Effects of Electrical Stimulation and Insulin on Na+–K+-ATPase ([3H]Ouabain Binding) in Rat Skeletal Muscle

Exercise has been reported to increase the Na+–K+-ATPase (Na+–K+ pump) α2 isoform in the plasma membrane 1.2- to 1.9-fold, purportedly reflecting Na+–K+ pump translocation from an undefined intracellular pool. We examined whether Na+–K+ pump stimulation, elicited by muscle contraction or insulin, increases the plasma membrane Na+–K+ pump content ([3H]ouabain binding) in muscles from young rats. Stimulation of isolated soleus muscle for 10 s at 120 Hz caused a rapid rise in intracellular Na+ content, followed by an 18-fold increase in the Na+ re-extrusion rate (80 % of theoretical maximum). Muscles frozen immediately or 120 s after 10–120 s stimulation showed 10–22 % decrease in [3H]ouabain binding expressed per gram wet weight, but with no significant change expressed per gram dry weight. In soleus muscles from adult rats, [3H]ouabain binding was unaltered after 10 s stimulation at 120 Hz. Extensor digitorum longus (EDL) muscles stimulated for 10–60 s at 120 Hz showed no significant change in [3H]ouabain binding. Insulin (100 mU ml−1) decreased intracellular Na+ content by 27 % and increased 86Rb uptake by 23 % soleus muscles, but [3H]ouabain binding was unchanged. After stimulation for 30 s at 60 Hz soleus muscle showed a 30% decrease in intracellular Na+ content, demonstrating increased Na+–K+ pump activity, but [3H]ouabain binding measured 5 to 120 min after stimulation was unchanged. Stimulation of soleus or EDL muscles for 120–240 min at 1 Hz (continuously) or 10 Hz (intermittently) produced no change in [3H]ouabain binding per gram dry weight. In conclusion, the stimulating effects of electrical stimulation or insulin on active Na+, K+-transport in rat skeletal muscle could not be even partially accounted for by an acute increase in the content of functional Na+–K+ pumps in the plasma membrane

Fetal myosin heavy chains in regenerating muscle.

There are several lines of evidence for the existence of a distinct class of myosins in developing muscle. Using various biochemical and immunological approaches, Whalen et al. recently suggested that two myosin heavy chain isozymes appear sequentially in rat muscle development, preceding the definitive adult myosins. It is unknown whether these myosins are present only in developing fast muscles or whether they also occur in developing slow muscles. Pyrophosphate of gel electrophoresis studies have suggested that fast-twitch and slow-twitch muscles synthesize the same fetal myosin isozymes early in development. Immunocytochemical studies with antibodies directed against adult fast and slow myosins show differences in myosin composition between fetal muscle fibres but interpretation of these findings is complicated by cross-reactions of these antibodies with fetal isomyosins. We have used a more direct immunocytochemical approach to identify the myosin types present in developing muscle fibres. An antibody specific for bovine fetal myosin and cross-reactive with rat fetal myosin has been prepared. We report here that the fetal myosin heavy chains recognized by this antibody show a heterogeneous fibre distribution in fetal and neonatal rat muscle, disappear progressively during postnatal development and are transiently expressed in regenerating muscle