The membrane potential of rat diaphragm muscle fibres and the effect of denervation

Abstract

1. Resting membrane potentials of rat diaphragm muscles were measured in vitro after previous denervation for 0-10 days. In some experiments denervated muscles were incubated in vitro for 3 hr while in others they were cultured for 15-24 hr to allow adequate exposure to drugs before recording. 2. It was found that resting membrane potentials, within 2-5 mm of the site of nerve section were significantly lower, within 3 hr, than resting membrane potentials measured more than 9 mm away from site of nerve section. This difference could be reduced or abolished by bathing preparations in solutions containing adrenaline (10 muM), noradrenaline (10 muM) or isoprenaline (10 muM) or dibutyryl cyclic AMP (10 muM-0-25 mM in the presence of 2 mM theophylline). Cyclic AMP (0-5 mM) was ineffective. 3. Application of solutions containing dibutyryl cyclic AMP for 3 hr also raised the resting membrane potential of muscles denervated 4-5 days previously. Culture studies showed that this effect was sustained when the time of incubation was 24 hr. 4. Incubating freshly denervated preparations with cycloheximide (22 mug/ml.) or actinomycin D (1 mug/ml.) did not prevent the development of the early (3 hr) fall in resting membrane potential despite a concomitant inhibition of RNA or protein synthesis. Culturing freshly denervated muscles in solutions containing cycloheximide (10 or 25 mug/ml.) which blocked 93% of protein synthesis, did not prevent the expected drop in resting membrane potential after 15 or 24 hr. 5. It was found that exposure to ouabain (1 or 5 mM) produced a rapid (15 min) fall in resting membrane potential in innervated and denervated preparations treated with dibutyryl cyclic AMP but not denervated preparations. After 5 days denervation cyclic AMP levels in muscle were increased by about 40%. 6. It is suggested that upon denervation an electrogenic action of a NA+-pump is blocked and that dibutyryl cyclic AMP and catecholamines are capable of stimulating this pump.