Differential effect of intraterminal sodium on spontaneous quantal release of transmitter in two neuromuscular...

Nerve terminals on the superficial abdominal extensor muscle and the claw opener muscle of small crayfish were loaded with sodium by bath application of 100 mumol/l veratridine in superfusates where normal Ca2+ was removed (low-Ca2+ superfusate). In both preparations this caused an increase in spontaneous quantal release of excitatory and inhibitory transmitter which was evaluated by means of the noise analysis technique. About 2.5 min after application of veratridine, when spontaneous quantal release had largely ceased, the normal Ca2+ concentration was reestablished. This increased transiently the quantal release rate a second time. However, release activated by Ca2+ application was much more vigorous at the superficial abdominal extensor muscle than at the claw opener. At the superficial abdominal extensor muscle on average about 8% of the total number of quanta could be released in low Ca2+ and 92% in normal Ca2+ superfusate, while at the claw opener 75% of the quanta were released in low Ca2+ and 25% in normal Ca2+ superfusate. The experiments suggest that intraterminal sodium has a differential effect in the terminals of the two preparations. Possibly, the intraterminal source from which Na+ may liberate Ca2+ is more restricted in the superficial abdominal extensor muscle than in the opener muscle of the claw.