INTERACTION OF HEART-RATE AND HYPOTHERMIA ON GLOBAL MYOCARDIAL-CONTRACTION OF THE ISOLATED RABBIT HEART

We studied the effects of mild hypothermia on cardiac contractility in isolated rabbit hearts perfused with Krebs-Henseleit solution accordi ng to the technique of Langendorff. Isovolumetric left ventricular pre ssure (LVP) was measured with a fluid-filled balloon. Hearts were pace d after induction of atrioventricular block. At low heart rates (< 30 bpm) mild hypothermia (cooling to 30 degrees C) induced a 32% increase in LVP (146.5 +/- 10 mm Hg at 30 degrees C vs 110.7 +/- 13 mm Hg at 3 7 degrees C) but this positive inotropic response was progressively lo st by increasing heart rate. At pacing rates greater than or equal to 90 bpm, lower systolic LVP, higher diastolic LVP, and lower positive a nd negative LV dP/dt were obtained in hypothermic (93 +/- 12 mm Hg, 55 +/- 18 mm Hg, 584 +/- 137 mm Hg/s, and 323 +/- 57 mm Hg/s at 210 bpm, respectively) compared to normothermic hearts (123 +/- 4 mm Hg, 10 +/ - 4 mm Hg, 1705 +/- 145.5 mm Hg/s, and 1155 +/- 78 mm Hg/s at 210 bpm, respectively). The duration of mechanical diastole was reduced or sup pressed in these hearts. Exposure to the beta-adrenoceptor agonist, is oproterenol, improved this diastolic dysfunction during hypothermia an d pacing at high rates, suggesting that the sarcoplasmic reticulum Ca2 + uptake might be involved. Our data are also consistent with an incre ase in myofilament Ca2+ sensitivity that is opposed by isoproterenol d uring hypothermia.