Transient hyperpolarization of non-contracting muscle fibres in anaesthetized rats

1. Following laminectomy, the L5 ventral roots of anaesthetized rats were split and approximately half of the nerve fibres were stimulated at 40 Hz. Resting membrane potentials were then measured in previously contracting and non-contracting soleus muscle fibres. 2. In the non-contracting soleus fibres there was a post-tetanic increase in mean resting potential (from -82.2 +/- 7.1 (S.D.) to -91.6 +/- 8.7 mV) which was similar to that in contracting fibres (from -83.02 +/- 6.1 to -91.3 +/- 7.3 mV). In both types of fibres the hyperpolarizing responses (HRs) were evidently due to increased sodium pump activity since they could be abolished by the addition of ouabain (1 x 10(-4) M) to the bathing fluid. 3. The beta-adrenergic antagonist, propranolol, completely suppressed HRs in the non-contracting fibres and produced moderate reductions in the contracting ones. The alpha-adrenergic blocking agent, phentolamine, had no effect on the contracting fibres and only a modest, inhibitory, one on the non-contracting fibres. 4. On the basis of the above drug actions it appeared that catecholamines were necessary for the full development of HRs in contracting and non-contracting fibres; noradrenaline, released from intramuscular sympathetic nerve endings, may have been involved. 5. The increased sodium pump activity in the non-contracting fibres would serve to moderate the rise in interstitial [K+] caused by K+ efflux from the contracting fibres. By preventing passive depolarization, due to the rise in interstitial [K+], the sodium pump would also maintain the availability of non-contracting fibres for subsequent recruitment.