Properties of spontaneous potentials at denervated motor endplates of the frog

Some properties of spontaneous miniature potentials at denervated frog motor endplates (Schwann cell-min.e.p.ps) have been investigated. Schwann cell-min.e.p.ps showed a slower and more variable time course than min.e.p.ps at innervated endplates. The occurrence of Schwann cell-min.e.p.ps usually obeyed Poisson statistics and in the absence of experimental treatment their mean frequency remained stable for several hours. Schwann cell-min.e.p.p. frequency increased with increasing temperature. A logarithmic relation was observed with a mean Q$_{10}$ of 4.87 $\pm $ 0.37. In other experiments a Q$_{10}$ of 9.50 $\pm $ 0.63 was found for min.e.p.ps at innervated endplates. Lanthanum and manganese ions, black widow spider venom, ethanol, diamide, ouabain, theophylline and acid Ringer's solution, all of which increased normal min.e.p.p. frequency, failed to raise the frequency of Schwann cell-min.e.p.ps. Lanthanum, ethanol, ouabain and low pH depressed the frequency, while black widow spider venom, diamide, theophyline and manganese were without significant effect. Adenosine, which depresses min.e.p.p. frequency at innervated endplates, had no effect on spontaneous miniature potentials at denervated endplates. Removal of Ca$^{2+}$ from the external medium reduced the frequency of Schwann min.e.p.ps; and a decrease was also seen in one experiment where the Ca$^{2+}$ concentration was raised from 1.8 to 10 mM. Diluting the Ringer's solution raised Schwann cell-min.e.p.p. frequency transiently. This increase resulted from a reduction in osmolarity, not in the ionic strength, of the medium.