Identification and functional assay of an extracellular calcium-sensing receptor in Necturus gastric mucosa

In mammals and amphibians, increases in extracellular Ca²⁺ can activate bicarbonate secretion and other protective functions of gastric mucosa. We hypothesized that the recently cloned extracellular Ca²⁺- sensing receptor (CAR) is functioning in the gastric mucosa. In Necturus maculosus gastric mucosa, reverse transcription-polymerase chain reaction using primers based on previously cloned CaR sequences amplified a 326-bp DNA fragment that had 84% nucleotide sequence identity with the rat kidney CaR. Immunohistochemical localization of the CaR using specific anti-CaR antiserum revealed its presence on the basal aspect of gastric epithelial cells. In microelectrode studies of Necturus antral mucosa, exposure to elevated Ca²⁺ (4.8 mM) and the CaR agonists NPS-467 and neomycin sulfate resulted in significant hyperpolarizations of basal membrane electrical potentials and increases in apical-to-basal membrane resistance ratios. Circuit analysis revealed that these changes reflected specific decreases in basolateral membrane resistance. Inhibition of prostaglandin synthesis using indomethacin significantly attenuated these effects. We conclude that the CaR is present and functioning in Necturus gastric antrum.