Potential Responses of Nutrient Membrane of Frog's Stomach to Step Changes in External K+ and Cl- Concentrations

With an in vitro chamber method experiments were performed to determine the relative ionic conductances of the nutrient membrane (membrane facing muscularis mucosa). The concentration of a given ion in the nutrient bathing solution was changed, and the ensuing time course of the change in transmucosal potential difference (PD) was recorded. Changing K+ from 4 to 79 mM produced a response in PD which occurred markedly faster than the response for the reverse change, and similar results were obtained by changing the Cl- concentration. It was found that these differences were predicted by the analysis of an idealized model consisting of a membrane in series with a diffusion barrier. When both the K+ and Cl- were changed, such that the product of their concentrations remained constant, the time courses of the responses were again similiar to those predicted on the basis of the model. From the magnitudes of the total PD responses it is shown that in the presence of a 4 mM K+ nutrient solution, the conductivity of the nutrient membrane appears to be entirely due to the K+ and Cl- conductances, the K+ conductance being about twice that of the Cl-. It is also shown that with a 79 mM K+ nutrient solution the parameters of the membrane were changed such that the conductances of the two ions were approximately equal. The time constant for diffusion of KCl or NaCl across the barrier consisting of the submucosa, muscularis mucosa, and lamina propria is about 1min