Effect of inositol 1,4,5-trisphosphate and GTP on calcium release from pituitary microsomes

Abstract

Microsomal vesicles from bovine anterior pituitary accumulate Ca 2+ and maintain a steady-state ambient Ca 2+ level of 200 nM. IP 3 and GTP both induce calcium release from the microsomal vesicles. The effect of IP 3 is inhibited by polyethylene glycol (PEG), and the effect of GTP is absolutely dependent on PEG. Half-maximal effect of IP 3 (without PEG) is 0.26 μm, the maximal calcium release attaining 7% of the A23187-releasable pool. The same values for GTP (in the presence of PEG) are 80 μM and 10%, respectively. GTP potentiates the effect of IP 3. This potentiation is not mediated by protein phosphorylation.