Potential difference responses to nutrient K+, Na+ and Cl- changes with varying nutrient HCO3- in resting frog stomach

The effects of changes in nutrient concentrations of K+, Na+ and Cl- on the transmucosal potential difference (PD) and resistance were compared for 25 and 5 mM nutrient HCO3- in resting fundus. With 25 mM HCO3-, increase of K+ from 4 to 40 mM, decrease of Na+ from 100 to 10 mM and decrease of Cl- from 81 to 8.1 mM gave, 10 min after the change, delta PD values of -23.2, -15.1 and -21.3 mV, respectively. With 5 mM HCO3-, the same changes in nutrient ion concentration gave delta PD values of -11.9, -9.4 and -10.0 mV, respectively. From these results, in going from 25 to 5 mM HCO3-, it follows that the resistances of the ionic pathways for K+, Na+ and Cl- increased. The anomalous PD response following the increase in nutrient K+ from 4 to 40 mM with 5 mM nutrient HCO3- gave further evidence that the resistance of the simple K+ conductance pathway increased prior to the increase to 40 mM K+. The fact that 2 mM Ba2+ in the 25 mM HCO3- nutrient gave a smaller increase in resistance, compared to the decrease in nutrient HCO3- from 25 to 5 mM, supported the inference that resistances of ion pathways other than that of the K+ pathway increased.