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

Acid secretion in isolated rabbit gastric glands was monitored by the accum ulation of [C-14] aminopyrine. Stimulation of the glands with carbachol syn ergistically augmented the response to dibutyryl cAMP. The augmentation per sisted even after carbachol was washed out and was resistant to chelated ex tracellular Ca2+ and to inhibitors of either protein kinase C or calmodulin kinase II. Cytochalasin D at 10 muM preferentially blocked the secretory e ffect 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 recep tor (the major subtype in the parietal cell) from the fraction containing m embranes of large size to the microsomal fraction, suggesting a dissociatio n of the store from the plasma membrane. These findings suggest that intrac ellular Ca2+ release by cholinergic stimulation is critical for determining synergism with cAMP in parietal cell activation and that functional coupli ng between the Ca2+ store and the receptor is maintained by actin microfila ments.