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Voltage-gated potassium ([K.sub.v]) channels in vascular smooth muscle cells (VSMC) are critical regulators of membrane potential and vascular tone. These channels exert a hyperpolarizing influence to counteract the depolarizing effects of intraluminal pressure and vasoconstrictors. However, the contribution of [K.sub.v] channel activity to the functional regulation of cerebral (parenchymal) arterioles within the brain is not known. Thus [K.sub.v] channel properties in parenchymal arteriolar SMCs were characterized. Isolated, pressurized parenchymal arterioles and arterioles in cortical brain slices exhibited robust constriction in the presence of the [K.sub.v] channel inhibitor 4-aminopyridine (4-AP). 4-AP also decreased the amplitude of [K.sub.v] currents recorded from SMCs. The steady-state activation and inactivation properties of [K.sub.v] currents suggested that these channels are composed of [K.sub.v]1.2 and 1.5 subunits, which was confirmed by RT-PCR. [K.sub.v] channels can be regulated by extracellular glucose, which may be involved in the functional hyperemic response in the brain. Thus the effects of glucose on [K.sub.v] channel activity and arteriolar function were investigated. Elevation of glucose from 4 to 14 mM significantly decreased the peak [K.sub.v] current amplitude and constricted arterioles. Arteriolar constriction was prevented by inhibition of protein kinase C (PKC), consistent with previous studies showing enhanced PKC activity in the presence of elevated glucose. In cortical brain slices, the dilation generated by neuronal activity induced by electrical field stimulation was decreased by 54% in 14 mM glucose when compared with the dilation in 4 mM glucose. In anesthetized mice the whisker stimulation-induced increase in local cerebral blood flow was also significantly decreased in 14 mM glucose, and this effect was similarly prevented by PKC inhibition. These findings point to a critical role for [K.sub.v] channels in the regulation of intracerebral arteriolar function and suggest that changes in perivascular glucose levels could directly alter vascular diameter resulting in a modulation of local cerebral blood flow. cerebral blood flow; protein kinase C; vascular smooth muscle cells

Previous work has shown that orthostatic hypotension associated with cardiovascular deconditioning results from inadequate peripheral vasoconstriction. We used the hindlimb-unloaded (HU) rat in this study as a model to induce cardiovascular deconditioning. The purpose of this study was to test the hypothesis that 14 days of HU diminishes vasoconstrictor responsiveness of mesenteric resistance arteries. Mesenteric resistance arteries from control (n = 43) and HU (n = 44) rats were isolated, cannulated, and pressurized to 108 cm [H.sub.2]O for in vitro experimentation. Myogenic (intralumenal pressure ranging from 30 to 180 cm [H.sub.2]O), KCl (2-100 mM), norepinephrine (NE, [10.sup.-9]-[10.sup.-4] M) and caffeine (1-20 mM) induced vasoconstriction, as well as the temporal dynamics of vasoconstriction to NE, were determined. The active myogenic and passive pressure responses were unaltered by HU when pressures remained within physiological range. However, vasoconstrictor responses to KCl, NE, and caffeine were diminished by HU, as well as the rate of constriction to NE (C, 14.8 [+ or -] 3.6 [micro]m/s vs. HU 7.6 [+ or -] 1.8 [micro]m/s). Expression of sarcoplasmic reticulum [Ca.sup.2+] ATPase 2 and ryanodine 3 receptor mRNA was unaffected by HU, while ryanodine 2 receptor mRNA and protein expression were diminished in mesenteric arteries from HU rats. These data suggest that HU-induced and microgravity-associated orthostatic intolerance may be due, in part, to an attenuated vasoconstrictor responsiveness of mesenteric resistance arteries resulting from a diminished ryanodine 2 receptor [Ca.sup.2+] release mechanism. hindlimb unloading; vasoconstrictor responsiveness

Vascular damage signals smooth muscle cells to proliferate, often exacerbating existing pathologies. Although the role of changes in 'global' [Ca.sup.2+] in vascular smooth muscle (VSM) cell dedifferentiation has been studied, the role of specific [Ca.sup.2+] signals in determining VSM phenotype remains relatively unexplored. Earlier work with cultured VSM cells suggests that inositol 1,4,5-trisphosphate receptor ([IP.sub.3]R) expression and sarcoplasmic reticulum (SR) [Ca.sup.2+] release may be linked to VSM cell proliferation in native tissue. Thus we hypothesized that SR [Ca.sup.2+] release through [IP.sub.3]Rs in the form of discrete transient signals is necessary for VSM cell proliferation. To investigate this hypothesis, we used mouse cerebral arteries to design an organ culture system that permitted examination of [Ca.sup.2+] dynamics in native tissue. Explanted arteries were cultured in normal medium with 10% FBS, and appearance of individual VSM cells migrating from explanted arteries (outgrowth cells) was tracked daily. Initial exposure to 10% FBS increased [Ca.sup.2+] waves in myocytes in the arteries that were blocked by the [IP.sub.3]R antagonist 2-aminoethoxydiphenylborate (2-APB). Inhibition of [IP.sub.3]R opening (via 100 [micro]M 2-APB, 10 [micro]M xestospongin C, or 25 [micro]M U-73122) dramatically reduced outgrowth cell number compared with untreated or ryanodine-treated (10 [micro]M) arteries. Consistent with this finding, 2-APB inhibited cell proliferation, as measured by reduced proliferating cell nuclear antigen immunostaining within 48 h of culture but did not inhibit cell migration. These results indicate that activation of [IP.sub.3]R [Ca.sup.2+] release is required for VSM cell proliferation in these arteries. calcium signaling; explant culture; calcium channels; sarcoplasmic reticulum

Elevations in arterial pressure associated with hypertension, micro-gravity, and prolonged bed rest alter cerebrovascular autoregulation in humans. Using head-down tail suspension (HDT) in rats to induce cephalic fluid shifts and elevate arterial pressure, this study tested the hypothesis that 2-wk HDT enhances cerebral artery vasoconstriction and that an enhanced vasoconstriction described in vitro will alter regional cerebral blood flow (CBF) and vascular resistance (CVR) during standing and head-up tilt. To test this hypothesis, basal tone and vasoconstrictor responses to increases in transmural pressure, shear stress, and [K.sup.+] were determined in vitro in middle cerebral arteries (MCAs) from HDT and control rats. All in vitro measurements were done in the presence and absence of the nitric oxide synthase (NOS) inhibitor [N.sup.G]-nitro-L-arginine methyl ester (L-NAME; [10.sup.-5] M) and with endothelium removal. Endothelial NOS (eNOS) mRNA and protein expression levels were measured by RT-PCR and immunoblot, respectively. Regional CBF and CVR were determined with a radiolabeled tracer technique and quantitative autoradiography. Basal tone and all vasoconstrictor responses were greater in MCAs from HDT rats. L-NAME and endothelium removal abolished these differences between groups, and HDT was associated with lower levels of MCA eNOS protein. CBF in select regions was lower and CVR higher during standing and head-up tilt in HDT rats. These results indicate that chronic cephalic fluid shifts enhanced basal tone and vasoconstriction through alterations in the eNOS signaling mechanism. The functional consequence of these vascular alterations with HDT is regional elevations in CVR and corresponding reductions in cerebral perfusion. orthostatic intolerance; cerebral blood flow; hindlimb unloading; middle cerebral artery; vascular remodeling; nitric oxide synthase

Acute and chronic head-down tail suspension diminishes cerebral perfusion in rats. Am J Physiol Heart Circ Physiol 282: H328-H334, 2002; 10.1152/ajpheart.00727.2001. --The purpose of this study was to test the hypothesis that regional brain blood flow and vascular resistance are altered by acute and chronic head-down tail suspension (HDT). Regional cerebral blood flow, arterial pressure, heart rate, and vascular resistance were measured in a group of control rats during normal standing and following 10 min of HDT and in two other groups of rats after 7 and 28 days of HDT. Heart rate was not different among conditions, whereas mean arterial pressure was elevated at 10 min of HDT relative to the other conditions. Total brain blood flow was reduced from that during standing by 48, 24, and 27% following 10 min and 7 and 28 days of HDT, respectively. Regional blood flows to all cerebral tissues and the eyes were reduced with 10 min of HDT and remained lower in the eye, olfactory bulbs, left and right cerebrum, thalamic region, and the midbrain with 7 and 28 days of HDT. Total brain vascular resistance was 116, 44, and 38% greater following 10 min and 7 and 28 days of HDT, respectively, relative to that during control standing. Vascular resistance was elevated in all cerebral regions with 10 min of HDT and remained higher than control levels in most brain regions. These results demonstrate that HDT results in chronic elevations in total and regional cerebral vascular resistance, and this may be the underlying stimulus for the HDT-induced smooth muscle hypertrophy of cerebral resistance arteries. cerebral blood flow; microgravity; hindlimb unloading; unweighting

Delp, Michael D., Patrick N. Colleran, M. Keith Wilkerson, Matthew R. McCurdy, and Judy Muller-Delp. Structural and functional remodeling of skeletal muscle microvasculature is induced by simulated microgravity. Am J Physiol Heart Circ Physio1 278: H1866-H1873, 2000.-- Hindlimb unloading of rats results in a diminished ability of skeletal muscle arterioles to constrict in vitro and elevate vascular resistance in vivo. The purpose of the present study was to determine whether alterations in the mechanical environment (i.e., reduced fluid pressure and blood flow) of the vasculature in hindlimb skeletal muscles from 2-wk hindlimb-unloaded (HU) rats induces a structural remodeling of arterial microvessels that may account for these observations. Transverse cross sections were used to determine media cross-sectional area (CSA), wall thickness, outer perimeter, number of media nuclei, and vessel luminal diameter of feed arteries and first-order (1A) arterioles from soleus and the superficial portion of gastrocnemius muscles. Endothelium-dependent dilation (ACh) was also determined. Media CSA of resistance arteries was diminished by hindlimb unloading as a result of decreased media thickness (gastrocnemius muscle) or reduced vessel diameter (soleus muscle). ACh-induced dilation was diminished by 2 wk of hindlimb unloading in soleus 1A arterioles, but not in gastrocnemius 1A arterioles. These results indicate that structural remodeling and functional adaptations of the arterial microvasculature occur in skeletal muscles of the HU rat; the data suggest that these alterations may be induced by reductions in transmural pressure (gastrocnemius muscle) and wall shear stress (soleus muscle). acetylcholine; arteriole; endothelium; shear stress; smooth muscle

I. INTRODUCTION A. Historical Perspective Corpora (bodies) lutea (yellow) were named by Marcello Malpighi (1628--1694) and first accurately described by Regnier de Graaf (1641--1673) who noted that after coitus 'globular [...]

Voltage-gated potassium (Kv) channels in vascular smooth muscle cells (VSMC) are critical regulators of membrane potential and vascular tone. These channels exert a hyperpolarizing influence to counteract the depolarizing effects of intraluminal pressure and vasoconstrictors. However, the contribution of Kv channel activity to the functional regulation of cerebral (parenchymal) arterioles within the brain is not known. Thus Kv channel properties in parenchymal arteriolar SMCs were characterized. Isolated, pressurized parenchymal arterioles and arterioles in cortical brain slices exhibited robust constriction in the presence of the Kv channel inhibitor 4-aminopyridine (4-AP). 4-AP also decreased the amplitude of Kv currents recorded from SMCs. The steady-state activation and inactivation properties of Kv currents suggested that these channels are composed of Kv1.2 and 1.5 subunits, which was confirmed by RT-PCR. Kv channels can be regulated by extracellular glucose, which may be involved in the functional hyperemic response in the brain. Thus the effects of glucose on K channel activity and arteriolar function were investigated. Elevation of glucose from 4 to 14 mM significantly decreased the peak Kv current amplitude and constricted arterioles. Arteriolar constriction was prevented by inhibition of protein kinase C (PKC), consistent with previous studies showing enhanced PKC activity in the presence of elevated glucose. In cortical brain slices, the dilation generated by neuronal activity induced by electrical field stimulation was decreased by 54% in 14 mM glucose when compared with the dilation in 4 mM glucose. In anesthetized mice the whisker stimulation-induced increase in local cerebral blood flow was also significantly decreased in 14 mM glucose, and this effect was similarly prevented by PKC inhibition. These findings point to a critical role for Kv channels in the regulation of intracerebral arteriolar function and suggest that changes in perivascular glucose levels could directly alter vascular diameter resulting in a modulation of local cerebral blood flow. [ABSTRACT FROM AUTHOR]

Previous work has shown that orthostatic hypotension associated with cardiovascular deconditioning results from inadequate peripheral vasoconstriction. We used the hindlimb-unloaded (HU) rat in this study as a model to induce cardiovascular deconditioning. The purpose of this study was to test the hypothesis that 14 days of HU diminishes vasoconstrictor responsiveness of mesenteric resistance arteries. Mesenteric resistance arteries from control (n = 43) and HU (n = 44) rats were isolated, cannulated, and pressurized to 108 cm H2O for in vitro experimentation. Myogenic (intralumenal pressure ranging from 30 to 180cm H2O), KCI (2-100 mM), norepinephrine (NE, 10-9-10-4 M) and caffeine (1-20 mM) induced vasoconstriction, as well as the temporal dynamics of vasoconstnction to NE, were determined. The active myogenic and passive pressure responses were unaltered by HU when pressures remained within physiological range. However, vasoconstrictor responses to KCI, NE, and caffeine were diminished by HU, as well as the rate of constriction to NE (C, 14.8 ± 3.6 μm/s vs. HU 7.6 ± 1.8 μm/s). Expression of sarcoplasmic reticulum Ca2+ATPase 2 and ryanodine 3 receptor mRNA was unaffected by HU, while ryanodine 2 receptor mRNA and protein expression were diminished in mesenteric arteries from HU rats. These data suggest that HU-induced and microgravity-associated orthostatic intolerance may be due, in part, to an attenuated vasoconstrictor responsiveness of mesenteric resistance arteries resulting from a diminished ryanodine 2 receptor Ca2+ release mechanism. [ABSTRACT FROM AUTHOR]