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

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Abstract
An analysis of neuromuscular transmission has been made in phrenic nerve/diaphragm preparations from male rats aged 30 days or 110 days. The amplitude of miniature end-plate potentials was found to decrease with age, being 0.969±SEM 0.058 mV at 30 days and 0.510±SEM 0.031 mV at 110 days. Over the same period, the quantum content of the first end-plate potential of a train of 40 at 10 Hz, increased from 144.5, SEM+11.1, −10.4 to 346, SEM+41.4, −37.0. A corresponding change was observed also in the average quantum contents of the last 30 end-plate potentials of each train; from 50.6, SEM+ 3.5, −3.2, to 138.9, SEM+15.0, −13.6. The safety factor for neuromuscular transmission, calculated from these measured parameters, was found at 30 days to be only 70–80% of that at 110 days. It was estimated that the lower safety factor found in young rats was approximately equivalent to the neuromuscular blocking action of a dose of, at least, 0.0225 mg/kg of d-tubocurarine. Extrapolation of these results to man would support previous reports of increased sensitivity to d-tubocurarine inneonates.

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