Stimulatory Pathways of the Calcium-Sensing Receptor on Acid Secretion in Freshly Isolated Human Gastric Glands

Abstract

Gastric acid secretion is not only stimulated via the classical known neuronal and hormonal pathways but also by the Ca²⁺-Sensing Receptor (CaSR) located at the basolateral membrane of the acid-secretory gastric parietal cell. Stimulation of CaSR with divalent cations or the potent agonist Gd³⁺ leads to activation of the H⁺/K⁺-ATPase and subsequently to gastric acid secretion. Here we investigated the intracellular mechanism(s) mediating the effects of the CaSR on H⁺/K⁺-ATPase activity in freshly isolated human gastric glands. Inhibition of heterotrimeric G-proteins (G_i and G_o) with pertussis toxin during stimulation of the CaSR with Gd³⁺ only partly reduced the observed stimulatory effect. A similar effect was observed with the PLC inhibitor U73122. The reduction of the H⁺/K⁺-ATPase activity measured after incubation of gastric glands with BAPTA-AM, a chelator of intracellular Ca²⁺, showed that intracellular Ca²⁺ plays an important role in the signalling cascade. TMB-8, a ER Ca²⁺store release inhibitor, prevented the stimulation of H⁺/K⁺-ATPase activity. Also verapamil, an inhibitor of L-type Ca²⁺-channels reduced stimulation suggesting that both the release of intracellular Ca²⁺ from the ER as well as Ca²⁺ influx into the cell are involved in CaSR-mediated H⁺/K⁺-ATPase activation. Chelerythrine, a general inhibitor of protein kinase C, and Gö 6976 which selectively inhibits Ca²⁺-dependent PKC_α and PKC_βI-isozymes completely abolished the stimulatory effect of Gd³⁺. In contrast, Ro 31-8220, a selective inhibitor of the Ca²⁺-independent PKCε and PKC-δ isoforms reduced the stimulatory effect of Gd³⁺ only about 60 %. On the other hand, activation of PKC with DOG led to an activation of H⁺/K⁺-ATPase activity which was only about 60 % of the effect observed with Gd³⁺. Incubation of the parietal cells with PD 098059 to inhibit ERK1/2 MAP-kinases showed a significant reduction of the Gd³⁺ effect. Thus, in the human gastric parietal cell the CaSR is coupled to pertussis toxin sensitive heterotrimeric G-Proteins and requires calcium to enhance the activity of the proton-pump. PLC, ERK 1/2 MAP-kinases as well as Ca²⁺ dependent and Ca²⁺-independent PKC isoforms are part of the down-stream signalling cascade.

Calcimimetic R-568 effects on activity of R990G polymorphism of calcium-sensing receptor

“…Moreover, we can speculate that calcium transients remained active in presence of PKC modulators because they are triggered by the PLC-IP 3 pathway, upstream PKC (Young & Rozengurt 2002). Studies on CASR and PKC isoforms demonstrated that both conventional and novel PKC isoforms are activated by [Ca 2C ] o increases (Remy et al 2007) and PKC-specific inhibitors did not completely abolish ERK1/2 phosphorylation (Sakwe et al 2004). Novel PKCs (d, 3, h, and q) are activated by [Ca 2C ] i and diacyl glycerol, whereas conventional PKCs (a, b, and g) are activated only by [Ca 2C ] i , so we can speculate that the oscillatory response of CASR 990G could require only one PKC group (Oancea & Meyer 1998, Young et al 2002.…”

Impact of Vitamin D Receptor VDR rs2228570 Polymorphism in Oldest Old

“…Vitamin D deficiency may limit mobility in community dwelling older adults [74], which may impact on instrumental activity of daily life (IADL)-score. In theory the influence on the prevalence of peptic ulcers may be secondary to stimulation of intestinal Ca 2+ absorption with subsequent increase of extracellular Ca 2+ and stimulation of gastric H + secretion by the Ca 2+ sensing receptor [75,76]. The known influence of VDR on the immune response [6,7,8] may have contributed to or even accounted for the impact of the VDR rs2228570 polymorphism on allergy.…”

Molecular characterisation and expression analysis of SEREX-defined antigen NUCB2 in gastric epithelium, gastritis and gastric cancer

“…The main function of chief and parietal cells is the secretion of pepsinogens and gastric acid, respectively. Both secretion processes are coupled and mediated by the intracellular Ca 2+ signalling (Hou and Schubert 2006;Remy et al 2007;Kimura et al 2001;Gritti et al 2000). The localisation of NUCB2 in the secretory granules of chief cells and its loss in atrophic glands suggests a role in the secretion processes by establishing an agonistreleasable Ca 2+ store in ER or Golgi apparatus, signalling via heterotrimeric Gα proteins and/or mediating the exocytosis of the secretory granules.…”