Effects of adrenaline on excitation-induced stimulation of the sodium-potassium pump in rat skeletal muscle.

Experiments were performed on isolated rat soleus and extensor digitorum longus (EDL) muscles of 4-week-old rats. In the soleus, direct electrical stimulation for 10 min induced a frequency-dependent increase in the ouabain-suppressible 86Rb+ uptake, which was maximal (+110%) at a frequency of 2 Hz. In the EDL this frequency only induced a 31% increase. A supramaximal concentration of adrenaline (10 mumol l-1) stimulated ouabain-suppressible 86Rb+ uptake by 80% and 27% in soleus and EDL, respectively. The combined effect of stimulation at 2 Hz and adrenaline was not significantly larger than each of the interventions alone in either of the muscles. The fractional loss of 22Na+ from soleus muscle was increased by around 50% by the exposure to adrenaline, electrical stimulation at 2 Hz or a combination of both. The effect of electrical stimulation on 22Na+ efflux was not prevented by addition of propranolol (1 or 10 mumol l-1). The results indicate that the stimulation of active Na+-K+ transport induced by adrenaline or electrical stimulation is much more pronounced in soleus (slow-twitch) muscle than in EDL (fast-twitch) muscle. Since it has been suggested that an accumulation of K+ ions in the extracellular space may play a role in the development of fatigue (Bigland-Ritchie 1984), our findings might be related to the fact that slow-twitch muscles have a much higher resistance to fatigue than fast-twitch muscles (Burke et al. 1971).