EFFECTS OF CATECHOLAMINES ON CHOLINERGIC NEUROMUSCULAR TRANSMISSION IN FISH RED MUSCLE

1. The effects of adrenaline and noradrenaline on neuromuscular transmission and the question of whether these catecholamines act on the presynaptic nerve terminal or on the post‐synaptic membrane were investigated using the fin muscles of the silver carp. 2. Noradrenaline (10−8‐10−6 g/ml.) had no effect on membrane potential or input resistance of the membrane. In contrast, adrenaline (10−8‐10−6 g/ml.) hyperpolarized the membrane and increased the input resistance. 3. Noradrenaline had no effect on the amplitude of the miniature junction potentials but caused them to become more frequent. 4. Adrenaline enhanced the amplitude of the miniature junction potentials and slightly increased their frequency. These effects of adrenaline were partly due to the increased input resistance of the post‐synaptic membrane, and probably did not involve the presynaptic terminals. 5. Both noradrenaline and adrenaline increased the amplitude and prolonged the falling phase of the excitatory junction potentials (e.j.p.s). 6. Noradrenaline enhanced the amplitude and prolonged the duration of the extracellularly recorded excitatory junction current. No such effect was observed with adrenaline. 7. Adrenaline but not noradrenaline enhanced the amplitudes of acetylcholine potentials evoked by iontophoretic application of acetylcholine. 8. Noradrenaline and adrenaline had no effect on the compound action potentials of the nerve supplying the red muscle, recorded by the sucrose gap method. © 1969 The Physiological Society