At concentrations >0.01 μM, thapsigargin (ThG) dose-dependently caused an increase in cytosolic free Ca
2+ concentration ([Ca
2+]
i) in rat parotid acinar cells, as measured by the fluorescent Ca
2+-indicator fura-2. In the absence of extracellular Ca
2+, a transient increase in [Ca
2+]
i by ThG was observed, and subsequent addition of carbachol (CCh) did not produce a further [Ca
2+]
i response, suggesting that ThG released Ca
2+ from the CCh-sensitive intracellular Ca
2+ pool. Since ThG did not stimulate formation of inositol phosphates, the ThG-induced Ca
2+ mobilization is independent of phosphoinositide breakdown. High concentrations (>0.1 μM) of ThG induced amylase release from rat parotide acini, but the effect was very poor as compared with that of CCh or the protein kinase C activator, PMA(phorbol 12-myristate 13-acetate). Combined addition of ThG and PMA modestly potentiated amylase release induced by PMA alone. These results support the view that amylase release by muscarinic stimulation is mediated mainly by activation of protein kinase C rather than a rise in [Ca
2+]
i although Ca
2+ may modulate the secretory response.
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