cAMP-Dependent potentiation of the Ca(2+)-activated release of the anionic fluorescent dye, calcein, from rat parotid acinar cells

A recent study indicates that elevation of [Ca(2+)](i) enhances the release of calcein, an anionic fluorescent dye, from isolated exocrine acinar cells, so cytoplasmic calcein is useful for monitoring the secretion of organic anions. In this study, we investigated the effect of cAMP on the calcein release evoked by elevation of [Ca(2+)](i). Isoproterenol, forskolin and dibutyryl cyclic AMP (dbcAMP) did not induce the release of calcein from isolated parotid acinar cells, but they potentiated the carbachol-induced release of calcein. Although cytoplasmic calcein is released through an increase in [Ca(2+)](i), isoproterenol potentiated the carbachol-induced release of calcein without affecting the increase in [Ca(2+)](i) evoked by a high concentration of carbachol (10(-6) M). Charybdotoxin, a K(+) channel blocker, inhibited both the carbachol-induced release and the potentiation by isoproterenol. However, the calcein permeation pathways mediating the carbachol-induced release and the isoproterenol-potentiated release exhibited distinct sensitivities to anion channel blockers. Our results indicate that the calcein release induced by carbachol is potentiated through an increase in intracellular levels of cAMP. Although both the Ca(2+)-activated release and the cAMP-potentiated release may be coupled to Ca(2+)-activated K(+) efflux, increases in both [Ca(2+)](i) and [cAMP](i) may activate the calcein conduction pathway which is not activated by an increase in [Ca(2+)](i) alone.