Effect of hypertonic saline on quantal size and synaptic vesicles in identified neuromuscular junction of the frog

Miniature endplate potential amplitude distributions, miniature endplate potential frequencies and the percentage of sub-miniature endplate potentials were studied during treatment with hypertonic saline (with sucrose) during the initial high frequencies of release and after fatigue. Small muscle fibers were selected which had normal miniature endplate potential frequencies of 0.1/s to 1/s so that the miniature endplate potential amplitude distributions could be determined at the height of the hypertonic effect (first 5-15 min) at which time the miniature endplate potential frequency increased two-hundredfold. During the first few minutes of the effect, there was little change in miniature endplate potential amplitude or in the profiles of their amplitude histograms. Subsequently, after the occurrence of as few as 10(4) miniature endplate potentials, the size of the mean bell-miniature endplate potentials decreased. Later (25 min) the amplitude profiles became uniform, and finally (45 min) the percentage of sub-miniature endplate potentials and smaller miniature endplate potentials increased until many miniature endplate potentials (30-70%) were of the sub-miniature endplate potential class and the overall distributions were skewed. The mean sub-miniature endplate potential amplitude did not appear to change. After the initial high frequency of release, many miniature endplate potentials showed a definite break on the rising phase and the amplitude of the break was usually that of the sub-miniature endplate potential. The rapid decrease in miniature endplate potential size, change in miniature endplate potential amplitude profile and breaks on the miniature endplate potential rising phase can be explained with the subunit hypothesis. The edge fibers of the sartorius muscle were used so that physiologically studied edge junctions that were producing various miniature endplate potential histograms could be identified for electron microscopy. Synaptic vesicle diameters and the coefficient of variation of vesicle diameters were not changed either during high miniature endplate potential frequencies or in those junctions that generated mainly sub-miniature endplate potentials. Thus, the quantal class (i.e. sub-miniature endplate potential or bell-miniature endplate potential) cannot be determined from the vesicle diameter.