Rubidium influx into rat skeletal muscles in relation to electrical activity

1. Rates of (86)Rb influx were compared in vivo over 2, 4 and 6 hr periods in various tonic and phasic muscles of rat following its I.P. injection. During the 2 hr period its influx rate into soleus was about 4 times that of the vastus with the EDL muscles at an intermediate rate. Uptake by diaphragm was fastest reaching equilibrium within 2 hr.2. Unilateral section of the sciatic nerve 48 hr before (86)Rb injection reduced isotope uptake into soleus to about 50% of its contralateral control muscle over a 4 hr period. In EDL muscles on the other hand nerve section increased influx by about 75% of control in conscious rats and more than doubled influx in anaesthetized rats.3. Tenotomy of soleus reduced (86)Rb influx to 40% of control, but tenotomy in EDL was without effect in influx.4. Uptake of urea into muscles within 5 min of its I.V. injection was used to determine the possibility of muscle blood flow determining (86)Rb influx. Accumulation of urea was not significantly different in control and denervated EDL muscles nor between soleus and vastus muscles in anaesthetized rats, so it seems unlikely that blood flow is important here.5. Membrane depolarization in response to addition of 30 mM rubidium to external bathing fluid was greater in the case of denervated than in control EDL muscles which was in keeping with the greater (86)Rb influx seen in the former muscles. The ouabain sensitivity of rubidium-induced depolarization in the denervated EDL muscles would suggest, however, that rubidium enters the fibres actively.