In frog a variety of treatments have been shown to increase quantal size at the neuromuscular junction (NMJ), apparently by releasing more acetylcholine (ACh) per quantum. The present experiments were undertaken to see whether similar changes occur at the mouse NMJ. 2. None of the hormones tested, adrenaline, insulin or corticosteroids, significantly increased quantal size at the mouse NMJ. 3. Soaking diaphragms in hypertonic solution (about 475 mosmol/kg) for 15-30 min roughly doubled the size of miniature endplate potentials (MEPPs), miniature endplate currents (MEPCs), and uniquantal endplate potentials (EPPs). We will refer to these as 'large quanta'. Note that all of the measurements were made after returning the preparation to normal (Tyrode) solution. 4. In frog hypertonic solution made with sodium gluconate replacing NaCl increases quantal size four- rather than two-fold. In mouse there was little difference in the effects of solutions made with the two anions. In Cl(-)-free hypertonic solution, made with sodium gluconate and SO4(2-) solutions, quantal size increase is somewhat less, so there may be a role for Cl- in enlarging quantal size. 5. After hypertonic treatment, quantal size remained elevated for at least 1 h and then gradually declined back to usual levels. The data suggest a gradual decrease in mean quantal size, rather than the appearance of a new subpopulation of smaller quanta. 6. Hypertonic treatment did not change the endplate depolarization in response to ionophoretically applied ACh. This suggests that the increased quantal size is not due to a postsynaptic change. Large MEPP's disappear in the presence of tubocurarine and reappear when the drug is washed away. 7. Vesamicol is an inhibitor of active ACh uptake into isolated synaptic vesicles. 5 microM-vesamicol has no detectable postjunctional effect. However, when vesamicol was included in the Tyrode and the hypertonic solutions the increase in quantal size was blocked. This is further evidence that the large quanta are produced by the release of more ACh per quantum. 8. Even when added after the hypertonic treatment, vesamicol soon decreased quantal size back to the normal level. Two other inhibitors of active ACh uptake into vesicles, tetraphenylboron (TPB) and hexanitrodiphenylamine (HNPA) also decreased quantal size after hypertonic treatment, apparently by a presynaptic action. This suggests that the additional ACh that produces large miniatures may be incorporated into the quanta shortly before release.(ABSTRACT TRUNCATED AT 400 WORDS)
