INS(1,4,5)P3 INDUCES CA-2+ RELEASE FROM BRAIN MICROSOMES LOADED EITHER BY THE CA-2+ ATPASE OR BY THE NA+/CA-2+ EXCHANGER

In this study we investigated the release of Ca2+ in brain microsomes after Ca2+ loading by the Ca2+-ATPase or by the Na+/Ca2+ exchanger. The results show that in microsomes loaded with Ca2+ by the Ca2+-ATPase, Ins(1,4,5)P3 (5 muM) released 21 +/- 2% of the total Ca2+ accumulated, and that in the microsomes loaded with Ca2+ by the Na+/Ca2+ exchanger, Ins(1,4,5)P3 released 28 +/- 3% of the total Ca2+ accumulated. These results suggest that receptors of Ins(1,4,5)P3 may be co-localized with the Na+/Ca2+ exchanger in the endoplasmic reticulum membrane or that there are Ins(1,4,5)P3 receptors in the plasma membrane where the Na+/Ca2+ exchanger is normally present, or both. We also found that Ins(1,4,5)P3 inhibited the Ca2+-ATPase by 33.7%, but that it had no significant effect on the Na+/Ca2+ exchanger.