Na+ transport by mammalian stomach.

Gastric mucosas from newborn pigs (0--20 days) and rabbits (0--20 days) were used for in vitro investigation of active Na+ transport during resting (no HCl secretion) conditions. As measured with 22Na+, these tissues actively absorb Na+ from the mucosal-to serosal (m-t-s) bathing solution during both open-circuit and short-circuit current (Is) conditions. In the nonsecreting state, net Na+ transport accounts for 40--60% of Isc. The remaining current is provided by net s-to-m flux of Cl-. Amiloride (2-5 X 10(-5) M) in the mucosal solution abolishes this active Na+ transport by inhibiting m-to-s fluxes of Na+ (JNams). In vivo-in vitro experiments showed that active Na+ transport is a normal function of the resting mammalian stomach. Decreasing pH of the mucosal solution below pH 5 reversibly causes decreased current-generating capability of the tissue. Pretreatment of the tissue with amiloride abolishes this pH effect. The implication is that the low pH affects the Na+-entry step into cells. "Titration curves" of current vs. pH had an apparent pK approximately 4.0. Ouabain and K+-free solutions both cause decreases in active Na+ and Cl- current. Calculations indicate that a shunt may account for only a small (less than 30%) percentage of total transepithelial conductance.