Effects of temperature on chronotropic and inotropic responses of isolated canine atria to five sympathomimetic amines

The effects of temperature on both atrial pacemaker and contractile activities were investigated, using the isolated, blood-perfused canine atrial preparation with heparinised arterial blood conducted from the support dog. Lowering the temperature from 37° to 27°C consistently caused a linear decrease in sinus rate (107 to 51 beats·min−1) and progressive increase in the developed tension (100 to 175%). However the maximum rate of developed tension was hardly altered, since the duration of shortening and relaxation was longer than that in normothermia. The positive chronotropic and inotropic effects of dopamine, l-noradrenaline, l-adrenaline, l-isoprenaline and tyramine were compared at 37°, 32°, and 29°C. The order of potencies of chronotropic and inotropic effects of each amine was l-isoprenaline > l-noradrenaline ≳ l-adrenaline > dopamine > tyramine at each examined temperature. Percentage changes in positive inotropic effects at sub-maximal doses were slightly but not significantly depressed at a low temperature, but the positive chronotropic effects were markedly suppressed both in absolute and percentage values at a low temperature. The threshold doses for inducing positive inotropic effects of these amines were not very different when the temperature was lowered. However the threshold doses for inducing positive chronotropic effects were increased when the temperature was lowered. From these results, differential chronotropic and inotropic actions observed by sympathomimetic amines at different temperatures lead to the speculation that β1-type cardiac adrenergic receptors may not be homogenous.