Abstract

Quipazine, a nonselective serotonin (5-HT) agonist, has been shown to increase plasma renin activity (PRA). The present study examined the effects of quipazine on mean arterial pressure (MAP), heart rate (HR) and PRA in conscious, chronically catheterized male rats. Quipazine caused dose (0.3-3.0 mg/kg i.v.)-and time (up to 30 min)-dependent increases in MAP and PRA. The maximum increases in MAP (control = 94 +/- 2 mm Hg, 3 mg/kg = 155 +/- 1 mm Hg) and PRA (nanograms of angiotensin 1 per milliliter per hour; control = 2.5 +/- 0.2, 3 mg/kg = 25.2 +/- 5.9) were observed 5 min after quipazine. HR tended to decrease, but a significant bradycardia was observed only 15 min after 3 mg/kg. The selective 5-HT2 antagonist LY 53857 (1 mg/kg i.v.) did not affect MAP, HR or PRA per se, but at 0.03 to 1.0 mg/kg totally abolished the pressor response to quipazine (3 mg/kg). At 0.01 mg/kg, LY 53857 attenuated quipazine-induced hypertension, whereas 0.003 mg/kg was ineffective. Total blockade of quipazine-induced renin secretion was produced by LY 53857 at 0.003 mg/kg, and the response was still reduced by 50% at 0.001 mg/kg. In summary, although quipazine increases arterial pressure and renin secretion, endogenous 5-HT does not tonically control MAP or PRA in conscious, unrestrained, normotensive rats through 5-HT2 receptors. The 10-fold difference in the dose of LY 53857 necessary to block the pressor and renin responses may be due to subtle differences in receptor subtypes, or to pharmacokinetic properties favoring antagonism of quipazine-induced renin secretion.