Mechanisms of secretagogue action in isolated parietal cells

• The mechanisms of secretagogue action and various intracellular events in isolated pig and rat gastric parietal cells were investigated. The response to gastrin in aminopyrine accumulation, an index of the acid produced and n·apped by the cells, was ilifferent in pig and rat parietal cells. In pig, gastrin alone stimulated (unaffected by H2 antagonist ranitidine) and potentiated the action of histamine, IBMX, DBcAMP and Sp-cAMP[S]. In rat cells, gastrin alone was ineffective but potentiated the actions of histamine, DBcAMP and Sp-cAMP[S]. The stimulation of aminopyline accumulation by the acetylcholine analogue carbachol, or by gastrin (in pig), was dosedependently inhibited by the protein kinase A inhibitor Rp-cAMP[S]. In rat cells, histamineandDBcAMP-stimulated aminopyrine accumulations were dose-dependently inhibited by the intracellular Ca2+ chelator BAPT A. The basal intracellular cAMP content in pig palietal cells was 3.5-fold higher than that in rat parietal cells. Although IBMX slightly increased cAMP content, it was not enough to increase aminopyrine accumulation in rat. Histamine combined with IBMX increased the cAMP content by 8- to 38-fold. The aminopyrine accumulation, however, was not stimulated further than that observed with histamine-stimulation alone. Gasn·in increased cytosolic free Ca2+ in both pig and rat palietal cell. Basal Ca2+ was reduced by the intracellular Ca2+ chelator BAPTA, and both gasn·in- and carbachol-induced increases in cytosolic free Ca2+ were abolished by the inclusion of BAPT A. Gastrin was as efficient as histamine in inducing the formation of vacuolar/canalicular spaces in the parietal cells, i.e., inducing a secretion-associated morphology. Ranitidine abolished histamine- but not gasn·in-induced morphological n·ansformation. In the presence of BAPTA, the morphological transformations induced by either gastrin, the cAMP analogues DBcAMP or Sp-cAMP[S] were completely abolished. It is concluded that: I) gastrin has a direct action on the parietal cells; 2) species difference of gastrin action seems to be related to different basal cAMP contents; 3) Ca2+. dependent morphological transformation is essential for aminopyrine accumulation; and 4) a threshold level of either Ca2+ or cAMP seems to be required for the stimulation by the other second messenger.

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