Naloxone-resistant effects of opioids on the twitch in frog skeletal muscle

Morphine and several other opioid agonists including the enkephalins caused a dual action on the twitch in the isolated, curarized, and electrically stimulated frog toe muscle: a potentiating action at low drug concentrations and a potentiation followed by an inhibitory action at higher concentrations. The twitch potentiation was found to be nonstereospecific and resistant to antagonism by naloxone. The inhibitory action too was naloxone-resistant and is probably due to a nonspecific local anesthetic effect of the opioids on the electric properties of the frog skeletal muscle fibre membrane.