Na+/HCO(3)(-)cotransport and expression of NBC1 and NBC2 in rabbit gastricparietal and mucous cells

Background & Aims: The gastric epithelium protects itself against luminal a cid by secreting HCO3--rich fluid into the mucous layer and by HCO3--depend ent intracellular pH regulation, but the basolateral HCO3- uptake mechanism s are incompletely characterized. This study examined the expression and fu nctional significance of the Na+/HCO3- cotransporters NBC1 and NBC2 in rabb it gastric epithelial cells. Methods: Rabbit NBC1 and NBC2 complementary DN A fragments were cloned and sequenced, and cellular expression levels were assessed by semiquantitative polymerase chain reaction. Na+/HCO3- cotranspo rt activity was measured fluorometrically in cultured rabbit parietal and m ucous cells. Results: NBC1 expression was 4.5-fold lower in the stomach tha n kidney cortex and 5.5-fold higher in mucous than parietal cells. NBC2 exp ression in the stomach was much lower than in the eye, similar to 4-fold lo wer than NBC1 expression in the stomach, and 2.5-fold higher in mucous than parietal cells. The Na+- and HCO3--dependent, dimethylamiloride-insensitiv e (which at 500 mu mol/L completely inhibits all Na+/H+ exchanger isoforms) base influx rates were 4.6 -t- 0.02 and 16.2 +/- 0.04 mmol/L/min in acidif ied parietal and mucous cells, respectively, and were not significantly dif ferent in the absence of Cl-. Conclusions: This study shows that NBC1 and N BC2 are expressed in rabbit stomach, with high levels in mucous cells where Na+/HCO3- cotransport is the major base-importing mechanism in the presenc e of CO2/HCO3-.