Modulation of extracellular ATP-induced Ca2+ responses: role of protein kinases

Evidence for the modulation of the P2z-purinoceptor for extracellular ATP in dissociated rat parotid cells is presented in studies using compounds that inhibit protein kinases. Preincubation of acinar cells with the protein kinase catalytic-site inhibitors K-252a and staurosporine, as well as with the regulatory-domain inhibitor sphingosine, specifically potentiates the elevation in cytosolic Ca2+ concentration ([Ca2+]i) mediated by extracellular ATP, but has no effect on the [Ca2+]i elevation mediated by muscarinic receptors through phospholipase C activation. Phorbol dibutyrate (PDBu), which activates protein kinase C (PKC), has no modulatory effect on ATP-mediated [Ca2+]i elevation. Further, pretreatment with PDBu does not reverse or block the effects of K-252a or sphinogosine, arguing against the involvement of PKC. Other pharmacological manipulations indicate that neither calmodulin-dependent nor cyclic-AMP-dependent kinases are involved. Neither the peak intracellular Ca2+ mobilization nor the sustained Ca2+ entry in response to carbachol or to a Ca2+ ionophore (4-bromo-A23187) is altered by the kinase inhibitors that potentiate the [Ca2+]i response to ATP, indicating that effects on the ATP response are not due to non-specific permeability changes, nor to decreased Ca2+ removal from the cytosol. ATP-mediated influx of Mn2+ as well as ATP-induced membrane depolarization are potentiated in cells preincubated with K-252a, directly demonstrating that cation influx is enhanced through a P2z-specific route. These results show that P2z responses (or purinoceptors) can be modulated and suggest that phosphorylation events are involved.