DISTRIBUTION OF WATER, SODIUM, AND POTASSIUM IN RESTING AND STIMULATED MAMMALIAN MUSCLE

Abstract

Potassium and sodium concentration and inulin space were measured in samples of rat gastrocnemius after direct or indirect stimulation in situ for 1/2, 1, 3, or 6 hours. The muscles in each case were cut into three pieces (1) close to the bony origin (proximal), (2) about the middle part, and (3) close to the tendinous insertion (distal). During the first hour of stimulation the percentage change in intrafiber [Na] and [K] was marked and identical in all parts of the muscle. However, after prolonged stimulation at a rate of 10/second, the intrafiber Na and K distribution, was not uniform along the muscle fiber, with K depletion and Na accumulation being significantly greater in the middle than in either the distal or proximal parts of the muscle. In about 75% of the nearly fatigued muscles changes in intrafiber [Na] and [K] were extreme, although the glycogen content was still adequate (over 200 mg %). When the muscles were stimulated more slowly over a prolonged period (6 hours, 2/second) either isotonically or isometrically, the final cationic distributions of the proximal and middle parts of the muscle were almost identical. The distal portion, however, exhibited the most marked cationic "debt". The Na-K exchange phenomenon is evidently not exclusively related to the excitation process, but may play a role in the contractile process itself. The data support the idea of a relation between fatigue and "ionic debt" in muscle, and also cast some doubt on the uniform distribution of potassium in the muscle fibers.