AGENTS THAT PROMOTE PROTEIN-PHOSPHORYLATION INHIBIT THE ACTIVITY OF THE NA+ CA2+ EXCHANGER AND PROLONG CA2+ TRANSIENTS IN BOVINE CHROMAFFINCELLS/

The Na+/Ca2+ exchanger is an important element in the maintenance of c alcium homeostasis in bovine chromaffin cells. The Na+/Ca2+ exchanger from other cell types has been extensively studied, but little is know n about its regulation in the cell. We have investigated the role of r eversible protein phosphorylation in the activity of the Na+/Ca2+ exch anger of these cells. Cells treated with 1 mM dibutyryl cyclic AMP (db cAMP), 1 mu M phorbol 12,13-dibutyrate, 1 mu M okadaic acid, or 100 nM calyculin A showed lowered Na+/Ca2+ exchange activity and prolonged c ytosolic Ca2+ transients caused by depolarization. A combination of 10 nM okadaic acid and 1 mu M dbcAMP synergistically inhibited Na+/Ca2exchange activity. Conversely, 50 mu M 1-(5-isoquinolinylsulfonyl)-2-m ethylpiperazine, a protein kinase inhibitor, enhanced Na+/Ca2+ exchang e activity. Moreover, we used cyclic AMP-dependent protein kinase and calcium phospholipid-dependent protein kinase catalytic subunits to ph osphorylate isolated membrane vesicles and found that the Na+/Ca2+ exc hange activity was inhibited by this treatment. These results indicate that reversible protein phosphorylation modulates the activity of the Na+/Ca2+ exchanger and suggest that modulation of the exchanger may p lay a role in the regulation of secretion.