DIAZEPAM POTENTIATES THE POSITIVE INOTROPIC EFFECT OF ISOPRENALINE IN RAT VENTRICLE STRIPS - ROLE OF CYCLIC-AMP

The responses of the electrically driven right ventricle strip of the rat heart to isoprenaline and other cyclic AMP-related inotropic agents were recorded in the absence and in the presence of diazepam. Isoprenaline, in concentrations ranging from 10 nM to 1 mu M, significantly increased, in a concentration-dependent manner, the contractile force in this preparation. Diazepam (10 mu M) produced a leftward shift in the isoprenaline concentration-response curve and significantly reduced its EC(50). Higher concentrations of diazepam (100 mu M) produced no further shift, but reduced the maximum of the concentration-response curve of isoprenaline. Forskolin (0.5-10 mu M), which directly stimulates adenyl cyclase, also produced a concentration-dependent increase in cardiac contractility. Diazepam (10 mu M) displaced to the left the concentration-response curve for forskolin and reduced its EC(50). The cyclic AMP analogous dibutyryl cyclic AMP (0.1-1 mM) produced concentration-dependent positive inotropic effects which were not significantly modified in the presence of diazepam (10 mu M). Diazepam (10 mu M) significantly enhanced the cyclic AMP production induced by isoprenaline (0.1 mu M) and forskolin (10 mu M) by about 136% and 35% respectively. These results indicate that diazepam potentiates the positive inotropic effect induced by beta-adrenoceptor agonists, probably by increasing cyclic AMP production induced by these agents.