title: Naloxone-resistant effects of opioids on the twitch in frog skeletal muscle.
abstract: 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.
mesh_terms: D000818:Animals; D004305:Dose-Response Relationship, Drug; D004351:Drug Resistance; D066298:In Vitro Techniques; D009020:Morphine / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects*; D009270:Naloxone / Q000494:pharmacology*; D009294:Narcotics / Q000494:pharmacology*; D011894:Rana pipiens; D013237:Stereoisomerism
chemical_list: D009294:Narcotics; D009270:Naloxone; D009020:Morphine