Studies on the control of cation permeability in skeletal muscle of the laboratory rat

Abstract

The present study was undertaken in an attempt to examine the importance of the NaKMgATPase (the 'sodium pump' enzyme) in cation movements in rat skeletal muscle. Though the 'sodium pump' was first postulated in an attempt to explain some of the cation movements recorded in muscle tissue, at the commencement of this study clear evidence of the presence of the NaKMgATPase was still awaited. Several approaches to the problem were made, using physiological and biochemical preparations. Isolated preparations of muscle, maintained in optimum conditions, were subjected to treatments with reagents known to affect the NaKMgATPase and the changes in cation distribution were observed. Similar preparations were studied with respect to their oxygen consumption, to determine whether treatments known to cause changes in NaKMgATPase activity would in turn lead to alterations in oxidative metabolism as had been reported in a variety of other tissues, e.g. kidney, bain. Biochemical approaches consisted largely of attempts to isolate a fraction from skeletal muscle which demonstrated; the properties of the NaKMgATPase that had been isolated from a wide range of tissues, and was especially active in those tissues in which a great deal of active cation movement was known to occur. The studies of cation distribution were largely inconclusive, as though modified cation movements were clearly seen in various conditions known to inhibit NaKMgATPase activity, it was not possible to identify firmly the position of such an enzyme or indeed that it was the sole site of action. Measurements of oxygen uptake failed to reveal any portion of oxidative metabolism which could be ascribed to metabolic activity linked to NaKMgATPase activity. After a variety of isolation techniques had failed to produce a fraction of skeletal muscle containing a clearly demonstrable NaKMgATPase, a procedure involving exposure of the muscle homogenate to 2 M NaI was successful in separating a membrane fraction exhibiting NaKMgATPase activity from rat diaphragm and hind-limb muscle. Whilst isolation of a 'pure' enzyme was not made, the study showed that the properties of the fraction isolated resembled those described for NaKMgATPases isolated from other mammalian tissues.