title: Contractile effects of 4-aminopyridine on isolated frog rectus abdominis muscles.
abstract: The contractile effects of 4-aminopyridine (4-AP) on isolated frog rectus abdominis muscles were examined, and compared with KCl-induced contractures. 4-AP (1-40 mM) caused slowly developing, concentration-dependent contractures which were not modified by (+)-tubocurarine (2.7-13.3 microM). The contractures were prolonged and very slowly relaxed (greater than 30 min) on washout. KCl-induced contractures developed more rapidly and relaxation was equally rapid, both occurring within 90 s of application and washout, respectively. KCl contractures were slightly but significantly (P less than 0.05) attenuated by (+)-tubocurarine in concentrations that blocked carbachol contractures. In calcium-free Ringer's solution, KCl (10-120 mM) responses were completely abolished, but 4-AP concentration-response curves were shifted to the right three- to four-fold. The results show that 4-AP causes contracture of the frog rectus abdominis. It is suggested that at the lower concentration employed (less than 10 mM), 4-AP increases extracellular calcium entry into the muscle, while larger concentrations produce contractures by a direct intracellular mechanism. 4-AP contractures were independent of postjunctional nicotinic cholinoceptor activation.
mesh_terms: D015761:4-Aminopyridine; D000631:Aminopyridines / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects*; D009467:Neuromuscular Depolarizing Agents / Q000494:pharmacology*; D011189:Potassium Chloride / Q000494:pharmacology; D011896:Rana temporaria; D014403:Tubocurarine / Q000494:pharmacology
chemical_list: D000631:Aminopyridines; D009467:Neuromuscular Depolarizing Agents; D011189:Potassium Chloride; D015761:4-Aminopyridine; D002118:Calcium; D014403:Tubocurarine