Dual effect of calmidazolium (R 24571) on transmitter release at the frog neuromuscular junction

The effect of the non-phenothiazine calmodulin-inhibitor R 24571 on transmitter release at the frog neuromuscular junction has been investigated. MEPP frequency was reduced by 25-40% at low concentrations of the drug (2-5 X 10(-7) M) but increased at higher concentrations. These effects were independent of [Ca2+]o. EPP quantal content was increased at low levels of release (quantal content 0.5-1.5) and was unaffected at higher levels (quantal content 30-40). It is concluded that R 24571 has both inhibitory and stimulatory effects, but that evoked release of transmitter is insensitive to the inhibitory action of the drug.