Effects of catecholamines on the neuromuscular junction of the somatic muscle of the earthworm Pheretima communissima.

Abstract

ABSTRACT Effects of catecholamines and adrenergic blocking agents on the neuromuscular junction of the somatic muscle of the earthworm, Pheretima communissima were studied by the microelectrode technique. Effects of catecholamines and their blocking agents on the post-junction membrane could be summarized as follows: (a) Adrenaline and isoprenaline (10−8–10−5 g/ml) hyperpolarized the membrane in proportion to the applied concentration. However, noradrenaline (10−8–10−5 g/ml) had no effect on the membrane potential, (b) Changes of the membrane resistance which appeared on treatment with catecholamines were in parallel with the changes of the membrane potential, (c) In the presence of excess calcium ions and in sodium-free (tris) solution, adrenaline and isoprenaline had no effect on the membrane potential or on the input resistance of the membrane. An increase of potassium ions and reduction of chloride ions had no relation to the actions of adrenaline and isoprenaline on the post-junctional muscle membrane, (d) Propranolol competitively inhibited the action of isoprenaline on the post-junctional muscle membrane but phentolamine had no effect on this action. Effects of catecholamines and their blockers on the m.e.j.p. and m.i.j.p. were observed, (a) After treatment with picrotoxin, the m.e.j.p. could be recorded in sodium-free solution. The frequency of the m.e.j.p. was increased by noradrenaline and adrenaline but not by isoprenaline. In normal Ringer’s solution isoprenaline slightly increased the frequency of m.e.j.p. but this appeared to be due to the effect of isoprenaline on the post-junctional membrane, (b) Phentolamine increased the mean amplitude of the m.e.j.p. but had no effect on the frequency of the m.e.j.p. After treatments with phentolamine effects of noradrenaline were inhibited, (c) After pre-treatment with d-tubocurarine, noradrenaline and adrenaline increased the frequency of the m.i.j.p. but only adrenaline increased the mean amplitude. Isoprenaline had no effect on the frequency but increased the mean amplitude of the m.i.j.p. From the above results it is concluded that the α-response to catecholamines increased the release of chemical transmitter from the nerve terminals; the β-response increased the membrane potential and the input resistance of the post-junctional muscle membrane. Both α-and β-responses to the catecholamines facilitated the transmission mechanism of the earthworm somatic muscle.