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 elevation of cytosolic free Ca2+ concentration ([Ca2+]i) just after stimulation with the muscarinic agonist carbachol (CCh), but they suppressed the sustained increase in [Ca2+]i. In the absence of extracellular Ca2+, CCh produced a transient increase in [Ca2+]i due to Ca2+ release from intracellular Ca2+ stores, and this increase in [Ca2+]i was unaffected by the phosphatase inhibitors. When Ca2+ was added to the external medium after the transient [Ca2+]i response, the increase in [Ca2+]i in the cells treated with the phosphatase inhibitors was significantly smaller than that in the control cells, indicating that the Ca2+ entry was reduced. Similar suppression of Ca2+ entry by the phosphatase inhibitors was observed when intracellular Ca2+ stores were previously depleted by the microsomal Ca(2+)-ATPase inhibitor thapsigargin (TG). In addition, the phosphatase inhibitors reduced the Mn2+ (Ca2+ surrogate) influx following the addition of CCh or TG. The enhancement of Ca2+ entry by the protein kinase inhibitor staurosporine was significantly attenuated by the phosphatase inhibitors. These results suggest that the phosphatase inhibitors suppressed the Ca2+ entry mechanism activated by depletion of intracellular Ca2+ stores in rat parotid acinar cells. The capacitative Ca2+ entry may be regulated by protein phosphorylation/dephosphorylation.