The K+-sensitive hyperpolarization of rat skeletal muscle and metabolic inhibitors

The electrogenic contribution of Na-pump to the resting potential was investigated in isolated "Na-rich" rat soleus muscles exposed to K-free medium for more than 4 hr at 37 degree C. When the resting potentials of fibres fell to around - 75 mV in the K-free medium, the addition of K+ to the bathing medium evoked a transient hyperpolarization. The K+-sensitive hyperpolarization was reproducible in repeated K+ challenged at 45 or 60 min cycles. The K+-sensitive hyperpolarization was completely abolished by cooling, by adding ouabain, or by substituting Li+ for Na+, and reduced by low and high pH. DNP and hypoxia had little effect on the K+-sensitive hyperpolarizations.