Blockade of angiotensin pressor activity in the freshwater turtle

Abstract

In order to study the pharmacological characteristics of the renin-angiotensin system in the turtle, Pseudemys scripta elegans, angiotensin I (AI) and angiotensin II (AII) were administered intravenously to conscious turtles along with angiotensin antagonists, a converting enzyme inhibitor, and adrenergic blockers. Both AI and AII produced dose-dependent vasopressor responses. Infusion of the AII analogs [Sar 1, Ala 8] AII or [Sar 1, Ile 8] AII at 10 μg/kg/min significantly reduced the pressor responses to both AI and AII while the pressor action of norepinephrine (3 μg/kg) was unaffected. Neither AII analog exhibited a sustained pressor action at this infusion rate. The converting enzyme inhibitor SQ 20881 (1 mg/kg bolus + 0.5 mg/kg/hr) did not reduce the pressor effect of AII but significantly reduced the response to AI. The α-adrenergic antagonists phentolamine (0.5 mg bolus + 1 mg/kg/hr) and phenoxybenzamine (0.25 mg/kg/min) almost abolished the pressor response to norepinephrine and significantly decreased the vasopressor action of AI and AII. The results suggest that an angiotensin-converting enzyme-like mechanism may exist in the turtle and that the pressor response to angiotensin in the turtle may be due largely to catecholamines.