The Physiological Role of Calmodulin in Rat Parotid Amylase Release

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Using dispersed rat parotid cells, the effects of three calmodulin antagonists trifluoperazine (TFP), W-7 and W-5, on amylase release and acinar cell structure were examined. TFP and W-7 strongly inhibited both isoproterenol (ISO)- or DBcAMP-stimulated amylase release at a concentration of 50 or 100 muM, while W-5, a weak calmodulin antagonist, had a little effect. Cyclic AMP level was markedly elevated by ISO even in the presence of TFP and W-7.Electron micrographs demonstrated that treatment of parotid cells with either TFP or W-7 caused a loss of luminal microvilli and surface folds. When cells were stimulated by ISO in the presence of TFP or W-7, the enlarged lumina did not recover to their original size and the discharged secretory material was retained in the lumina. The structural changes were similar, in part, to those evoked by cytochalasin D, a microfilament disrupting agent. W-5 affected the acinar cell structure only a little.Amylase release induced by ISO or DBcAMP was significantly enhanced by pretreatment with Ca^<2+>. Depletion of intracellular Ca^<2+> by pre-treatment with the Ca^<2+>-chelators EGTA and BAPTA-AM reduced the amylase release.The effects of ISO on cytosolic free Ca^<2+>([Ca^<2+>]i)were examined using the fluorescent Ca^<2+>-indicator fura-2. At concentrations up to 1 mM, ISO caused a rapid increase in [Ca^<2+>]i, while 1 muM ISO, which stimulates the maximum amylase release, had little effect. Since the alpha-adrenoceptor antagonist phentolamine blocked the ISO-induced increasg+in [Ca^<2+>]i better than the beta-adrenoceptor antagonist, the increase in [Ca^<2+>]i is due to an activation of alpha-adrenoceptors rather than beta-adrenoceptors.This study suggests that calmodulin is involved in the exocytosis of parotid amylase through the regulation of the cytoskeletal system. Calmodulin may be functional even at resting levels of [Ca^<2+>]i.