HYPERTONIC TREATMENT REVERSIBLY INCREASES THE RATIO OF GIANT SKEW MINIATURE END-PLATE POTENTIALS TO BELL-MINIATURE END-PLATE POTENTIALS

Miniature endplate potentials were recorded from single frog muscle fi bers before, during and after treatment with hypertonic saline (200-50 0 mM NaCl or Na gluconate added to frog saline). Miniature endplate po tential amplitude distributions were plotted from small muscle fibers so that the modes and ratios of the skew-miniature endplate potential to bell-miniature endplate potential classes could be defined. Muscle fibers were voltage clamped with two electrodes to determine the input resistance before, during and after treatment. Input resistance incre ased from two to 100 times during treatment and rapidly fell towards c ontrol values (no more than 30% greater) when preparations were return ed to normal frog saline. Short duration treatments with 200-300 mM hy pertonic salines immediately increased frequencies (100-fold) of both skew-miniature endplate potential and bell-miniature endplate potentia l classes. Preparations when returned to normal frog saline after a fe w minutes of treatment showed control miniature endplate potential dis tributions within minutes. One to two hour treatments left only the sk ew-miniature endplate potential class and with hour-long recovery peri ods bell-miniature endplate potentials reappeared and ratios of skew-m iniature endplate potential to-bell-miniature endplate potential class es returned to control values. Treatment with 500 mM NaCl added to fro g saline immediately increased the percentage of skew-miniature endpla te potentials (from 2 to 50%) with little or no increase in overall mi niature endplate potential frequencies. The mode of the skew-miniature endplate potential class was unchanged after hypertonic treatment, wh ereas that of the bell-miniature end plate potential class either rema ined about the same size or decreased depending on the duration of tre atment. The number and percentage of giant-miniature endplate potentia ls belonging to the skew-miniature endplate potential class increased as a function of the duration of 200-300 mM hypertonic saline treatmen ts. Most giant-miniature endplate potentials had a slow rising phase w ith a foot and/or breaks demonstrating a composite structure. Sequenti ally recorded giant-miniature endplate potentials had similar initial slopes indicating either repetitive releases from single sites or rele ases from cooperative sites. After hypertonic treatment the bell-minia ture endplate potential size was never more than that expected with th e increase (under 30%) in input resistance. The results presented here are completely different from those of Yu and Van der Kloot [(1991) J . Physiol. 433, 677-704] who reported that the bell-miniature endplate potential amplitude was increased two- to four-fold after hypertonic treatment. The wide range of results in the ratio of skew-miniature en dplate potential to bell-miniature endplate potential classes is discu ssed in regards to the quantal hypothesis which is based on a single c lass of immutable amounts of transmitter; and, a hypothesis based on a dynamical process that meters transmitter in subunit amounts to contr ol miniature endplate potential size and class during release.