2,5-Di(TERT-BUTYL)-1,4-BENZOHYDROQUINONE - TRISPHOSPHATE-SENSITIVE Ca2+ POOL A NOVEL MOBILIZER OF THE INOSITOL 1,4,5-

Isolated hepatocytes and the isolated perfused rat liver have been used to study the alterations of cytosolic free Ca’+ concentration ([Ca”],) produced by 2.5-di(tert-butyl)-l.4-benzohydroquinone (tBuBHQ), a potent inhibitor of hepatic microsomal Ca’+ sequestration (Moore. G.A., McConkey. D.J., Kass, G.E.N., OBrien, P.J. and Orrenius, S. FEBS Lett.. 224, 331-336). (1987). Addition of tBuBHQ to isolated hepatocytes caused a rapid increase in [Ca’+], which was similar in magnitude to the [CaL+], elevation induced by the Ca’+ mobilizing hormone, vasopressin. In contrast with vasopressin which caused a Ca” transient, tBuBHQ elevated [Ca”], to a new steady state that was maintained for up to 15-20min. When vasopressin was administered during the t BuBHQ-induced period of elevated [Ca” I,. [Ca” 1, rapidly returned to basal levels. Similarly, if vasopressin was administered just prior to tBuBHQ, the resultant tBuBHQ-dependent change in [Ca’+ 1, was transient. and not sustained. The hydroquinone mobilized the same intracellular Ca’+ pool as inositol I ,4.5-trisphosphate. but tBuBHQ did not produce any detectable inositol polyphosphate accumulation. IBuBHQ stimulated glucose release from perifused hepatocytes. mimicking the effect of vasopressin. In the perfused liver, tBuBHQ infusion produced a single, slow and prolonged release of Ca” into the perfusate and inhibition of subsequent vasopressin-induced Ca” effluxes. Inhibition of the response to vasopressin was reversed over time, and closely correlated with the extent of inhibition of both Ca’+ sequestration and (Ca”-Mg‘+ )-ATPase activity in microsomes isolated from the isolated perfused liver. The present results are consistent with tBuBHQ inhibiting ATP-dependent Ca” sequestration by a direct effect on the endoplasmic reticular Ca” pump, which results in net Ca2+ release and elevation of [Ca’+ 1,. Furthermore. vasopressin appears to stimulate active removal of increased [Ca’+] from the hepatocyte cytosol by a mechanism which does not depend on reuptake of Ca’+ into the endoplasmic reticulum.