Cellular and molecular biology of chloride secretion in the shark rectal gland: regulation by adenosine receptors.

The rectal gland of the dogfish shark (Squalus acanthias) is a sodium chloride secreting epithelial organ whose function was discovered in 1959 by Wendell Burger. The gland, composed of homogenous tubules of a single cell type, is an important model for secondary active chloride transport. Hormonal stimulation of chloride secretion in this system activates asymetrically arranged transport proteins (apical cAMP-activated CFTR-like Cl- channels, basolateral Na/K/2Cl cotransporters, Na/K-ATPase activity, and K+ channels). Five receptors, hormones, and membrane proteins of the shark rectal gland involved in chloride secretion have been cloned recently. Because the intact gland can be perfused via a single artery and vein, it has been possible to examine precisely the metabolic regulation of chloride transport by endogenous adenosine. Rectal gland cells have a high density of both stimulatory A2 type and inhibitory A1 type adenosine receptors. When stimulated by secretagogues, chloride secretion and venous adenosine concentrations increase in parallel, with chloride secretion increasing from approximately 150 to 2100 microEq/hr/g, and adenosine concentrations increasing from approximately 5 to approximately 890 nM. This work of ion transport is accompanied by a marked fall in intracellular ATP activity and a rise in both intracellular AMP and adenosine activity. Agents that prevent the interaction of endogenous adenosine with extracellular receptors significantly increase the chloride transport response to secretagogues. When chloride transport is inhibited by blocking the Na/K/2Cl cotransporter with bumetanide, both adenosine release and chloride secretion fall to basal values. We recently cloned a unique adenosine receptor subtype that is distinct from previously cloned mammalian adenosine receptors. Because of its highly specialized function, single cell type, and simple vascular system, the shark rectal gland is an ideal model system for examining the metabolic regulation of chloride secretion by adenosine receptors.