Seasonal changes in the response of fast and slow mammalian skeletal muscle fibers to zero potassium.

While investigating the decline in resting membrane potential (RMP) of rat skeletal muscle fibers in zero potassium solution, we discovered that there is seasonal variation in the response of the extensor digitorum longus muscle (EDL). In January, most EDL fibers hyperpolarize in zero K+; in September, most depolarize; the distribution of RMPs recorded in May is bimodal, with some fibers hyperpolarizing and some depolarizing. Fibers from the soleus muscle depolarize in zero K+ irrespective of the season. The ability of EDL fibers to hyperpolarize appears during the 7th and 8th weeks postpartum, and is dependent upon the presence of a functional nerve, since denervation abolished the response. As possible explanations for these findings, inactivation of K(+)-channels and inhibition of the Na-K pump by zero K+ are discussed.