Effects of pethidine and nalorphine on the mechanical and electrical activities of mammalian isolated ventricular muscle.

1. The strength of the isometric mechanical contraction of electricallydriven ventricular muscle has been recorded simultaneously with the resting and action potentials; the effects of pethidine and of nalorphine on these parameters have been studied.2. When lower concentrations of pethidine (0.22-6.5 mug/ml) were perfused, isometric peak tension was decreased in parallel with the maximum upstroke velocity of the action potential; these actions are considered to result from membrane stabilization. At higher concentrations (11.8-109 mug/ml) pethidine usually produced, in addition, a progressive decrease in the resting and action potentials associated with marked irregularities in, or even abolition of, the mechanical response. It is suggested that these effects of the higher doses might be due to a depression of ATPase activity in the myocardial membrane.3. Compared with pethidine, nalorphine had similar, but weaker, actions.