CHARACTERIZATION OF THE CO-AGONIST EFFECTS OF STRONTIUM AND CALCIUM ON MYOINOSITOL TRISPHOSPHATE-DEPENDENT ION FLUXES IN CEREBELLAR MICROSOMES

Using sheep cerebellum microsomes previously loaded with Ca-45(2+) Or Sr-90(2+), we measured the dependence of inositol 1,4,5-trisphosphate (InsP(3))-induced efflux of these ions on Ca2+ or Sr2+ On the cytosoli c side, At a low InsP(3) concentration, Ca2+ in the submicromolar rang e only poorly activated Ca-45(2+) or Sr-90(2+) efflux, and higher Ca2 concentrations were inhibitory. In contrast, Sr2+ in the micromolar r ange activated release efficiently, while only very high Sr2+ concentr ations were inhibitory, Experiments were repeated in the presence of a high InsP(3) concentration, which allowed increasing free Ca2+ to mic romolar concentrations without inducing complete inhibition of the Ins P(3)-dependent efflux, Under these conditions, micromolar Ca2+ was fou nd to activate efflux to a large extent, similar to that previously fo und with Sr2+. Optimal activation by Ca2+ of the InsP(3)-dependent cha nnel occurs at micromolar rather than submicromolar free Ca2+ concentr ations, but at too low an InsP(3) concentration, Ca2+-induced activati on is counteracted by Ca2+-induced inactivation. Separate measurements of [H-3]-InsP(3) binding at a low concentration showed that Sr2+ and Ca2+ did not enhance the amount of bound [H-3]-InsP(3), implying that the activating effect of Sr2+ and Ca2+ in cerebellar microsomes is med iated by an increase in the channel opening probability and not by an increase in the receptor's affinity for InsP(3). A similar relationshi p also holds in the case of the activating effect of nucleotides.