Loading dependence of inositol 1,4,5-trisphosphate-induced Ca2+ release in the clonal cell line A7r5. Implications for the mechanism of quantal Ca2+ release.

The Ca2+ content of the intracellular Ca2+ stores controls the inositol 1,4,5-trisphosphate receptor (InsP3R) in the clonal cell line A7r5. This regulation was characterized with respect to the understanding of the "quantal" release phenomenon. Independent of the loading protocol used, increasing the Ca2+ content of the stores increased the sensitivity of the inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ release until a Ca2+ content of about 30% of the steady-state value was reached. Loading of the stores to higher levels had only a marginal effect on the Ca2+ release. The effects of luminal Ca2+ were still observed in the presence of 10 mM BAPTA (1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid), excluding the possibility that luminal Ca2+ acted indirectly via cytoplasmic binding sites. Conditions were developed to simultaneously measure [3H]InsP3 binding to the InsP3R and the 45Ca2+ content of the stores in the same cells. [3H]InsP3 binding to a high affinity binding site was potentiated by luminal Ca2+. Analysis at the molecular level revealed the simultaneous expression of different splice variants of InsP3R-I, as well as the expression of InsP3R-III, and of the putative InsP3R-IV. We conclude that the control of the InsP3R by luminal Ca2+ could account for quantal release and that the observed heterogeneity of the InsP3R may also contribute to this behavior, especially at high levels of store loading.