Changes in Na+, K(+)-adenosinetriphosphatase, citrate synthase and K+ in sheep skeletal muscle during immobilization and remobilization.

The K+ balance and muscle activity seem to interact in a complex way with regard to regulating the muscle density of Na(+)-K+ pumps. The effect of immobilization was examined in ten sheep that had low muscle K+ content. Three additional sheep served as untreated controls. After being brought from pasture to sheep stalls one hindlimb was immobilized in a plaster splint for 9 weeks, and in five of the animals remobilization was carried out for a further 9 weeks. The weight bearing of the leg in plaster was recorded by a force plate. Open muscle biopsies from the vastus lateralis muscle were obtained before the study, after 9 weeks of immobilization, and after another 9 weeks of remobilization. The Na(+)-K+ pump density was measured as [3H]-ouabain binding to intact tissue, and citrate synthase activity was measured in tissue homogenate. The tissue content of K+ was measured in fat-free dried tissue. Muscle K+ content increased linearly by almost 70% through the 18-week period independent of intervention. Immobilization reduced thigh circumference by 8% (P < 0.05). A slight decrease in the area of type I fibres at 9 weeks and a slight increase at 18-weeks was found. The [3H]-ouabain binding was reduced by 39% and 22% in the immobilized and control legs, respectively, whereas citrate synthase activity was reduced by about 30% in both legs after 9 weeks of immobilization. During remobilization both the [3H]-ouabain binding and the citrate synthase activity increased to the same level as in the control animals. The plaster cast significantly reduced mass bearing of the immobilized leg, and a corresponding reduction in muscle activity must be assumed to have occurred in both legs as judged from citrate synthase activity. We concluded from this study that the reduction in the [3H]-ouabain binding during immobilization independent of an increase in muscle K+ content points to muscle activity as a strong stimulus for control of Na(+)-K+ pump density.