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Platelets have been implicated in the pathophysiology of ischemia-reperfusion injury. In this study, antiplatelet effects of cyclic GMP (cGMP)- and cyclic AMP (cAMP)-mediated agents were evaluated in renal ischemia in pentobarbital-anesthetized rats. Renal ischemia was induced by unilateral occlusion of the left renal artery (40 min) followed by reperfusion (30 min) with the contralateral kidney serving as control. 111Indium-labeled platelets, drugs or vehicle were administered 30 min before induction of renal ischemia. Occlusion of the left renal artery for 20, 40 or 60 min resulted in a 100, 300 and 600% increase (over contralateral right kidney) in the platelet-associated 111indium activity in the ischemic kidney. In all subsequent studies the kidney was occluded for 40 min to test the antiplatelet activity of individual agents. 8-Br-cGMP (0.1 and 0.3 mg/kg/min i.v.), zaprinast (0.1 mg/kg/min i.v.) and sodium nitroprusside (0.003 and 0.01 mg/kg/min i.v.) significantly attenuated platelet accumulation in renal ischemia, whereas 8-Br-cAMP (0.3 mg/kg/min i.v.) or milrinone (0.1 mg/kg i.v. bolus, plus 0.01 mg/kg/min) did not. Minoxidil (0.01 and 0.03 mg/kg/min i.v.), a vasodilator which produced equihypotensive effects as the cGMP-mediated agents, and milrinone failed to prevent platelet accumulation. These results demonstrate that modulation of the platelet function by cGMP agents can be dissociated from their blood pressure lowering effects. cGMP is known to inhibit both platelet adhesion and aggregation, whereas cAMP is only active against aggregation. The present findings provide further evidence that cGMP-mediated drugs may afford effective antiplatelet action in an in vivo model of ischemia-reperfusion injury.

Endothelin (ET)-1 is a potent vasoactive peptide elaborated by the vascular endothelial cells. In the present study we examined the effects of ischemia-hypoxia (I/H) on ET-1 release from isolated perfused guinea pig lungs and heart. Guinea pig lungs subjected to 15 min I/H followed by reperfusion and reventilation significantly (P less than .05) augmented ET-1 release from 14.1 +/- 2.7 to 30.4 +/- 5.6, 27.3 +/- 4.0 and 28.0 +/- 5.0 pg/g of dry weight of lung at 15, 30 and 45 min after I/H, respectively. Pretreatment of guinea pigs with phosphoramidon (10 mg/kg i.v.), an ET converting enzyme inhibitor, 10 min before the removal of lungs abrogated the I/H-induced increases in ET-1 release without affecting the base-line values of ET-1. Phosphoramidon also attenuated the elevations in pulmonary insufflation pressure (PIP) produced by I/H. Moreover, infusion of big ET-1 (BET-1; 30 micrograms/over 15 min) into isolated perfused guinea pig lungs enhanced PIP that was abolished by phosphoramidon. Isolated guinea pig hearts subjected to 15 or 30 min of global ischemia exhibited no disturbances in ET-1 release or mechanical activity. In addition, the increases in perfusion pressure elicited by BET-1 infusion (12 micrograms/over 30 min) into isolated guinea pig hearts was unaffected by phosphoramidon. In a separate study in anesthetized guinea pigs, phosphoramidon significantly attenuated the increases in blood pressure and PIP elicited by BET-1 (10 micrograms/kg i.v.); the pressor and PIP responses to ET-1 (4 micrograms/kg i.v.) were not affected.(ABSTRACT TRUNCATED AT 250 WORDS)

The possible role of sympathoadrenal stimulation and endothelin release in cyclosporine (CS)-induced hypertension was ascertained in intact and pithed rats. CS (20 and 40 mg/kg), administered by i.v. infusion over 10 min, produced a dose-dependent increase in blood pressure: 19 +/- 5 and 31 +/- 2 mm Hg in intact rats and 13 +/- 4 and 18 +/- 2 mm Hg in pithed rats. In intact rats, pretreatment with reserpine (5 mg/kg, i.p.) or hexamethonium (10 mg/kg, i.v.) greatly blunted the pressor responses to CS (40 mg/kg) (7 +/- 3 and 11 +/- 2 mm Hg, respectively). In pithed rats, the blood pressure responses to CS (40 mg/kg) were significantly impaired, but were not further modified by phenoxybenzamine (3 mg/kg, i.v.), whereas adrenalectomy completely abolished the CS-induced pressor responses (0 +/- 1 mm Hg). CS (40 mg/kg) did not potentiate pressor responses to sympathetic nerve stimulation (0.1 and 0.3 Hz) or vasoconstrictors, including angiotensin II (0.03 microgram/kg, i.v.), phenylephrine (1 microgram/kg, i.v.) and arginine vasopressin (0.075 microgram/kg) in pithed rats. In addition, CS (40 mg/kg, i.v.) did not cause elevation of plasma immunoreactive endothelin-1 and -3. Furthermore, phosphoramidon (0.25 mg/kg/min x 30) abolished pressor response to big endothelin-1 (5 micrograms/kg, i.v.) but failed to affect CS-induced hypertension. It is concluded that the acute blood pressure response to CS manifests great dependence on sympathetic nervous system but appears independent of endothelin release.

The antisecretory and cytoprotective actions of SCH 28080 (2-methyl-8-(phenylmethoxy)imidazol[ 1,2-a ]pyridine-3-acetonitrile), an antiulcer compound, were characterized. In the isolated guinea-pig gastric mucosa, SCH 28080 at 5 X 10(-8) M abolished the acid secretory responses to histamine and methacholine but at 5 X 10(-7) M also inhibited the responses to dibutyryl cyclic AMP plus theophylline. The data suggest that the antisecretory effect of SCH 28080 involves a direct action on the parietal cells distal to the primary events mediating the cholinergic and H2 histaminergic secretory mechanisms. In the histamine-stimulated dogs, a prolonged suppression of acid secretion was accompanied by only a transient fall in the ratio of mucosal plasma flow to acid output after i.v. dosing of SCH 28080, 1 mg/kg, indicating that the antisecretory action of SCH 28080 was not secondary to changes in gastric blood flow. Mechanisms mediating the cytoprotective effects of SCH 28080 were investigated. SCH 28080 (10 mg/kg p.o.) increased total mucus as determined by measurements of N-acetylneuraminic acid in rat stomach. In the isolated guinea-pig gastric mucosa, bicarbonate secretion was augmented in a dose-dependent (10(-6)-10(-4) M) manner. Concomitant stimulation of the gastric mucus and bicarbonate by SCH 28080 may lead to strengthening of the gastric mucosal barrier and account for its gastric cytoprotective activity against injuring agents. In conclusion, the antisecretory and cytoprotective activities of SCH 28080 are due to a direct action on the parietal cells and a stimulatory effect on mucus and bicarbonate secretion by the mucosal epithelial cells, respectively.

SCH 39370 (N-[N-[1-(S)-carboxyl-3-phenylpropyl]-(S)-phenyl-alanyl]-(S)-isoserin e) is a potent and specific inhibitor of neutral metalloendopeptidase (NEP) from rabbit kidney (IC50 = 11.2 +/- 1.9 nM) and is devoid of angiotensin-converting enzyme inhibitory activity at 1 microM. We evaluated the effect of NEP inhibition with SCH 39370 on the inactivation of atrial natriuretic factor (ANF) and on cardiovascular function in rats. SCH 39370 effectively prevented in vitro degradation of ANF (99-126) by a purified rabbit kidney NEP. SCH 39370 (30 mg/kg s.c) significantly delayed the disappearance of immunoreactive (ir) ANF from plasma in rats after an i.v. infusion of ANF (1 microgram/kg/min for 30 min): the plasma ir ANF level at 15 min postinfusion was 1.5 +/- 0.3 ng/ml vs. 0.3 +/- 0.04 ng/ml in the control. SCH 39370 also delayed the disappearance of ir ANF after infusion of the peptide (0.1 microgram/kg/min for 30 min) which increased plasma levels to those observed during volume expansion. This effect was accentuated markedly in rats with bilateral nephrectomy. The hypotensive response to injection of ANF (30 micrograms/kg i.v.) in spontaneously hypertensive rats (-38 +/- 6 mm Hg vs. -13 +/- 2 mm Hg in the control) and the diuretic and natriuretic responses to ANF in normal rats were potentiated by SCH 39370 (30 mg/kg s.c.), respectively. The results suggest that NEP can play a role in ANF disposition in vivo and that potentiation of the biological activities of high doses of ANF by SCH 39370 may be consequent to its inhibitory effect on ANF degradation.(ABSTRACT TRUNCATED AT 250 WORDS)

Phorbol esters activate protein kinase C and induce contraction in vascular smooth muscle. The role of Ca and the sensitivity of this response to nitrovasodilators were evaluated in rabbit aortic rings. Phorbol dibutyrate (PDB) (0.01-10 microM) elicited a concentration-dependent contraction of rabbit aortic rings. Responses to PDB in a salt solution (SS) containing 2.54 mM CaCl2 were not significantly different from those in SS containing 0 Ca and 2 mM ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid. Contractions to PDB (1 microM) in zero Ca SS were not reduced by depletion of the norepinephrine-sensitive pool of intracellular Ca. The Ca entry blockers verapamil (100 microM) and nifedipine (100 microM) did not affect PDB (1 microM) responses. Nitroprusside (0.1-10 microM) and nitroglycerin (0.1-10 microM) inhibited PDB contractions in both normal SS and Ca-free SS. 8-Br-cyclic GMP (100 microM) also inhibited PDB responses. Effects of PDB on 45Ca fluxes were evaluated in separate experiments. PDB (1 and 10 microM) elicited contraction, but no change in 45Ca uptake or efflux. In contrast KCl (80 mM) and norepinephrine (10 microM) elicited an increase in both influx and efflux, reflecting a rise in cytosolic Ca. The data suggest that PDB-induced contractions in rabbit aorta are independent of extracellular Ca and are not associated with a rise in cytosolic Ca as detected by calcium flux studies.(ABSTRACT TRUNCATED AT 250 WORDS)

The gastrointestinal protective action of SCH 12223 [3-(n-butyl)-4-hydroxy-1-phenyl-1,8-naphthyridin-2(1H)-one hydrate, sodium salt] against various noxious stimuli was characterized in rats. SCH 12223 inhibited the ethanol-induced gastric lesions when given p.o. (0.3-3 mg/kg) or i.v. (1 and 3 mg/kg). Indomethacin pretreatment (10 mg/kg p.o.) did not interfere with this effect. SCH 12223 was also active p.o. in five gastric erosion models: aspirin (1-10 mg/kg), aspirin + acid (1-30 mg/kg), indomethacin (1-10 mg/kg), stress (1 and 3 mg/kg) and reserpine (0.3-10 mg/kg)-induced erosions. The compound lacked antisecretory activity in the pylorus-ligated rats (10 mg/kg p.o.) but increased total gastric mucus (N-acetylneuraminic acid measurements) (1-10 mg/kg p.o.). SCH 12223 (10 and 30 mg/kg p.o.) attenuated intestinal lesions provoked by indomethacin (20 mg/kg p.o. or s.c.) and the protected animals showed better weight gain and food intake than controls. In a 38-day study, SCH 12223 also improved survival from indomethacin lethality (6 of 16 vs. 16 of 16 in the control, P less than .05). The protection cannot be attributed to changes in biliary excretion and plasma levels of indomethacin. We conclude that SCH 12223 is a unique agent which protects both gastric and intestinal epithelia from noxious stimuli, a feature shared by prostaglandins.

SCH 28080 (2-methyl-8-(phenylmethoxy)imidazo[1,2-a]pyridine-3-acetonitrile) is a novel antiulcer agent which has both antisecretory and cytoprotective activities. The antisecretory ED50 values in the pylorus-ligated rat were 3.7 mg/kg p.o. and 2.8 mg/kg i.p., being 7 and 10 times more potent than cimetidine, respectively. In dogs, SCH 28080 was effective in inhibiting acid secretion stimulated by histamine (ED50 of 0.09 mg/kg i.v. and 4.4 mg/kg p.o.), dimaprit, pentagastrin, insulin and feeding. The cytoprotective activity of SCH 28080 was demonstrated by inhibition of ethanol-induced gastric lesions in a dose-dependent manner in rats (ED50:3.0 mg/kg p.o.). SCH 28080 was active in similar dose ranges (1-10 mg/kg) by both p.o. and i.v. routes of administration. This gastric cytoprotective activity was not affected by indomethacin pretreatment. Furthermore, the gastric potential difference was effectively sustained by SCH 28080 (3, 10 and 30 mg/kg p.o.) after intragastric ethanol. SCH 28080 (1-30 mg/kg p.o.) also inhibited gastric ulcers provoked by aspirin, aspirin + acid, indomethacin and stress (cold-restraint) in rats. The data support the concept that it is possible to have combined antisecretory and cytoprotectant actions in a single molecule which is not a prostaglandin.