Ca release induced by cyclic adenosine diphosphoribose (cADPr) in sea urchin egg homogenates: mechanisms of release and heterogeneity of the Ca compartments

Abstract

A rapid superfusion system measuring the amounts, kinetics, and Ca dependencies of released 45Ca, was used to examine the effects of ryanodine (RY), caffeine (CF), and cyclic ADP ribose (cADPr) on sea urchin egg homogenates. The RY-sensitive compartment had more than twice the Ca release capacity of the CF-sensitive or cADPr-sensitive compartment. cADPr-stimulated 45Ca release required calcium with half-maximal activation at ∼0.2 to 0.6 μM [Ca 2+]. K 1/2 for cADPr activation was ∼100 nM, and in spite of the Ca requirement for cADPr-stimulated release, the cADPr affinity was not affected by [Ca 2+]. Peak 45Ca release rate with cADPr (3 μM) was greater than with CF (20 mM), yet the release amounts were similar and both were [Ca 2+]-dependent. When activated with CF and cADPr simultaneously, 45Ca release was large and, no longer [Ca 2+]-dependent. Mg competitively inhibited the Ca activation site(s), yet did not inhibit the activation with CF-plus-cADPr. Pre-release of 45Ca by cADPr with low (∼0.1 μM) [Ca 2+] right-shifted the [Ca 2+] dependence of the remaining cADPr-response. These data suggest that (a) only a portion of RY-sensitive compartments empty when stimulated with cADPr or CF, (b) Ca and cADPr act on non-interacting sites, and (c) cADPr-sensitive compartments represent a heterogeneous population with different [Ca 2+] dependencies.