Effects of converting enzyme inhibition and alpha receptor blockade on the angiotensin pressor response in the American alligator.

This study examines the effects of two converting enzyme inhibitors (captopril and enalaprilat) and two alpha-adrenergic receptor antagonists (phentolamine and phenoxybenzamine) on the pressor response produced by exogenous angiotensin I ([Asp1, Val5, Ser9] ANG I, fowl) and [Val5] angiotensin II (ANG II) in the American alligator (Alligator mississippiensis). Bolus administration of ANG I at 0.1, 0.5, and 1.0 micrograms/kg; ANG II at 0.05, 0.1, and 0.5 micrograms/kg; or norepinephrine (NE) at 2 micrograms/kg elicited dose-dependent increases in arterial blood pressure. Captopril (0.5 mg/kg/hr) and enalaprilat (300 micrograms/kg/hr) significantly reduced the response to ANG I, but not ANG II or NE. Both phenoxybenzamine (0.25 mg/kg/min) and phentolamine (1 mg/kg/hr) effectively blocked the NE pressor response (84 and 88%, respectively) and attenuated (42-80%) the pressor effects of ANG I and ANG II. These results support previous work suggesting the alligator may possess a renin-angiotensin system with characteristics similar to those found in mammals and other vertebrates. In addition, the pressor response to exogenously administered ANG I and ANG II was attenuated by alpha adrenergic receptor blockade and thus may be due, in part, to secondary catecholamine release.