The effects of adrenocorticotropin hormone and angiotensin II on adrenal corticosteroid secretions in the freshwater turtle Pseudemys scripta

Abstract

This study examined the effects of angiotensin II (ANG II) and mammalian adrenocorticotropin hormone (ACTH) on adrenal corticosteroid secretions in the freshwater turtle, Pseudemys scripta. Synthetic turtle ANG 11 ([Asp 1, Val 5] ANG II) was infused at rates of 1, 10, and 100 ng/kg/min in conscious turtles while monitoring blood pressure (BP). One 60-min saline (0.6%) infusion preceded the ANG II infusions; two followed. Blood samples were collected at 30- and 60-min intervals and the plasma was frozen at −20° until assay. Mammalian ACTH was infused at doses of 0.1 and 1.0 IU/min; the procedures were followed as delineated above. The plasma was assayed for corticosterone, cortisol, and aldosterone utilizing radioimmunoassay techniques. Infusions of exogenous, native ANG II at subpressor and pressor rates elicited dose-dependent increases in BP, which rose from a control mean of 22.6 ± 5.8 mm Hg to a maximum mean value of 38.2 ± 11.0 mm Hg ( P < 0.05 compared to control), and plasma corticosterone concentrations, which rose from a control mean of 6.6 ± 2.8 ng/ml to a maximum mean value of 27.2 ± 2.6 ng/ml ( P < 0.05 compared to control). Furthermore, both BP and corticosterone levels returned toward control levels during the final saline recovery period, suggesting that neither physical stress suffered by the animal nor blood volume changes due to infusions and blood sampling affected these parameters. ACTH failed to alter either BP or corticosterone. Neither ANG II nor ACTH had any effect on plasma cortisol or aldosterone concentrations—which fell below the minimal detection levels for these assays. The results suggest that corticosterone may be a primary mineralocorticoid in P. scripta, and perhaps in reptiles in general.