Effects of pH, Ca, ADH, and theophylline on kinetics of Na entry in frog skin.

The short circuit current as a function of Na concentration in both solutions was found to obey Michaelis-Menten kinetics under a variety of experimental conditions. Values of maximal transport rate (Im) and half-maximal Na concentrations (Kt) were determined from these experiments. Three type of results were obtained: 1) Im and Kt both decreased by approximately the same fraction when the pH of both solutions was reduced by increasing PCO2, 2) Im decreased and Kt increased when the external pH was decreased, and 3) Im increased with ADH and theophylline, decreased with external Ca, and Kt remained unchanged. Various criteria were utilized to determine that these were properties of the entry barrier for Na into the "transport pool." The results are explained in terms of a model that separates three different types of actions on the entry barrier: 1) competition of Na with other ions in the external solution for entry, 2) modulation of the number of sites available for Na translocation by changing the cytoplasmic pH, and 3) alterations in the rate of Na translocation caused by changes in the Na permeability or the electrochemical gradient across the entry barrier.