The regulation of intestinal bicarbonate secretion by marine teleost fish

• Main Author: Whittamore, Jonathan Mark
• Other Authors: Wilson, Roderic William
• Published: University of Exeter 2008
• Subjects: 571.1; bicarbonate secretion : osmoregulation : calcium-sensing receptor : chloride-bicarbonate exchange : teleost fish : carbonate precipitation : water transport : calcium : in vitro : in vivo
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.546911

In seawater, drinking is a fundamental part of the osmoregulatory strategy for teleost fish, and presents a unique challenge. The intestine has an established role in osmoregulation, and its ability to effectively absorb fluid from imbibed seawater is crucial to compensating for water losses to the surrounding hyperosmotic environment. Alongside solute-linked water transport (driven by NaCl cotransport), intestinal bicarbonate (HCO3-) secretion also benefits fluid absorption directly (via apical Cl-/HCO3- exchange), and indirectly through the formation of calcium carbonate (CaCO3) thus removing the osmotic influence of Ca2+ within the gut fluid. For the European flounder (Platichthys flesus), elevated luminal Ca2+ has proven to be a specific, potent stimulator of HCO3- secretion both in vitro and in vivo where these actions are presumably modulated by an extracellular Ca2+-sensing receptor (CaR). The focus of this work was to learn more about how intestinal HCO3- secretion is regulated, the role of Ca2+, and more specifically the CaR. To achieve this, in vitro ‘gut sac’ experiments investigated how luminal Ca2+ influenced HCO3- secretion, and associated ion and fluid transport. Contrary to expectation, increasing Ca2+ from 5 to 20 mM did not stimulate HCO3- secretion. In an attempt to elucidate the role of CaCO3 precipitation in fluid absorption, and further explore the physiological implications of HCO3- secretion, the intestine was perfused in vivo with salines containing varying concentrations of Ca2+ (10, 40 and 90 mM). The production and secretion of HCO3-, in addition to CaCO3 formation increased accordingly with Ca2+, and was associated with a dramatic 25 % rise in the fraction of fluid absorbed by the gut. Additional in vitro experiments, utilising the Ussing chamber, helped establish some of the characteristics of intestinal HCO3- secretion by the euryhaline killifish (Fundulus heteroclitus), but was unresponsive to elevated mucosal Ca2+. Further attempts to potentiate the activity of the CaR, and application of the receptor agonists gadolinium (Gd3+) and neomycin, failed to produce responses consistent with the effect of Ca2+ observed previously, either in vitro or in vivo. With no evidence supporting a direct role for an extracellular, intestinal CaR in HCO3- secretion it was argued that secretion would be principally regulated by two factors, the ability of the epithelia to generate high levels of intracellular HCO3- and the rate of CaCO3 formation.