Cardiac and hemodynamic effects of the centrally acting analgesics tramadol and pentazocine in anaesthetized rabbits and isolated guinea-pig atria and papillary muscles.

Cardio-hemodynamic and direct cardiac effects of 1-(m-methoxyphenyl)-2-(dimethylaminomethyl)- cyclohexan -1-ol (tramadol, Tramal ), and pentazocine were investigated in anaesthetized rabbits and in isolated guinea-pig atria and papillary muscles. Tramadol in anaesthetized rabbits at 1 and 4.64 mg/kg i.v. slightly increased arterial blood pressure and heart rate and at 10 mg/kg i.v. transiently decreased blood pressure, LV dp/dtmax and cardiac output and increased LV enddiastolic pressure. Pentazocine at 1 mg/kg i.v. increased blood pressure and peripheral vascular resistance, while with 2.15 and 4.64 mg/kg exerted dose-dependent cardiodepressive effects with initial fall of blood pressure, heart rate, LV dp/dtmax and cardiac output and increase of LV enddiastolic pressure. In spontaneously beating atria and electrically driven papillary muscles tramadol in the concentration range from 1 X 10(-6)-1 X 10(-4) mol/l showed concentration-dependent positive inotropic effects which where abolished and reversed to negative inotropic actions in catecholamine-depleted papillary muscles. In contrast, pentazocine in the same concentration range showed pure negative inotropic and negative chronotropic effects which were intensified in catecholamine-depleted papillary muscles. These in vitro results suggest that both tramadol and pentazocine interact with endogenous catecholamines in a way counteracting direct negative inotropic effects. While in the case of tramadol the sum of both properties leads to positive inotropic effects at least in vitro, with pentazocine negative inotropic action prevails.