Effects of nutrient HCO3- and Cl- on potential difference responses to nutrient Na+ changes in secreting and inhibited states of frog stomach

Previously, it was found that a decrease in nutrient (serosal) Na+ decreased the transmucosal potential difference (PD) in the secreting and inhibited states of fundi of stomachs of Rana pipiens. This PD response, designated as anomalous, was explained by two different models, an electrogenic NaCl symport and an electrogenic NaHCO3 symport. To test these hypotheses, experiments were done at pH similar to 7.4 in which the PD response to a change in nutrient Na+ was compared in 25, 5, and 0 mM nutrient HCO3- solutions. In the H+ secreting state, all HCO3- concentrations gave, for a 10-fold decrease in nutrient Na+, about the same PD response (9-12 mV). The PD response was 8.5, 2.9, and 0 mV higher in the inhibited state for 25, 5, and 0 mM HCO3-, respectively. Experiments with decreasing nutrient Cl- decreased the anomalous PD response to Na+ changes, and the response became insignificant at Cl- concentrations of 20 mM or less in the secreting state. The anomalous PD response to a decrease in Na+ concentration was obtained for all Cl- concentrations (0-79 mM), becoming less with decreasing Cl- in the inhibited state. These results support a NaCl symport to explain the PD response in the secreting state and a NaHCO3 symport to explain the enhancement in the inhibited state.