Contractile response of the isolated dorsal aorta of the snake to angiotensin II and norepinephrine.

Abstract

Vasopressor action is a phylogenetically old function of angiotensin (ANG) II. The action can be ascribed to both direct activation of vascular ANG II receptors and through catecholamine release. In the Rat snake, Ptyas korros, the possible presence of common adrenergic-ANG receptors and their involvement in this action have been proposed. In order to elucidate the vasopressor mechanism of ANG II in the snake, and to test the presence of common adrenergic-ANG receptors, the contractile response of isolated snake dorsal aorta to [Val5]ANG II and norepinephrine (NE) was studied using the cobra, Naja naja and the Rat snake, Ptyas korros. Both ANG II (3 X 10(-11) to 10(-6) M) and NE (10(-8) to 10(-5) M) produced a dose-dependent increase in tension in the aortic strips of Naja, which were more sensitive to ANG II. Phenotolamine, an alpha-adrenergic antagonist, competitively inhibited NE without altering the response to ANG II. [Sar1, Ala8]ANG II inhibited ANG II but did not affect the response to NE. A similar dose-dependent increase in tension in the aortic strips of Ptyas was seen with NE (10(-8) to 10(-5) M) but these strips did not respond to ANG II. These results suggest that functionally separate receptors for ANG II and NE exist in the dorsal aorta of the cobra and that local liberation of catecholamines from the adrenergic nerve terminals do not play a role in the expression of ANG II action. There also exist differences in the nature of ANG II action/receptors among different species of snakes and different vascular beds in the same species.