INHIBITION OF NA+ CA2+ EXCHANGE IN RENAL BLMV BY IP3 DEPENDS ON SITE OF ACTION AND DIRECTION OF CA2+ FLUX/

It has previously been shown in synaptosomes that inositol 1,4,5-trisp hosphate (1,4,5-IP3) inhibits Ca2+ transport by the plasma membrane-bo und Na+/Ca2+ exchanger. The present study was therefore designed to de termine if the effect of 1,4,5-IP3 was dependent on its site of action at the plasma membrane or on the direction of Ca2+ flux. To investiga te this possibility, studies were performed in basolateral membrane Ve sicles (BLMV) isolated from rat renal cortex. As with synaptosomes, Ca 2+ transport was inhibited by 1,4,5-IP3 in a concentration-dependent m anner. At a concentration of 10(-6) M, 1,4,5-IP3 significantly (P < 0. 005) inhibited Ca2+ transport by 36%. When Ca2+ transport was carried out in inside-out vesicles, 10(-6) M 1,4,5-IP3 significantly (P < 0.00 2) increased the degree ofinhibition by an additional 75% (63 vs. 36%) . However, 1,4,5-IP3 had no significant effect on Ca2+ transport in in side-out vesicles when Ca2+ flux was reversed (i.e., Ca2+ efflux). The se data in renal BLMV confirm the novel action of 1,4,5-IP3 on the Na/Ca2+ exchanger previously described in brain synaptosomes. These resu lts also suggest that the action of 1,4,5-IP3 depends on both its site of action at the plasma membrane and on the direction of Ca2+ flux.