The role of cytosolic Ca2+ in the secretion of NaCl in isolated in vitro perfused rectal gland tubules of Squalus acanthias.

In many exocrine glands cytosolic Ca2+ ([Ca2+]i) plays a pivotal role in stimulation-secretion coupling. In the rectal gland of the dogfish Squalus acanthias this appears not to be the case and it is believed that secretion is mainly controlled by the Cl- conductance of the luminal membrane. We have examined this question in a study of isolated in vitro perfused rectal gland tubules (RGT). Three types of measurements were performed: (1) measurements of [Ca2+]i by the fura-2 technique; (2) measurements of transepithelial electrical parameters, i.e. transepithelial voltage (Vte), transepithelial resistance (Rte), the equivalent short-circuit current (Isc) and the voltage across the basolateral membrane (Vbl), and (3) whole-cell patch-clamp measurements of cellular voltage (Vm), conductance (Gm) and membrane capacitance (Cm). The data indicates that carbachol (CCH) increases [Ca2+]i by increasing store release and Ca2+ influx. Other agonists, producing cytosolic cAMP, also increased [Ca2+] by enhancing Ca2+ influx. CCH hyperpolarized these cells and enhanced Gm significantly. The effect of CCH on Vte and Isc was most marked under control conditions and disappeared in RGT otherwise stimulated by agonists that lead to cAMP production. It is concluded that [Ca2+]i plays a major role in the stimulation of NaCl secretion in RGT by enhancing the basolateral K+ conductance. cAMP-producing agonists enhance [Ca2+]i by increased Ca2+ influx. CCH releases Ca2+ from respective stores. CCH, unlike the cAMP-producing agonists, only increases basolateral K+ conductance. It modulates secretion especially under conditions in which the cAMP pathway is not fully activated.