Gastric HCO3--secretion in the guinea pig.

Measurement of gastric intraluminal PCO2 and pH in the anesthetized guinea pig enabled simultaneous determination of total H+ and HCO3- gastric secretions. There was quantitative agreement between the release of CO2 and decrease in HCO3- after intragastric instillation of HCl. The basal rate of HCO3- secretion (approximately 40 mueq-h-1) was, in most cases, smaller than spontaneous H+ secretion, but gastric net secretory output was alkaline (HCO3- greater than H+) after inhibition of acid secretion with histamine H2-receptor antagonists (cimetidine 20 mg-kg-1 or metiamide 35 mg-kg-1). Carbachol (1-2 microgram-kg-1) stimulated secretion of both HCO3- and H+; only the latter response was sensitive to the histamine antagonists. Atropin (100 microgram-kg-1) blocked stimulation of HCO3- secretion but did not affect the basal output of HCO3-. An increase in HCO3- secretion was associated with an equivalent increase in net Na+ influx and an increase in the net influx of Cl- with H+ plus K+. Intragastric neutralization of H+ by HCO3- is likely to occur at the mucosal surface and may protect the mucosa from the damaging effects of intraluminal acid.