Volume Concentration of Rat Muscle Measured by the Phosphate Partition Method

Abstract

A simple method, based on the assumed impermeability of muscle cells to phosphate, has recently been described for finding the volume concentration (relative volumes of interspaces and cells) in frog muscle. 1 This investigation was undertaken with the idea that the method might be applicable to rat muscle.

The method used was essentially that of M. G. Eggleton. Adult rats were decapitated, and paired leg muscles (extensor digitorum longus or tibialis anticus) were quickly dissected out and weighed.

One muscle of a pair was placed in mammalian Ringer's solution of a given phosphate content, and the other in a solution of a different phosphate content. The phosphate (4 parts Na₂HPO₄ to 1 part KH₂PO₄) was added in M/15 concentration to insure isotonicity. The muscles were left in the solutions for 1, 2, 3, or 9 hours, and at the end of the period they were again weighed and the P of the solution was determined by the method of Martland and Robison. 2 From the weights of the muscles and from the initial and final phosphate concentrations in the Ringer's solution, one can calculate c, the initial concentration of phosphate in the interspaces, and a, that fraction of the muscle water contained in the interspaces. 3 The volume concentration is (1-a), and Eggleton found a mean value of about 23% for frog muscle.

In many cases results similar to Eggleton's were obtained (Table 1.) In these experiments. a has the mean value of 0.35 and varies between 0.20 and 0.53.