The "intermediate component" of the radiosodium efflux from frog skeletal muscle

The influence of diversity in the size of the cells of the frog's sartorius on the radiosodium efflux from the muscle was investigated. Morphometric analyses of light micrographs of complete cross sections of the muscle were done in the proximal and distal regions. The results were used to predict the shape of the radiosodium washout curve under the following assumptions: the cells differ in size and shape, but each has a single internal pool of exchangeable sodium; the sodium exchange properties of the limiting membranes are the same for all cells; and the diversity of the true areas of the limiting membranes is reflected by the diversity of the apparent areas measured at the light microscopic level. Radiosodium efflux measurements were performed on similar muscles. The model correctly predicted the occurrence of a continuous decline of the fractional loss of radiosodium, which was not due to diffusional delay and which would be interpreted as a second internal compartment in a compartmental analysis, and an effect of short versus long isotope loading intervals on the efflux. It was concluded that the existence of cell size diversity satisfactorily explains the flux data. No "special region" must be postulated.