Extracellular calcium and the inotropic effect of epinephrine on isolated skeletal muscle

Abstract

The purpose of these experiments was twofold: 1) to determine the concentration-effect relationship between epinephrine and peak isometric twitch tension in isolated skeletal muscle and 2) to determine if extracellular calcium influx plays an important role in mediating the inotropic effect of epinephrine in isolated muscle. In the first experiment, single sartorius muscle of small male grassfrogs (Rana Pipiens) were incubated serially in increasing concentrations of epinephrine (10 nM to 1 mM). During each incubation, single twitches were elicited every minute and analyzed for peak twitch tension (PT), and the peak rates of tension increase and decrease (+dP/dt and -dP/dt, respectively). Exposure to the various concentrations of epinephrine resulted in sigmoidal relationships between epinephrine concentration and PT, +dP/dt and -dP/dt. At the highest two epinephrine concentrations (10 uM and 1 mM) PT, +dP/dt and -dP/dt were increased by 40, 20 and 35%, respectively. The second experiment was designed to determine if extracellular calcium influx is an important mediator of the inotropic effect of epinephrine on isolated skeletal muscle. This was accomplished by exposing muscles to conditions known to inhibit calcium influx while simultaneously recording the muscle's twitch response to epinephrine. It was found that removal of extracellular calcium or replacement of extracellular calcium with magnesium inhibited epinephrine's effect on contractility. Re-introduction of calcium to media containing epinephrine caused progressive increases in PT, +dP/dt and -dP/dt. Likewise, the calcium channel blockers D-600 and diltiazem prevented the epinephrine-induced changes in muscle contractility. When these compounds were removed from the media containing epinephrine, PT, +dP/dt and -dP/dt were increased. It was also found that quiescent incubation in epinephrine did not cause an immediate increase in the muscle twitch once stimulation was initiated. From these experiments, the following conclusions seem warranted: 1. Epinephrine exerts a positive inotropic effect on isolated amphibian muscle, the magnitude of which is dependent on the concentration of the hormone. 2. Extracellular calcium appears to mediate this inotropic effect...