cAMP-dependent potentiation of the Ca2+-activated release of the anionic fluorescent dye, calcein, from rat parotid acinar cells

A recent study indicates that elevation of [Ca2+](i) enhances the release o f calcein, an anionic fluorescent dye, from isolated exocrine acinar cells, so cytoplasmic calcein is useful for monitoring the secretion of organic a nions. In this study, we investigated the effect of cAMP on the calcein rel ease evoked by elevation of [Ca2+](i). Isoproterenol, forskolin and dibutyr yl 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 [ Ca2+](i), isoproterenol potentiated the carbachol-induced release of calcei n without affecting the increase in [Ca2+](i) evoked by a high concentratio n of carbachol (10(-6) M). Charybdotoxin, a K+ channel blocker, inhibited b oth the carbachol-induced release and the potentiation by isoproterenol. Ho wever, the calcein permeation pathways mediating the carbachol-induced rele ase and the isoproterenol-potentiated release exhibited distinct sensitivit ies to anion channel blockers. Our results indicate that the calcein releas e induced by carbachol is potentiated through an increase in intracellular levels of cAMP. Although both the Ca2+-activated release and the cAMP-poten tiated release may be coupled to Ca2+-activated K+ efflux, increases in bot h [Ca2+](i) and [cAMP], may activate the calcein conduction pathway which i s not activated by an increase in [Ca2+](i) alone. (C) 2000 Elsevier Scienc e B.V. All rights reserved.