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AK Ho, T Ogiwara and CL Chik
Department of Physiology, Faculty of Medicine, University of Alberta, Edmonton, Canada. aho@physio.med.ualberta.ca
The role of mobilization of intracellular Ca2+ in the adrenergic- stimulated cAMP accumulation in rat pinealocytes was investigated with thapsigargin, an agent that inhibits endoplasmic reticulum Ca2+-ATPase. It was found that although thapsigargin alone had no effect on the basal cAMP accumulation, it potentiated the beta-adrenergic-stimulated cAMP response by isoproterenol in a dose-dependent manner. The potentiation was abolished with ethylene glycol bis(beta-aminoethyl ether)-N, N,N',N'-tetraacetic acid-acetoxymethyl ester (EGTA-AM) but persisted in the presence of isobutylmethylxanthine, indicating that thapsigargin enhances cAMP synthesis through elevation of cytosolic intracellular Ca2+ concentration ([Ca2+]i). However, when the pinealocytes were stimulated by norepinephrine, a mixed alpha 1- and beta-adrenergic agonist, thapsigargin dose-dependently inhibited the cAMP response. To investigate this inhibitory effect of thapsigargin, we substituted ionomycin, a [Ca2+]i-elevating agent, and 4 beta-phorbol- 12-myristate 13-acetate, an activator of protein kinase C, for the alpha(1)-adrenergic component of the norepinephrine-stimulated response. Although thapsigargin had no effect on the potentiation of the isoproterenol-stimulated cAMP accumulation by ionomycin, it significantly inhibited the potentiation by 4 beta-phorbol-12-myristate 13-acetate. Furthermore, the inhibitory effect of thapsigargin was not affected by cotreatment with EGTA-AM or ionomycin, suggesting that this effect is independent of [Ca2+]i. Similar results were obtained when cyclopiazonic acid was used to inhibit the Ca(2+)-ATPase. Taken together, our results indicate that thapsigargin enhances the beta- adrenergic-stimulated cAMP accumulation through its action in elevating [Ca2+]i but inhibits the potentiation of the beta-adrenergic-stimulated cAMP response by protein kinase C, as a consequence of Ca(2+)-ATPase inhibition.
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