Ca2+- and inositol-1,4,5-triphosphate induced release of Ca2+ in the microsomal fraction of the rat brain

Using the fluorescent probes, Quin 2 and chlortetracycline, a comparative study of the Ca2+ and inositol-1.4.5-triphosphate (IP3)-induced Ca2+ release from rabbit skeletal muscle sarcoplasmic reticulum (SR) terminal cisterns and rat brain microsomal vesicles was carried out. It was shown that Ca2+ release from rat brain microsomal vesicles is induced both by IP3 and Ca2+, whereas that in SR terminal cisterns is induced only by Ca2+. Data from chlorotetracycline fluorescence analysis revealed that CaCl2 (50 microM) causes the release of 15-20% and 40-50% of the total Ca2+ pool accumulated in rat brain microsomal vesicles and rabbit SR terminal cisterns, respectively. Using Quin 2, it was found that IP3 used at the optimal concentration (1.5 mM) caused the release of 0.4-0.6 nmol of Ca2+ per mg microsomal protein, which makes up to 10-15% of the total Ca2+ pool. IP3 does not induce Ca2+ release in SR. Preliminary release of Ca2+ from brain microsomes induced by IP3 diminishes the liberation of this cation induced by Ca2+. It is suggested that brain microsomes contain a Ca2+ pool which is exhausted under the action of the both effectors, Ca2+ and IP3.