An endocrinological update in toads: disparity between the cardiovascular effects of two angiotensin II analogs.

This study was undertaken to test the hypothesis that, in toads, the pressor effects of angiotensin II (ANG II) are partly due to the secondary effects of catecholamines. In Bufo marinus, blood pressure responses to bullfrog ([Val(5)]) ANG II administration were measured in animals pretreated (experimental group) and not pretreated (control group) with phentolamine, an alpha-adrenergic antagonist. At 1, 2, and 4 min following ANG II administration (50 ngkg(-1)), mean arterial pressure (MAP) of the experimental group was significantly less than in the control group (27.9+/-1.0, 28.2+/-1.6, and 25.8+/-2.1 mmHg versus 45.1+/-1.0, 39.4+/-2.8, and 35.5+/-3.6, respectively; n=6). Heart rate (f(H)) was unaffected by phentolamine. These data support our hypothesis. Previous authors, using human ([Ile(5)]) ANG II, have concluded that alpha-adrenergic mechanisms are not involved in the pressor effects of ANG II in B. marinus. In a separate group of toads, we examined MAP and f(H) responses to similar doses of human and bullfrog ANG II (100 ngkg(-1)). Bullfrog ANG II caused MAP to reach higher peak values (56.5+/-2.6 versus 37.2+/-0.4 mmHg; n=7) in less time (approximately 1 min versus approximately 8 min) than human ANG II. Furthermore, bullfrog ANG II caused f(H) to significantly increase from 41.3+/-5.1 to 79.3+/-2.3 beats min(-1) (at 1 min) whereas human ANG II caused no significant changes in f(H) throughout the measured time course. Higher doses of human ANG II (100 microgkg(-1)) invoked slow but large increases in MAP with non-significant decreases in f(H). These additional data suggest that the choice of exogenously administered hormone is an important one to consider in comparative endocrinological studies.