SUPPRESSION OF CAPACITATIVE CA2+ ENTRY BY SERINE THREONINE PHOSPHATASE INHIBITORS IN RAT PAROTID ACINAR-CELLS/

The effects of three serine/threonine protein phosphatase inhibitors, calyculin-A, tautomycin and okadaic acid, on the Ca2+ entry across the plasma membrane was studied in Fura-2-loaded rat parotid acinar cells . These protein phosphatase inhibitors did not affect the peak elevati on of cytosolic free Ca2+ concentration ([Ca2+](i)) just after stimula tion with the muscarinic agonist carbachol (CCh), but they suppressed the sustained increase in [Ca2+](i). In the absence of extracellular C a2+, CCh produced a transient increase in [Ca2+](i) due to Ca2+ releas e from intracellular Ca2+ stores, and this increase in [Ca2+]i was una ffected by the phosphatase inhibitors. When Ca2+ was added to the exte rnal medium after the transient [Ca-2+](i) response, the increase in [ Ca2+](i) in the cells treated with the phosphatase inhibitors was sign ificantly smaller than that in the control cells, indicating that the Ca2+ entry was reduced. Similar suppression of Ca2+ entry by the phosp hatase inhibitors was observed when intracellular Ca2+ stores were pre viously depleted by the microsomal Ca2+-ATPase inhibitor thapsigargin (TG). In addition, the phosphatase inhibitors reduced the Mn2+ (Ca2+ s urrogate)influx following the addition of CCh or TG. The enhancement o f Ca2+ entry by the protein kinase inhibitor staurosporine was signifi cantly attenuated by the phosphatase inhibitors. These results suggest that the phosphatase inhibitors suppressed the Ca2+ entry mechanism a ctivated by depletion of intracellular Ca2+ stores in rat parotid acin ar cells. The capacitative Ca2+ entry may be regulated by protein phos phorylation/dephosphorylation.