title: Effect of ionophore X-537A on desensitization rate and tension development in potassium-depolarized muscle fibres.
abstract: The effects of the ionophore X-537A were studied on carbamylcholine (carbachol)-induced densitization and on tension development in relaxed potassium-depolarized frog sartorius muscles. 2 X-537A accelerated carbachol-induced desensitization in Ca2+-deficient solutions without having any effect on the conductance of the membrane in the absence of carbachol or on the extent of the carbachol-induced increase in conductance. 3 In Ca2+-deficient solution, the acceleration of desensitization by the ionophore was concentration-dependent. No effect was observed with concentrations less than 5 muM and maximal acceleration was evident with 10 muM. 4 The influence of X-537A on desensitization was time-dependent. At 20 muM X-537A, there was a marked acceleration of desensitization by the end of 5 min exposure. An additional gradual acceleration occurred during a 5 to 30 min treatment. No acceleration of desensitization was evident when X-537A was simultaneously applied with carbachol to the end-plate region without prior exposure to the ionophore. 5 Desensitization also was accelerated by 30 min exposure to 20 muM X-537A in solutions containing Ca2+ or deficient in both Mg2+ and Ca2+; the rate being increased 2.8-fold in Ca2+-containing solutions, 2.9-fold in Ca2+-deficient solutions containing Mg2+, and 2.5-fold in divalent cation-deficient solutions. 6 Tension development gradually occurred in relaxed potassium-depolarized muscle preparations exposed to 20 muM X-537A. The onset of tension development occurred only after approximately 25 min of exposure both in preparations kept in Ca2+-deficient or Ca2+-containing solutions. By the end of 90 min in the ionophore, the tension developed was approximately 12% and 23% of the initial potassium contracture in those preparations maintained in the Ca2+-deficient or Ca2+-containing solutions, respectively. 7 We assume that the increase in desensitization rate following exposure to X-537A results from an elevation of the intracellular Ca2+ concentration. That muscle tension gradually increased during exposure to the ionophore supports this conclusion. The acceleration of densitization by X-537A in the absence of external Ca2+ supports the view that the site of calcium acceleration is not on the external surface of the end-plate membrane either at or near the agonist-recognition site but rather on the inner surface.
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology; D002217:Carbachol / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D004553:Electric Conductivity; D066298:In Vitro Techniques; D007832:Lasalocid / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects; D009045:Motor Endplate / Q000187:drug effects*; D009119:Muscle Contraction / Q000187:drug effects; D009469:Neuromuscular Junction / Q000187:drug effects*; D011188:Potassium / Q000494:pharmacology; D011894:Rana pipiens; D013997:Time Factors
chemical_list: D000900:Anti-Bacterial Agents; D002217:Carbachol; D011188:Potassium; D002118:Calcium; D007832:Lasalocid