Neural regulation on the active sodium‐potassium transport in hypokalaemic rat skeletal muscles.

C.N.S.‐induced suppression of muscle Na‐pump activity was studied in fast ‘twitch’ muscle, extensor digitorum longus, of hypokalaemic rats which were fed a K‐deficient diet for 0‐9 weeks. The results were compared with those of slow ‘tonic’ muscle, soleus, reported previously. K‐deficient diet caused blood hypokalaemia and a considerable K+ loss and Na+ accumulation in the skeletal, heart and smooth muscles. The cellular K+ loss was in the order of soleus greater than extensor digitorum longus greater than diaphragm greater than duodenum greater than auricle greater than ventricle; C.N.S. organs such as cerebrum, cerebellum, medulla oblongata, spinal cord and liver were spared this K+ fall. Skeletal, heart and smooth muscles lost more K+ with prolongation of hypokalaemic periods, whereas plasma K+ concentration did not fall much below 1.6 mM during hypokalaemia. Peripheral nerve section, cervical and brain‐stem transection, decerebration and cortical spreading depression with 20% KCl, which activated the active Na+ and K+ transport in soleus muscles during hypokalaemia, could not enhance the pump activity in extensor digitorum longus muscles. Alpha‐adrenoreceptor antagonists such as phenoxybenzamine, phentolamine and dibenamine and a specific blocker of post‐synaptic alpha 1‐adrenoreceptor, prazosin, did not stimulate Na+ and K+ transport in the extensor digitorum longus muscles during hypokalaemia while the beta‐adrenoreceptor antagonist, propranolol, also had no effect. The sensitivity of the active Na+ and K+ transport system in rat muscles to ouabain applied intraperitoneally was greater in extensor digitorum longus muscles than in soleus muscles. The binding experiment with a radiolabelled ligand of alpha 1 adrenoreceptor antagonist, [3H]prazosin, demonstrated the presence of alpha 1‐adrenergic receptors on the soleus muscle membranes of hypokalaemic rats, but not of normal rats. alpha 1 Adrenergic receptors were not detected on the extensor digitorum longus muscle membranes prepared from either hypokalaemic or normal rats. The correlation between the C.N.S.‐induced inhibition on the Na pump in soleus muscle during hypokalaemia and the occurrence of alpha 1 adrenergic receptors on the muscle was discussed.