THE EFFECT OF AGE ON THE SAFETY FACTOR IN NEUROMUSCULAR TRANSMISSION IN THE ISOLATED DIAPHRAGM OF THE RAT

An analysis of neuromuscular transmission has been made in phrenicnerve/diaphragm preparations from male rats aged 30 days or110 days. The amplitude of miniature end-plate potentials wasfound to decrease with age, being 0.969±SEM 0.058 mVat 30 days and 0.510±SEM 0.031 mV at 110 days. Over thesame period, the quantum content of the first end-plate potentialof a train of 40 at 10 Hz, increased from 144.5, SEM+11.1, –10.4to 346, SEM+41.4, –37.0. A corresponding change was observedalso in the average quantum contents of the last 30 end-platepotentials of each train; from 50.6, SEM+ 3.5, –3.2, to138.9, SEM+15.0, –13.6. The safety factor for neuromusculartransmission, calculated from these measured parameters, wasfound at 30 days to be only 70–80% of that at 110 days.It was estimated that the lower safety factor found in youngrats was approximately equivalent to the neuromuscular blockingaction of a dose of, at least, 0.0225 mg/kg of d-tubocurarine.Extrapolation of these results to man would support previousreports of increased sensitivity to d-tubocurarine inneonates.