A proposed mechanism for the potentiation of cAMP-mediated acid secretion by carbachol

Acid secretion in isolated rabbit gastric glands was monitored by the accumulation of [C-14] aminopyrine. Stimulation of the glands with carbachol synergistically augmented the response to dibutyryl cAMP. The augmentation persisted even after carbachol was washed out and was resistant to chelated extracellular Ca2+ and to inhibitors of either protein kinase C or calmodulin kinase II. Cytochalasin D at 10 muM preferentially blocked the secretory effect of carbachol and its synergism with cAMP, whereas it had no effect on histamine- or cAMP-stimulated acid secretion within 15 min. Cytochalasin D inhibited the carbachol-stimulated intracellular Ca2+ concentration ([Ca2+](i)) increase due to release from the Ca2+ store. Treatment of the glands with cytochalasin D redistributed type 3 inositol 1,4,5-trisphosphate receptor (the major subtype in the parietal cell) from the fraction containing membranes of large size to the microsomal fraction, suggesting a dissociation of the store from the plasma membrane. These findings suggest that intracellular Ca2+ release by cholinergic stimulation is critical for determining synergism with cAMP in parietal cell activation and that functional coupling between the Ca2+ store and the receptor is maintained by actin microfilaments.