INOSITOL 1,4,5-TRISPHOSPHATE MAY REGULATE RAT-BRAIN CAI++ BY INHIBITING MEMBRANE-BOUND NA+-CA++ EXCHANGER

The role of inositol 1,4,5-trisphosphate (1,4,5-IP3) in regulating cytosolic Ca++ by stimulating Ca++ release from intracellular organelles is well established. However, other modes of intracellular Ca++ regulation by 1,4,5-IP3 have not been determined. To determine if 1,4,5-IP3 may regulate cell cytosolic Ca++ by acting on plasma membrane bound Na+ -Ca++ exchanger, we investigated Ca++ transport in synaptosomes using 45Ca++ as tracer. In the presence of either an inhibitor of voltage gated Na+ channels (tetrodotoxin) or the K+ ionophore (valinomycin), Ca++ uptake was significantly inhibited (P < 0.05) by 1,4,5-IP3 in a concentration dependent manner, with half-maximal inhibition occurring at submicromolar concentrations between 10-9 M and 10-10 M 1,4,5-IP3. Similarly, Ca++ efflux by the exchanger was significantly inhibited 40% by 1,4,5-IP3. The inhibitory effect of 1,4,5-IP3 on the Na+-Ca++ exchanger was observed in the presence of Ca++ channel blockers, and in vesicles pretreated with caffeine to deplete the 1,4,5-IP3-sensitive stores of Ca++. These results suggest that during signal transduction in brain, 1,4,5-IP3 may increase cytosolic [Ca++] in part by inhibiting the Na+ -Ca++ exchanger and thus, Ca++ efflux from cell.