Regulation of [³H]nitrendipine binding by phospholipases A₂ and C through direct and GTP-sensitive mechanisms

We compared the effects of Phospholipases A₂, C, B and D on [³H]nitrendipine binding to hamster cardiac membranes, in the absence and presence of ATP or GTP. Phospholipase A₂, competitively inhibited [³H]nitrendipine binding to hamster cardiac membranes unchanged by ATP or GTP (K_i = 5 ng/ml); as evidenced by complete and reversible displacement of [³H]nitrendipine binding and increase in K_D on Scatchard analyses. Phospholipase C also completely inhibited [³H]nitrendipine binding to hamster cardiac membranes (K_i = 5 ug/ml) with a decrease in B_max and no change in K_D on Scatchard analyses. The addition of GTP alone inhibited the PLC effect in EGTA-treated membranes. The addition of GTP with either CaCl₂, or ATP or both resulted in an equal and opposite enhancement of the PLC effect. Phospholipases B and D had no effect on [³H]nitrendipine binding. These data support: (1) Direct effect of PLA₂ on dihydropyridine binding. (2) Indirect regulation of dihydropyridine binding by Phospholipase C through a GTP and ATP-sensitive mechanism.