Calcium control on InsP3-induced discharge of calcium from permeabilised hepatocyte pools.
The control exerted by intralumenal and cytosolic Ca2+ on InsP3-induced release of Ca2+ from intracellular Ca2+ pools in suspensions of saponin-permeabilised rat hepatocytes was investigated by combined Quin-2 and 45Ca2+ measurements at 20 degrees C. We failed to detect a major effect of intralumenal Ca2+ in regulating this release, as various manipulations in which the load of the Ca2+ pools was varied by a factor of two did not significantly affect the apparent relative efficiency of InsP3 in releasing Ca2+; these manipulations included loading the Ca2+ pools up to various steady state levels by preliminary equilibration at various external free Ca2+ concentrations, as well as emptying them progressively through the blockade of pump-mediated Ca2+ uptake. As regards Ca2+ on the cytosolic side, in contrast with recent results obtained with other systems, we found that, at maximal doses, InsP3-induced Ca2+ release was not stimulated by raising Ca2+ from very low to submicromolar or micromolar concentrations, and that only relatively high concentrations of free Ca2+ inhibited this release (half-maximal inhibition was between 3 and 15 microM). Such elevated Ca2+ concentrations reduced the size of the InsP3-sensitive Ca2+ pool. We also noted that the apparent cooperativity of InsP3 activation of release at pCa 5 was noticeably 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 stimulated InsP3-mediated Ca2+ release. These results are discussed in the context of the current speculations about tissue specificity, heterogeneity, quantal release, oscillations, and the several different mechanisms that may control InsP3-induced Ca2+ release.
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Unité de Recherche U274, Institut National de la Santé et de la Recherche Médicale, Université Paris Sud, Orsay, France.
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D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002463:Cell Membrane Permeability; D005260:Female; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D008099:Liver / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction
D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium