both at low (5 Hz) and high (15 Hz) frequencies than in those of the normotensive controls. Vasoconstrictor responses to electrical nerve stimulation or exogenous norepinephrine were also enhanced in the DOCA-salt hypertension compared to those in the controls. The calmodulin antagonist (W-7) inhibited the stimulation-evoked norepinephrine overflow and pressor responses in a dose-dependent manner. The suppressive magnitudes in these responses by W-7 were significantly greater in the mesenteric vasculatures of the DOCA-salt hypertension than in those of the control rats. These results demonstrate that the vascular adrenergic neurotransmission might be enhanced in the mesenteric vasculatures of the DOCA-salt hypertensive rats. Further, it is suggested that the marked reduction in stimulation-evoked norepinephrine overflow and vasoconstrictor responses by W-7 showed the greater calmodulin-dependent regulation in the vascular adrenergic activity of DOCA-salt hypertension.

The purpose of the present study was to investigate the mechanisms of the hypotensive actions of an angiotensin converting enzyme inhibitor of the hypotensive actions of an angiotensin converting enzyme inhibitor (captopril) in hypertension. In perfused mesenteric vasculatures from spontaneously hypertensive rats (SHR, Okamoto and Aoki strain, 10-13 weeks of age) and age-matched normotensive Wistar Kyoto rats (WKY), the effects of captopril on vascular responsiveness and norepinephrine release from the adrenergic nerve endings were examined. The vasoconstrictor responses and norepinephrine release during the electrical nerve stimulation were significantly enhanced in SHR compared to those in age-matched WKY. Captopril reduced both vasoconstrictor responses and norepinephrine release during the electrical nerve stimulation, dose-dependently. These inhibitory effects of captopril were significantly greater in SHR than in WKY. The results demonstrate that captopril affects presynaptic sites on the resistance vessels and causes a decrease in electrically-stimulated norepinephrine release from the adrenergic nerve endings. The marked reduction of both pressor responses and norepinephrine release by captopril in SHR suggests an enhanced renin-angiotensin system in the vascular walls of hypertension.

This study investigated norepinephrine release during electrical nerve stimulation and inhibitory characteristics of presynaptic alpha 2-adrenoceptors in perfused mesenteric vasculature from deoxycorticosterone acetate (DOCA)-salt hypertensive rats (7-8 weeks after surgery). Electrical stimulation of sympathetic innervation caused a significantly greater release of endogenous norepinephrine into the mesenteric vasculature of DOCA-salt hypertensive rats than in age-matched normotensive controls. Pressor responses to electrical nerve stimulation were also enhanced in DOCA-salt hypertension. Yohimbine, a potent alpha 2-adrenoceptor blocking agent, potentiated the stimulation-evoked release of norepinephrine into the vasculature in normotensive rats. This effect was blunted in DOCA-salt hypertension. These results suggest that increased norepinephrine release from the sympathetic nerve endings in DOCA-salt hypertension might partly reflect an impaired presynaptic alpha 2-adrenoceptor-mediated inhibition, which could enhance vascular sympathetic tone in this model of hypertension.

In order to investigate the peripheral neuroeffector functions of renal hypertension, norepinephrine release from the sympathetic nerve endings and vascular responsiveness were evaluated in the mesenteric vasculatures from two-kidney, one-clip Goldblatt hypertensive rats (2K, 1C-HT). Norepinephrine release and pressor responses during periarterial nerve stimulation (40 V, 5 msec rectangular pulses for 1 min, 5, 10 and 15 Hz) were unchanged during the acute phase (3 weeks after surgery), and were rather reduced during the chronic phase (7-8 weeks after surgery) in 2K, 1C-HT compared to sham-operated normotensive control rats. By contrast, the vasoconstrictor responses to exogenously applied norepinephrine were significantly greater in 2K, 1C-HT than in the normotensive controls. From these observations, it seems unlikely that peripheral adrenergic function in the blood vessels plays an important role in the pathogenesis of 2K, 1C-HT. However, the increased sensitivity of vascular smooth muscle cells to norepinephrine may contribute to the maintenance of high blood pressure in the face of reduced vascular sympathetic tone in this form of hypertension.

To investigate the role of norepinephrine release from the sympathetic nerve endings and vascular responsiveness to endogenous and exogenous norepinephrine in the pathogenesis of hypertension, the perfused mesenteric preparations from spontaneously hypertensive rats (SHR) and DOCA-salt hypertension were used. In addition, the effects of two calcium antagonists on norepinephrine release by electrical stimulation and the vasoconstrictor responses to norepinephrine were examined to evaluate the calcium dependency of norepinephrine release and the vascular responsiveness in hypertension. The vascular responsiveness was increased in SHR and DOCA-salt hypertension in both early and later phases. Norepinephrine overflow from the sympathetic nerve terminals was enhanced only in young SHR and the chronic phase of DOCA-salt hypertension. After infusion of verapamil, a calcium antagonist, into the perfusate, the pressor responses and norepinephrine overflow by electrical stimulation were more inhibited in SHR, especially in young SHR, and the chronic phase of DOCA-salt hypertension than in controls. These results suggest that calcium dependency in both pre- and postsynaptic sites of neurotransmission may contribute to the pathogenesis of hypertension.

The purpose of the present study is to determine the role of Na+,K+-ATPase in adrenergic neurotransmission of hypertension. Isolated perfused mesenteric vasculatures were prepared from spontaneously hypertensive rats (SHR, Okamoto and Aoki, 7-10 weeks old) and age-matched normotensive Wistar Kyoto rats (WKY). The effects of ouabain, a Na+,K+-ATPase inhibitor, on the norepinephrine overflow from the sympathetic nerve endings were examined. Norepinephrine overflow from the nerve endings as well as pressor responses during electrical nerve stimulation were significantly greater in SHR than in WKY. Ouabain increased the norepinephrine overflow evoked by electrical nerve stimulation, even in the presence of an uptake-blocker of norepinephrine. Further, the facilitatory effect of ouabain on stimulation-induced norepinephrine overflow was more prominent in SHR than in WKY. These results suggested that ouabain-sensitive Na+,K+-ATPase on sympathetic nerve terminals could have an important role in the regulation of neurotransmitter release, and that its activity might be enhanced in SHR compared with WKY.

The purpose of the present study was to analyse the regulatory mechanisms of epinephrine and dopamine on norepinephrine release from the sympathetic nerve endings in the resistance vessels in hypertension. The perfused mesenteric arteries were used for the experiment in spontaneously hypertensive rats (SHR; 7-10 weeks old) and age-matched normotensive Wistar-Kyoto (WKY) rats. Norepinephrine overflow during electrical nerve stimulation was significantly greater in SHR than in WKY rats. A low concentration of epinephrine (5.5 X 10(-9) mol/l) potentiated norepinephrine overflow from the nerve endings in SHR, and this was antagonized by propranolol, while this overflow was reduced by the same concentration of epinephrine in WKY rats. Higher concentrations of epinephrine decreased the norepinephrine overflow both in SHR and WKY rats, and this was antagonized by yohimbine. Dopamine reduced the norepinephrine overflow during electrical nerve stimulation, suppression being significantly less in SHR than in WKY rats. These results suggest that in SHR the increased norepinephrine overflow from sympathetic nerve endings may be partly caused by the facilitatory effect of epinephrine (through presynaptic beta-adrenoceptors) and impaired dopamine-mediated inhibition on the nerve terminals, which might contribute to the enhanced adrenergic activity in hypertension.

The present study was designed to investigate whether Ca-antagonists influence norepinephrine release from the sympathetic nerve endings in resistance vessels. Isolated mesenteric vascular preparations of rats, perfused with Ringer-Locke solution, were used to determine the effects of verapamil or diltiazem on norepinephrine release from sympathetic nerve terminals. Pressor responses to exogenous norepinephrine and electrical nerve stimulation were recorded, and the norepinephrine contents of the perfusate were measured before and after the electrical nerve stimulation, using high pressure liquid chromatography with an electrochemical detector. Pressor responses to exogenous norepinephrine and electrical nerve stimulation were depressed dose-dependently by verapamil and diltiazem. The inhibition of the pressor responses for nerve stimulation was greater than exogenous norepinephrine. Both verapamil and diltiazem reduced norepinephrine release from the mesenteric arterial beds after the electrical nerve stimulation. These results indicate that Ca++-antagonists, such as verapamil and diltiazem, have inhibitory effects on norepinephrine release from the sympathetic nerve endings, in addition to their direct actions on the vascular smooth muscle. It is also suggested that the presynaptic inhibitory action of Ca++-antagonists could be due to the blockade of Ca++-channels in nerve cell terminals, which may be important in the clinical use of Ca-antagonists.

This study was carried out to elucidate the involvement of the peripheral sympathetic nervous system in the pathogenesis of reduced renal mass (RRM)-salt hypertension in rats. Male Wistar rats underwent subtotal nephrectomy (70-80% reduction in renal mass). On the 10th postoperative day, systolic blood pressure was higher in RRM-salt rats than in RRM-water rats. Perfusion of isolated mesenteric vasculature from normotensive rats, with the deproteinized plasma from RRM-salt hypertensive rats induced significantly higher pressor responses and noradrenaline overflow with electrical stimulation than the perfusate from RRM-water normotensive rats; the responses to exogenous noradrenaline showed no significant changes. In the presence of desipramine, noradrenaline overflow did not increase following perfusion with plasma from RRM-salt hypertensive rats. These data indicate that plasma from rats with RRM-salt hypertension might contain some noradrenaline re-uptake blocking factor.

This study was designed to investigate the role of calmodulin in adrenergic neurotransmission of resistance vessels. The effects of various calmodulin antagonists (trifluoperazine, W-7, calmidazolium, chlorpromazine, fluphenazine) on the vascular responsiveness and norepinephrine overflow from adrenergic nerve endings were examined in perfused rat mesenteric vasculature preparations. Pressor responses to electrical nerve stimulation or exogenous norepinephrine were inhibited dose-dependently by each calmodulin antagonist. Norepinephrine overflow from the sympathetic nerve endings during electrical nerve stimulation was also suppressed by calmodulin antagonists. These results indicate that calmodulin antagonists affected both pre- and post-synaptic sites of adrenergic neurotransmission, suggesting that calmodulin is involved both in neurosecretion and vascular smooth muscle contractions in peripheral resistance vessels.

It has been reported that salt loading induces a blood pressure rise in rats whose renal mass has been reduced. We examined the involvement of the peripheral sympathetic nervous system in the pathogenesis of hypertension in subtotally nephrectomized and salt-loaded rats. Male Wistar rats (200-240 g) underwent subtotal nephrectomy (removal of 70-80% of the renal mass). After surgery, 1% saline was given ad libitum as drinking water in the experimental group (E), while tap water was given to the controls (C). On the 10th day after nephrectomy, blood pressure was determined, and plasma samples were collected. Deproteinized plasma samples were diluted with Ringer-Locke solution (1/40 v/v), and were perfused into isolated mesenteric artery-intestinal loop preparations of normotensive male Wistar rats. Pressor responses and norepinephrine overflow induced by electrical nerve stimulation, and the pressor responses to exogenous norepinephrine were assessed before and after the addition of deproteinized plasma to the perfusate. Systolic blood pressure was significantly elevated in the experimental group (E: 150 +/- 5 mmHg, C: 109 +/- 4 mmHg, p less than 0.01). When deproteinized plasma from the experimental group was perfused, pressor responses to electrical nerve stimulation were significantly increased (E: 167.2 +/- 13.7%, C: 91.1 +/- 6.4%; p less than 0.01), as was norepinephrine overflow (E: 117.3 +/- 6.4%, C: 90.7 +/- 5.9%; p less than 0.05), while responses to exogenous norepinephrine were only slightly augmented compared with the control group (E: 120.0 +/- 2.7%, C: 105.5 +/- 9.0%; n.s.).(ABSTRACT TRUNCATED AT 250 WORDS)