THE SHARK RECTAL GLAND MODEL: A CHAMPION OF RECEPTOR MEDIATED CHLORIDE SECRETION THROUGH CFTR

The dogfish shark salt gland was predicted by Smith and discovered by Burger at the Mount Desert Island Biological Laboratory in Salisbury Cove, Maine. It is an epithelial organ in the intestine composed of tubules that serve a single function: the secretion of hypertonic NaCl. Many G protein receptors are present on the basolateral surface of these tubules, including stimulatory receptors for vasoactive intestinal peptide, adenosine A₂, growth hormone releasing hormone, and inhibitory receptors for somatostatin and adenosine A₁. An entirely different class of stimulatory receptors is present as C-type natriuretic peptide receptors. Each stimulatory receptor evokes powerful NaCl secretion. G protein receptors bind to G_αs to activate the catalytic unit of adenylate cyclase to form cyclic adenosine monophosphate (cAMP) and protein kinase A that phosphorylates the regulatory domain of cystic fibrosis transmembrane conductance regulator, opening the channel. The C-type natriuretic peptide receptor stimulates by activating guanylate cyclase and endogenous cyclic guanosine monophosphate which inhibits type 3 phosphodiesterase, the enzyme that breaks down cAMP, thereby elevating cAMP and activating the protein kinase A pathway.