Effect of temperature on endplate potential rundown and recovery in rat diaphragm

The amplitude of neuromuscular junction end-plate potentials (EPPs) decreas es quickly within a train but recovers nearly completely from train to trai n during intermittent stimulation. Rundown has been shown to be dependent n ot only on the rate of transmitter release but also on the rate of replenis hment of the depleted neurotransmitter at the site of release. Two groups o f processes have been proposed for synaptic vesicle recycling, both of whic h involve multiple energy-requiring steps and enzymatic reactions and which therefore would be expected to be very temperature-sensitive. The present study tested the hypothesis that low temperature therefore increases the ra te of EPP amplitude rundown. Studies were performed in vitro on rat diaphra gm and used mu -conotoxin to allow normal-sized EPPs to be recorded from in tact fibers. EPP amplitude rundown during intermittent stimulation at 20 an d 50 Hz (duty cycle 333 ms) was greater at 20 degreesC than it was at 37 de greesC. Initially, temperature affected only intra-train rundown but, over longer periods of stimulation, both intra- and inter-train rundown were sig nificantly accelerated by cold temperature. Cumulative EPP amplitudes were calculated by successively adding the amplitudes of each EPP during the sti mulation period to provide an estimate of total neurotransmitter release in the neuromuscular junction. The cumulative EPP amplitude was significantly lower at 20 degreesC than it was at 37 degreesC during both 20 and 50 Hz s timulation. These data indicate that the mechanism involved in EPP amplitud e rundown and recovery is temperature-sensitive, with a greater decrement i n EPP amplitude at cold than at warm temperatures.