OSCILLATIONS IN CYTOSOLIC-FREE CA2-RAT HEPATOCYTES DISPLAY DIFFERENTIAL SENSITIVITY TO APPLICATION OF PHORBOL ESTER( INDUCED BY ADP AND ATPIN SINGLE)

We have previously described differences in the osillatory responses o f cytosolic free Ca2+ concentration ([Ca2+](i)) in hepatocytes to ADP and ATP, which we have interpreted as evidence that these two nucleoti des are acting at distinct receptors. We show here that ADP- and ATP-i nduced oscillations are differentially sensitive to application of the phorbol ester 4 beta-phorbol 12,13-dibutyrate (PDB). ADP-induced [Ca2 +](i) oscillations are abolished by low concentrations of PDB (5-10 nM ), whereas ATP-induced oscillations of long duration are refractory to PDB, even at greatly elevated concentrations (100 nM). The data illus trate a further difference in the actions of ADP and ATP, strengthenin g the argument that these agonists are not acting at the same receptor on rat hepatocytes.