EFFECTS OF CATECHOLAMINES ON EXCITATIONCONTRACTION COUPLING IN FROG SINGLE TWITCH FIBER

Abstract

The effects of catecholamines (adrenaline and isoproterenol), dibutyryl cyclic AMP and propranolol on mechanical and electrical responses in curarized frog twitch fiber were investigated. Twitch tension and potassium contracture were potentiated by catecholamines or by dibutyryl cyclic AMP, without exerting any effect on the magnitude of the resting potential, the amplitude and duration of the action potential and the negative afterpotential. The effect on twitch tension was more pronounced with isoproterenol than with adrenaline. The action of isoproterenol was antagonized by propranolol in optimal concentrations. Caffeine contracture was not affected by catecholamines or by dibutyryl cyclic AMP. Dibutyryl cyclic AMP itself did not induce contracture, even in phospholipase A-treated fibers.On the other hand, twitch tension was abolished and potassium contracture was inhibited by propranolol in relatively higher concentrations (50μm), whereas the caffeine contracture and the resting and action potentials were not affected. The twitch tension once abolished in a solution with 50μm propranolol was completely recovered after immersion of the fibers in a solution with propranolol and either isoproterenol or dibutyryl cyclic AMP in high concentrations. Adenylate cyclase could not be detected in frog sartorius muscle, whereas the presence of the enzyme was found electron microscopically in guinea pig psoas muscle. On the basis of these results, the actions of catecholamines and their antagonist on excitation-contraction coupling in frog twitch fiber and the mechanism of their actions were discussed.