CHANGES IN NA-ADENOSINE-TRIPHOSPHATASE, CITRATE SYNTHASE AND K+ IN SHEEP SKELETAL-MUSCLE DURING IMMOBILIZATION AND REMOBILIZATION(, K+)

The K+ balance and muscle activity seem to interact in a complex way w ith regard to regulating the muscle density of Na+-K+ pumps. The effec t of immobilization was examined in ten sheep that had low muscle K+ c ontent. Three additional sheep served as untreated controls. After bei ng brought from pasture to sheep stalls one hindlimb was immobilized i n a plaster splint for 9 weeks, and in five of the animals remobilizat ion was carried out for a further 9 weeks. The weight bearing of the l eg in plaster was recorded by a force plate. Open muscle biopsies from the vastus lateralis muscle were obtained before the study, after 9 w eeks of immobilization, and after another 9 weeks of remobilization. T he Na+-K+ pump density was measured as [H-3]-ouabain binding to intact tissue, and citrate synthase activity was measured in tissue homogena te. The tissue content of K+ was measured in fat-free dried tissue. Mu scle K+ content increased linearly by almost 70% through the 18-week p eriod independent of intervention. Immobilization reduced thigh circum ference by 8% (P < 0.05). A slight decrease in the area of type I fibr es at 9 weeks and a slight increase at 18-weeks was found. The [H-3]-o uabain binding was reduced by 39% and 22% in the immobilized and contr ol legs, respectively, whereas citrate synthase activity was reduced b y about 30% in both legs after 9 weeks of immobilization. During remob ilization both the [H-3]-ouabain binding and the citrate synthase acti vity increased to the same level as in the control animals. The plaste r cast significantly reduced mass bearing of the immobilized leg, and a corresponding reduction in muscle activity must be assumed to have o ccurred in both legs as judged from citrate synthase activity. We conc luded from this study that the reduction in the [H-3]-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+ p ump density.