The relation between tonicity and impulse-evoked transmitter release in the frog

Abstract

1. The increase in miniature end-plate potential (m.e.p.p.) frequency in response to tetanic stimulation of the motor nerve at frog neuromuscular junctions in Ca(2+)-free, Mg(2+) EGTA-containing (0 Ca(2+)-Mg(2+) EGTA) solutions of varying tonicity has been studied. The response to stimulation is markedly increased in hypertonic solutions and is decreased in hypotonic solutions. Under these conditions changes in tonicity have comparable effects on stimulated and spontaneous quantal release.2. The tonicity was raised by adding sucrose, NaCl or glycine to the extracellular solution. The effects of the addition depended primarily on the increase in osmolarity of the solution, not on the chemical species producing it.3. The tonicity was decreased by lowering [NaCl](o). The hypotonic solution decreased the response to tetanic stimulation. When the tonicity of the solution with the low [NaCl](o) was restored to normal by adding sucrose, the response was restored to its usual level. These results suggest that in 0 Ca(2+)-Mg(2+) EGTA solutions stimulation does not enhance the probability of quantal release by raising [Na(+)](i).4. Repeated bouts of tetanic stimulation produced almost identical responses. In some instances the frequency continued to rise after the end of the tetanic stimulation, as reported by Erulkar & Rahamimoff (1978). This suggests that the stimulation of the nerve leads to the elevation within the terminal of a substance that in turn liberates an activator for quantal release.5. The Q(10) for the increase in probability of quantal release is as high as 7. High Q(10) values have also been reported for spontaneous m.e.p.p. frequencies. Tonicity and temperature appear to affect spontaneous and stimulated quantal release similarly.