Reduction of quantal size and inhibition of neuromuscular transmission by bafilomycin A.

The energy for uphill transport of neurotransmitters into synaptic vesicles is created by bafilomycin A- and concanamycin A-sensitive vacuolar H(+)-ATPase (V-ATPase). Both blockers (at 0.1-5 microM) depressed twitch tension and induced tetanic fade of mouse diaphragm on stimulation of the phrenic nerve. Axonal impulse conduction and depolarization of motor endplate by exogenous acetylcholine were not inhibited. The IC(50)s for bafilomycin A and concanamycin A were 1.1+/-0.2 and 0.7+/-0.1 microM, respectively. Contractile response evoked by stimulation of diaphragm, muscle resting membrane potential and membrane resistance were not altered. V-ATPase blockers decreased quantal size and shifted the distribution of miniature endplate potentials (mepps) to low amplitude direction. The increase of mepp events in high KCl medium was suppressed slightly. The blockers depressed endplate potentials (epps) with IC(50)s of 0.7+/-0.2 microM (bafilomycin A) and 0.4+/-0.1 microM (concanamycin A). On high frequency stimulation, the coefficient of variance and run-down of epps were increased. The inhibitory effects on mepps and epps were irreversible and augmented by nerve stimulation. The results suggest that inhibition of V-ATPase reduces the acetylcholine content of synaptic vesicles, leading to suppression of neuromuscular transmission.