Effect of autonomic agents on renin release in the turtle, Pseudemys scripta

Experiments were conducted in the freshwater turtle, Pseudemys scripta, to study the effect of autonomic agents on the control of renin in this primitive species. The unique finding in these studies was that, unlike mammals, isoproterenol infusion fails to increase renin activity even though heart rate and arterial pressure patterns indicated that systemic responses were similar to that in mammals. On the other hand, acetylcholine (ACh) infusion resulted in a prompt threefold elevation (P less than 0.01) of renin activity. This response was blocked by propranolol but not atropine. Other experiments demonstrated that ACh elicited a prompt elevation of circulating norepinephrine and epinephrine in these turtles and led to the hypothesis that the renin response to ACh was secondary to the release of endogenous catecholamines. This hypothesis was supported by the fact that, in reserpinized turtles, ACh failed to elicit the usual renin response. Further evidence stemmed from the fact that epinephrine administration led to a prompt fourfold increase (P less than 0.01) in renin, which could be blocked by propranolol. On the other hand, phenoxybenzamine failed to block the response under identical conditions. Taken as a whole, these data suggest that in this primitive species renin activity is elevated by endogenous catecholamines even though isoproterenol, a classical beta-adrenergic agonist, is without effect. These data illustrate again the need for caution when assuming analogy when comparing responses in primitive species with that of mammals.