Permeation and diffusion of K ions in frog muscle.

The movements of tracer K and net changes of K have been measured in frog muscle. The quantities moving can be linearly related to the square root of the time after a delay of 4 to 30 minutes depending on the external K concentration. The slope of the uptake-t((1/2)) line is increased when the external K concentration is raised. The Q(10) of the uptake is about 1.9 per unit t((1/2)). K uptake from 1 to 2 mM concentration is diminished by a factor of about 2 if strophanthin is applied. The output per unit t((1/2)) is increased by a factor of about 1.4 by strophanthin. Tetrabutylammonium substituted for 10 per cent of the Na in the medium causes a reversible slowing of K uptake and Na output. The rates of movement found in the tracer experiments can be used to calculate the net losses of K taking place in K-free or strophanthin-containing media. The results are interpreted on the basis of K movement being limited both by a resistive outer layer and by diffusion within a K-rich region. The internal diffusion constant is 10(-11) to 10(-10) cm.(2) sec.(-1) depending on the K concentration. The rate of movement of the K can be related to the electrochemical activity of the ion, the lability of the sites on which it is absorbed, and cation + anion pair diffusion within the cell. The surface resistance to K ions can be accounted for as the sum of a membrane resistance equal to that found by electrical methods and the resistance offered to the movement of K by an annulus sufficiently thick (ca. 3 micro) to accommodate the cell Na at a density equal to the mean density of cation within the cell through which K diffuses with the same diffusion constant as holds in the K-rich region. Na movement, if assumed to take place by diffusion from the annulus with diffusion constant equal to that for K ions, has a rate which agrees well with observed values. The influence of strophanthin and tetrabutylammonium on the ion movements is interpreted as being the result of these agents causing an expansion of the outer non-selective region, normally occupied mainly by Na, at the expense of the inner K-rich region.