TAYLOR, J.R.; GROSELL, M.; Univ. of Miami; Univ. of Miami: Feeding and osmoregulation: dual function of the marine teleost intestine

Experiments performed on Gulf Toadfish, Opsanus beta, demonstrate how feeding impacts HCO₃^– secretion and overall osmoregulation in the marine teleost intestine. Because Ca²⁺ has been shown to stimulate intestinal HCO₃^– secretion, Gulf Toadfish were fed a high Ca²⁺ diet of chopped pilchards, Sardina pilchardus ([Ca²⁺] 404.2 mMol kg^-1), and data were collected 1, 2, 3, 6, 9 and 12 days after feeding. Ion analysis of fluid in segments from the stomach to the rectum, particularly one and two days post-feeding, supports evidence that apical Cl^–/HCO₃^– exchange plays a vital role in Cl^– absorption and osmoregulation, particularly during digestion. Chloride concentrations one day post-feeding drop markedly from the stomach (182.5 mM) along the intestine to the rectum (46.9 mM), while [Na⁺] and [K⁺] remain relatively constant along the entire gastrointestinal tract. A pH increase from 3.95 to 7.35 between the stomach and anterior intestine one day post-feeding is the result of Cl^–/HCO₃^– exchange, which is also responsible for a notable rise in luminal HCO₃– concentration seen two days post-feeding, when intestinal pH has stabilized near 8.6. Bicarbonate secretion inhibits the absorption of Ca²⁺ by precipitating CaCO₃ as evidenced by Ca²⁺ concentrations that are elevated and constant across the intestine one day post-feeding and normalized by day two, while remaining consistently low in the blood plasma. Equivalent analysis is planned for a diet low in Ca²⁺ (squid, [Ca²⁺] 1.3 mMol kg-1), which may expose the critical stimulant of HCO₃^– secretion, be it high [Ca²⁺] or a low pH environment. A more comprehensive assessment of the osmoregulatory processes and potential alkaline tide that occur 0-48 hrs post-feeding is planned for both diets. (Supported by NSF-IBN 0416440 and a University of Miami graduate student fellowship).