Size of the Extracellular Compartment of Skeletal Muscle

Abstract

The water of the animal body may be roughly divided into that contained within the cells, the intercellular fluid, and that contained in the fluid which bathes the cells but is not enclosed by the cell membrane, the extracellular fluid. There have been numerous attempts in recent years to determine the relative size of each of these compartments. In general, the methods employed consist of either (1) the determination of the amount of dilution of an injected, non-toxic foreign substance which does not enter the cell but is distributed uniformly throughout the extracellular water and (2) by the measurement of the amount of some normal constituent which is limited to and uniformly distributed throughout the extracellular water. Sodium thiocyanate has been suggested as a suitable substance for the first method 1 2 3 4 and chloride determination most convenient for the second method. 5 6 7 8 9

Following the intravenous injection of sodium thiocyanate in the cat we find that this substance distributes itself through about 25 to 40% of the body weight. In 2 experiments on anesthetized cats sodium thiocyanate (148 mg and 177 mg per kilo body weight) was injected intravenously, 60 minutes allowed for its dilution, carotid blood taken for sodium thiocyanate and chloride analyses and the animals then sacrificed by asphyxia. Each animal was dissolved in normal KOH, digested in nitric acid and an aliquot taken for chloride analysis. Assuming the total chloride content of the animal body, with the exception of the red blood cell, is in the extracellular compartment and is in equilibrium with the blood plasma, we find that the thiocyanate and chloride available volumes are quite similar (Table I).