Some effects of pentazocine and cyclazocine on the isolated guinea-pig atria.

Experiments carried out on spontaneously beating and electrically driven guinea-pig atria, have shown that pentazocine and cyclazocine when used in equal concentrations induce a reduction in rate and contraction amplitude. An initial positive inotropic response always precedes with a negative deflection in electrically driven atria. Atropine and nalorphine do not antagonize the drug-induced reduction in rate and contractility. Practolol does not influence either the initial positive or the negative inotropic effects of pentazocine. An apparent tachyphylaxis to the pentazocine-induced positive inotropic response is observed even after the first exposure when the same concentration of pentazocine is tested on the same preparation. Increasing the Ca++ concentration in the medium antagonizes the negative inotropic and chronotropic effects of adenosine and acetylcholine. It is concluded that the negative inotropic effects of the tested drugs may be due to an interference with the Ca++ balance within the atrial cells and/or a direct nonspecific effect on the cell membrane.