Electromechanical coupling in a molluscan muscle, the radula protractor of <i>Busycon canaliculatum</i>

Abstract

Abstract The radula protractor can be made to be spontaneously rhythmic, although it is normally a quiescent, phasic muscle, with apparently multi‐unit organization. Acetylcholine and potassium depolarize with contracture. Once a radula muscle has been put into a long‐lasting contracture by acetylcholine, tryptamine can repolarize it with accompanying relaxation. In the continued presence of acetylcholine and tryptamine, the repolarization and relaxation gradually taper off while membrane oscillation accompanied by mechanical rhythmicity develops. Contrariwise, if a radula protractor is in a potassium contracture, tryptamine will potentiate the contracture without affecting the level of membrane depolarization. Potentiation of potassium contracture, without changing potential level, can also be effected by increased calcium concentrations or by caffeine. After a muscle has been soaked in artificial sea water lacking calcium, its twitch tension, tetanic tension, potassium contracture, and acetylcholine contracture become directly dependent on external calcium concentration.