CALCIUM CONTROL ON INSP(3)-INDUCED DISCHARGE OF CALCIUM FROM PERMEABILIZED HEPATOCYTE POOLS

The control exerted by intralumenal and cytosolic Ca2+ on InsP3-induce d release of Ca2+ from intracellular Ca2+ pools in suspensions of sapo nin-permeabilised rat hepatocytes was investigated by combined Quin-2 and Ca2+ measurements at 20-degrees-C. We failed to detect a major eff ect of intralumenal Ca2+ in regulating this release, as various manipu lations in which the load of the Ca2+ pools was varied by a factor of two did not significantly affect the apparent relative efficiency of I nsP3 in releasing Ca2+; these manipulations included loading the Ca2pools up to various steady state levels by preliminary equilibration a t various external free Ca2+ concentrations, as well as emptying them progressively through the blockade of pump-mediated Ca2+ uptake. As re gards Ca2+ on the cytosolic side, in contrast with recent results obta ined with other systems, we found that, at maximal doses, InsP3-induce d Ca2+ release was not stimulated by raising Ca2+ from very low to sub micromolar or micromolar concentrations, and that only relatively high concentrations of free Ca2+ inhibited this release (half-maximal inhi bition was between 3 and 15 muM). Such elevated Ca2+ concentrations re duced the size of the InsP3-sensitive Ca2+ pool. We also noted that th e apparent cooperativity of InsP3 activation of release at pCa 5 was n oticeably less than that observed at pCa 7. As a result, at low InSP3 concentrations, a rise in cytosolic Ca2+ from pCa 7 to pCa 5 stimulate d InSP3-mediated Ca2+ release. These results are discussed in the cont ext of the current speculations about tissue specificity, heterogeneit y, quantal release, oscillations, and the several different mechanisms that may control InsP3-induced Ca2+ release.