DIFFERENTIAL-EFFECTS OF PROTEIN-KINASE-C ACTIVATION ON CALCIUM STORAGE AND CAPACITATIVE CALCIUM-ENTRY IN NIH 3T3 CELLS

In NIH 3T3 cells, treatment with phorbol 12-myristate 13-acetate (PMA) reduced the release of Ca2+ by thapsigargin, but did not activate Ca2 + entry; Ca2+ influx was triggered after the residual pool was emptied by thapsigargin, and this Ca2+ influx was similar to that induced by thapsigargin in control cells. The effect of PMA was clue to decreased Ca2+ storage because 1) Ca2+ release by ionomycin was similarly affec ted by PMA, and in both control and PMA-treated cells, ionomycin did n ot release Ca2+ following thapsigargin treatment; 2) PMA reduced Ca-45 (2+) accumulation; and 3) studies with Ca2+ indicator compartmentalize d into the endoplasmic reticulum indicated that stored Ca2+ was reduce d by PMA. Although PMA did not itself activate Ca2+ entry, PMA potenti ated Ca2+ entry with low concentrations of cyclopiazonic acid. With a somewhat higher concentration of cyclopiazonic acid, PMA had no effect on calcium entry. Thus, protein kinase C has two apparent actions on calcium signaling in NIH 3T3 cells: 1) reduced intracellular Ca2+ stor age capacity and 2) augmented calcium entry with submaximal intracellu lar Ca2+ pool depletion. These actions indicate a complex and potentia lly important role for the protein kinase C system in calcium homeosta sis in this cell type.