Effect of inhibition of microsomal Ca(2+)-ATPase on cytoplasmic calcium and enzyme secretion in pancreatic acini

Endoplasmic reticulum Ca(2+)-ATPase inhibitors stimulate membrane guanylate cyclase in pancreatic acinar cells

In this study, we show that particulate guanylate cyclase (GC) is present in rat pancreatic acinar cells and is located both on plasma membrane and membranes of endoplasmic reticulum (ER). Western blot analysis indicates that the enzyme isoform GC-A is present in the acinar cell membranes. The specific inhibitors of ER Ca(2+)-ATPase thapsigargin, 2,5-di-(t-butyl)-1,4-hydroquinone (BHQ), and cyclopiazonic acid all activated particulate GC in pancreatic acini, both in membrane fractions and intact cells. These inhibitors also induced dephosphorylation of GC. Dose dependencies of Ca(2+)-ATPase inhibition and GC activation by BHQ are very similar, and those for thapsigargin partially overlap. ER Ca(2+)-ATPase and GC are coimmunoprecipitated both by antisera against membrane GC and by antisera against ER Ca(2+)-ATPase, suggesting a physical association between the two enzymes. The results suggest that thapsigargin and the other inhibitors act through ER Ca(2+)-ATPase to activate membrane GC in pancreatic acinar cells, although their direct effect on GC cannot be excluded.

Protection of cardiomyocytes by pinacidil during metabolic inhibition and hyperkalemia

The objective of this study is to understand the mechanism underlying the cardioprotective effects of pinacidil, an ATP-sensitive K+ channel (K(ATP)) opener. We examined the effects of 10 microM pinacidil in cultured chicken cardiomyocytes. Pinacidil caused a concentration-dependent delay in metabolic inhibition-induced increase in intracellular calcium concentration ([Ca2+]i) and creatine phosphokinase release, and this action was antagonized by glyburide, a K(ATP) blocker. Neither verapamil, an L-type Ca2+ channel blocker, nor bepridil, a Na+-Ca2+ exchange inhibitor, affected the time course of increase in [Ca2+]i induced by metabolic inhibition. Pinacidil did not have an effect on the amplitude of K+-induced increase in [Ca2+]i, but accelerated the rate of decline following peak stimulation. In contrast, glyburide reduced the amplitude of K+-induced increase in [Ca2+]i and prolonged the rate of decline. These results provide direct evidence that pinacidil protects cardiomyocytes from metabolic inhibition-induced injury by cyanide (CN) through a delay in the onset of increase in [Ca2+]i, rather than by inhibition of the L-type Ca2+-channels or by alteration of Na+-Ca2+ exchange.

Effects of Ca2+ channel blockers on Ca2+ loading induced by metabolic inhibition and hyperkalemia in cardiomyocytes

The effects of the L-type (nifedipine and verapamil) and the T-type (mibefradil) Ca2+ channel blockers on the increase in intracellular Ca2+ concentration ([Ca2+]i) induced by NaCN metabolic inhibition and hyperkalemia were examined in chicken cardiomyocytes using fluorescence imaging with Fura-2. NaCN induced a slow and sustained rise in [Ca2+]i, which was not affected by pretreating the cells for 5 min with nifedipine, verapamil, or mibefradil at 100 nM or 10 microM. Pretreatment of the cells with 10 microM nifedipine, verapamil, or mibefradil for 5 min remarkably inhibited the K+-induced increase in [Ca2+]i. These inhibitory effects diminished after 48-h pretreatment with nifedipine or verapamil but not with mibefradil. Ryanodine also induces an increase in [Ca2+]i, and this effect was enhanced by 48-h pretreatment of the cells with 10 microM verapamil but not with 10 microM mibefradil. We conclude that the NaCN-induced increase in [Ca2+]i is independent of the Ca2+ influx though the L-type or T-type Ca2+ channels. Chronic inhibition of the L-type Ca2+ channels but not T-type channels may enhance the ryanodine receptor-mediated Ca2+ release, which may be responsible for the development of tolerance to their inhibitory effects on K+-induced increase in [Ca2+]i.