Effects of anticholinesterases and of sodium fluoride on neuromyal desensitization

Abstract

Desensitization of amphibian neuromyal junction was obtained by either repetitive, brief (2 msec) iontophoretic pulses of acetylcholine (ACh) or by a prolonged (40 sec) iontophoretic application of ACh. When ACh was applied repetitively, ACh potentials diminished gradually at a rate dependent on ACh pulse frequency; the recovery time constant amounted to approximately 7 sec following 30 sec of brief ACh pulses applied at 20 Hz. In the case of prolonged, 40 sec ACh pulses, repolarization occurred within the duration of the pulse. Subsequent test (2 msec) pulses of ACh showed a diminished endplate response. Anticholinesterase drugs, the organophosphorus tetraethylpyrophosphate (TEPP) and the carbamate, neostigmine, markedly accelerated desensitization. This occurred even when the iontophoretic current was decreased following anticholinesterase treatment in order to generate ACh potentials similar in amplitude to those recorded prior to the treatment. These and additional results indicated that anticholinesterases exert a direct, desensitizing effect independently of the possibility of their causing accumulation of iontophoretically applied ACh. Sodium fluoride (NaF) employed in concentrations of 0.1 to 5 mM, significantly delayed the onset of desensitization whether the latter was induced by repetitive or prolonged pulses of ACh; it also accelerated the recovery of the endplate from desensitization. At concentrations ranging from 0.05 to 2 mM, NaF also antagonized the acceleration of desensitization induced by either TEPP or neostigmine; in the combined presence of the anticholinesterase and NaF, the rate constant of desensitization approximated to that recorded under control conditions. This action by NaF was exerted after prolonged (30 min) TEPP treatment; thus, this effect was not due to the reactivating potential of NaF. Repetitive (at 50 Hz) indirect stimulation of the endplate produced progressive diminution of the EPP's, with a plateau occurring at 60 sec. This was concomitant with marked diminution of the response to test pulses of ACh and a slight decrease in the quantal content of the EPP's; thus, it was not due to diminution of the release of ACh. The rate of this phenomenon was markedly increased by anticholinesterases and markedly decreased by NaF. It is emphasized in the Discussion that the antidesensitizing action of NaF does not depend on its chelation of Ca 2+ and that it may be due to the direct action of NaF on the recentor.