Relation of resting potential of rat gastrocnemius and soleus muscles to innervation, activity, and the Na‐K pump

The effect of nerve on resting membrane potential of rat muscle was studied. Denervation produces a decline from 83.5 mV in gastrocnemius and from 68.8 mV in soleus to a range of 60 mV. The rate of decline is more rapid with distal than with proximal nerve section. With reinnervation, potential returns to normal. Immobilization, posterior rhizotomy or spinal section produces a decline in gastrocnemius membrane potential to about 70 mV indicating that integrity of ventral root does not prevent a fall. Independence of resting potential and muscle fiber size is indicated by work hypertrophy, by near and distant section of nerve and by variation in resting potential for the same degree of atrophy produced by disuse or denervation. Existence of a plateau below which resting potential does not decline following denervation is compatible with the concept of two fractions of normal resting potential maintained by different mechanisms. The higher and more vulnerable is less in soleus than in gastrocnemius. It responds to diresct and indirect tetanus with a decline, the low point of which is similar to that which follows nerve section. Direct tetanization of denervated gastrocnemius muscle affects only the residual of the upper fraction of membrane potential not yet reduced by denervation. Topical or intravenous administration of ouabain produces a decline of resting potential to the range of 60 mV. Following nerve section only the residual portion of “upper fraction” is vulnerable to ouabain. At no time does administration of ouabain produce a fall of mean resting potential below the level achieved by long term nerve section alone.