Short-term exposure to 12‰ brackish water has significant effects on the endocrine physiology of juvenile American alligator (Alligator mississippiensis)

Abstract

American alligators (Alligator mississippiensis) mainly inhabit freshwater habitats but can be exposed to a wide range of salinities during storm surges, droughts or from alterations in freshwater flows. Although some salinization events last weeks, others only last a few days. This study assessed changes in the endocrine function of the renin-angiotensin-aldosterone system (RAAS) and steroid hormone production (steroidogenesis) in juvenile alligators exposed to brackish water (12‰) for 7 days. We quantified plasma levels of angiotensin II and the corticosteroids (aldosterone, corticosterone and 11-deoxycortisol). Various progestogens, androgens, and estrogens were further assessed. The protein expression for the RAAS enzymes, renin and angiotensin converting enzyme (ACE), was quantified immunohistochemically in kidney and lung tissue, respectively, and histology was performed on kidney, lung and gonad tissues. Finally, blood biochemistry parameters such as electrolyte levels and diagnostic indicators for dehydration, renal, and hepatic function were measured. Corticosterone, 11-deoxycortisol, Na+, Cl−, total protein, albumin, uric acid, and cholesterol levels were all significantly elevated in alligators exposed to brackish water compared with alligators in freshwater. The levels of 17β-estradiol and estrone were significantly lowered while histology showed alterations in gonad tissue in the brackish water exposed group. In contrast, while there were no effects of exposure on aldosterone levels, angiotensin II was significantly reduced in brackish water exposed alligators. These results correlated with significantly decreased expressions for both renin and ACE in kidney and lung tissue. Overall, this study showed that short-term exposure of alligators to 12‰ brackish water has significant endocrine effects on juvenile alligators.