Na+/H+ exchange and PLA2 activity act interdependently to mediate the rapid effects of 1 alpha, 25-dihydroxyvitamin D3.

1 alpha, 25-Dihydroxyvitamin D3 (1 alpha, 25-(OH)2D3) has been shown to rapidly increase cytosolic calcium in freshly isolated and cultured rat hepatocytes. The rise in cytosolic calcium is dependent on phospholipase A2 (PLA2) activation and cell alkalinization through the Na+/H+ antiport system. To further characterize the rapid effects of 1 alpha, 25-(OH)2D3, cultured hepatocytes were treated with inhibitors of PLA2 and the Na+/H+ antiport system. 1 alpha, 25-(OH)2D3 treatment caused a 31-66% increase in [32P]lysophosphatidylinositol (LPI) and a 0.04 increase in pH within 5 minutes. Inhibition of the Na+/H+ antiport system with amiloride or removal of extracellular sodium abolished the 1 alpha, 25-(OH)2D3 rise in LPI. Inhibition of PLA2 with bromophenacylbromide also blocked the 1 alpha, 25-(OH)2D3-induced rise in [32P]LPI and cytosolic alkalinization in response to 1 alpha, 25-(OH)2D3. The data indicate that 1 alpha, 25-(OH)2D3 rapidly increases the activity of PLA2 and the Na+/H+ antiport system. The production of LPI is dependent on PLA2 activation and cell alkalinization through the Na+/H+ antiport system. It appears that the two events are interdependent in hepatocytes.