The role of phosphoinositide 3‐kinase (PI3K) in the stimulation of acid secretion by gastric parietal cells

Abstract

Previous studies using the aminopyrine (AP) assay showed gastric glands treated with histamine (H) + the PI3K inhibitor, LY294002 (H+LY), had enhanced acid secretory response compared to glands stimulated by H alone, and nearly to the extent of maximum response produced by histamine + IBMX (H+I). AP uptakes were 3.5, 8, and 7-fold over resting for H, H+I, and H+LY, respectively. Our data further showed that LY is not acting on PI3K via the Ca2+/PKC pathway or the EGF pathway, and activation of G protein-coupled H2 receptor (H2R) prior to adenylate cyclase is required. We now report on studies to confirm the enhanced secretion of H+LY. Morphology of parietal cells was examined by DIC microscopy and expansion of canalicular area was used to index acid secretion. The increase in area was larger for H+LY than for H alone. Ultrastructural detail of stimulated cells was obtained by electron microscopy using quantitative stereology. For glands stimulated by H+LY microvilli and canalicular volume increased by 4 and 3-fold respectively and tubulovesicle volume decreased by 3-fold compared to resting, substantiating a major morphological response. Relaxation of H+LY stimulated glands by adding the H2R antagonist (lupitidine) was tested. Secretion in response to H+LY slowed after lupitidine suggesting enhanced stimulation by LY was not due to pumps remaining at the apical membrane, but cells were stimulated to a higher degree. Thus, H+LY stimulates the full acid secretory response to a higher degree than H alone, and similarities between H+LY and H+I stimulation suggest this is due to an increase in [cAMP]. We propose PI3K is activated during parietal cell stimulation via H2R and that PI3K modulates an inhibitory pathway that is overwhelmed by increased [cAMP]. Support by DK10141.