MODULATORY EFFECT OF 4-BETA-PHORBOL 12-MYRISTATE 13-ACETATE (PMA) ON CARBACHOL-INDUCED CA2-CELLS( MOBILIZATION IN RAT PAROTID ACINAR)

Treatment of rat parotid acinar cells with 4 beta-phorbol 12-myristate 13-acetate (PMA) significantly inhibited an increase in cytosolic fre e Ca2+ concentration ([Ca2+])(i) induced by carbachol (CCh), a muscari nic agonist. The CCh-induced increase in [Ca2+](i) was also inhibited by another active phorbol ester, 4 beta-phorbol 12,13-dibutyrate, but not by 4 alpha-phorbol 12,13-didecanoate, which does not activate prot ein kinase C. The treatment with PMA had no effect on increases in [Ca 2+](i) evoked by ionomycin and thapsigargin, which do not stimulate ph osphoinositide hydrolysis. In contrast, an increase in [Ca2+](i) induc ed by NaF, a direct activator of GTP-binding proteins, was delayed in the presence of PMA. The formation of inositol phosphates in response to CCh was suppressed significantly by PMA treatment. In radioligand b inding assays, PMA did not directly interfere with the specific bindin g of [H-3]quinuclidinyl benzilate ([H-3]QNB), a muscarinic antagonist, to plasma membranes. Furthermore, the [H-3]QNB binding to plasma memb ranes prepared from the PMA-pretreated cells was not different from th at to the control membranes. These results indicate that PMA attenuate d the CCh-induced increase in [Ca2+](i) through inhibition of phosphoi nositide hydrolysis. Activation of protein kinase C may play a role in negative-feedback control of the muscarinic pathway in rat parotid ac inar cells.