Rapid activation and partial inactivation of inositol trisphosphate receptors by adenophostin A

Adenophostin A, the most potent known agonist of inositol 1,4,5-trisphosphate (InsP(3)) receptors, stimulated Ca-45(2+) release from the intracellular stores of permeabilized hepatocytes. The concentration of adenophostin 4 causing the half-maximal effect (EC50) was 7.1+/-0.5 nM, whereas the EC50 for InsP(3) was 177+/-26 nM; both responses were positively co-operative. In rapid superfusion analyses of Ca-45(2+) release from the intracellular stores of immobilized hepatocytes, maximal concentrations of adenophostin A or InsP(3) evoked indistinguishable patterns of Ca2+ release. The Ca2+ release evoked by both agonists peaked at the same maximal rate after about 375 ms and the activity of the receptors then decayed to a stable, partially (60%) inactivated state with a half-time (t(1/2)) of 318+/-29 ms for adenophostin A and 321 +/- 22 ms for InsP(3). Dissociation rates were measured by recording rates of InsP(3)-receptor channel closure after rapid removal of agonist. The rate of adenophostin A dissociation (t(1/2), 840 +/- 195 ms) was only 2-fold slower than that of InsP(3) (t(1/2), 436 +/- 48 ms). We conclude that slow dissociation of adenophostin A from InsP(3) receptors does not underlie either its high-affinity binding or the reported differences in the Ca2+ signals evoked by InsP(3) and adenophostin A in intact cells.