AJP - Cell Physiology, Vol 235, Issue 1 35-C48, Copyright © 1978 by American Physiological Society
Effects of pH, Ca, ADH, and theophylline on kinetics of Na entry in frog skin
L. J. Mandel
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.
