Characterization of the co-agonist effects of strontium and calcium on myo-inositol trisphosphate-dependent ion fluxes in cerebellar microsomes

Abstract

Using sheep cerebellum microsomes previously loaded with 45Ca 2+ or 90Sr 2+, we measured the dependence of inositol 1,4,5-trisphosphate (InsP 3)-induced efflux of these ions on Ca 2+ or Sr 2+ on the cytosolic side. At a low InsP 3 concentration, Ca 2+ in the submicromolar range only poorly activated 45Ca 2+ or 90Sr 2+ efflux, and higher Ca 2+ concentrations were inhibitory. In contrast, Sr 2+ in the micromolar range activated release efficiently, while only very high Sr 2+ concentrations were inhibitory. Experiments were repeated in the presence of a high InsP 3 concentration, which allowed increasing free Ca 2+ to micromolar concentrations without inducing complete inhibition of the InsP 3-dependent efflux. Under these conditions, micromolar Ca 2+ was found to activate efflux to a large extent, similar to that previously found with Sr 2+ Optimal activation by Ca 2+ of the InsP 3-dependent channel occurs at micromolar rather than submicromolar free Ca 2+ concentrations, but at too low an InsP 3 concentration, Ca 2+-induced activation is counteracted by Ca 2+-induced inactivation. Seprate measurements of [ 3H]-InsP 3 binding at a low concentration showed that Sr 2+ and Ca 2+ did not enhance the amount of bound [ 3H]-InsP 3, implying that the activating effect of Sr 2+ and Ca 2+ in cerebellar microsomes is mediated by an increase in the channel opening probability and not by an increase in the receptor's affinity for InsP 3. A similar relationship also holds in the case of the activating effect of nucleotides.