The physiological role and the control mechanism of Ca^<2+>-mobilization in stimulation-secretion coupling in parotid gland

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Carbachol (CCh), a cholinergic agonist, increased both cytosolic free calcium concentration ([Ca^<2+>]_i) and amylase release in rat parotid acinar cells in a dose-dependent manner. Treatment of acinar cells with the intracellular Ca^<2+> antagonist TMB-8 or the intracellular Ca^<2+> chelator BAPTA strongly attenuated the increase in [Ca^<2+>]_i evoked by CCh, but did not significantly suppress amylase release. A combined addition of the Ca^<2+> ionophore ionomycin and the microsomal ATPase inhibitor thapsigargin to cell suspension caused a noticeable increase in [Ca^<2+>]_i, but the effect on amylase release was much smaller than that of CCh. When ATP was added to cell suspension, a rapid elevation of [Ca^<2+>]_i was observed. This [Ca^<2+>]_i response is unlikely to be mediated by PI breakdown, because ATP had little or no effect on IP_3 formation. Despite the marked increase in [Ca^<2+>]_i, amylase release was not induced by extracellular ATP. The protein kinase C activator PMA stimulated amylase release in quantities similar to those induced by CCh. Staurosporine, a protein kinase C inhibitor, similarly inhibited both the CCh-and TPA-induced amylase release. These results suggest that an increase in [Ca^<2+>]_i does not play an essential role in amylase release by muscarinic stimulation. The amylase release may be primarily mediated by activation of protein kinase c.On the other hand, treatment of cells with TMB-8 or BAPTA strongly suppressed the CCh-induced K^+ release. A combined addition of Iono and ThG caused a marked release of K^+, but PMA did not affect basal K^+ release or potentiate the CCh-induced K^+ release. These results indicate that the CCh-induced K^+ release is mediated by a rapid increase in [Ca^<2+>]_i but is not associated with activation of protein kinase C