Deep Hypothermia Inverts the Inotropic Effect of Isoproterenol in the Rat Myocardium

The effects of the β-adrenergic receptor agonist isoproterenol on the contractile activity of the papillary muscle of the rat heart right ventricle were studied both at a temperature close to physiological (30°C) and under conditions of deep hypothermia (10°C). Isoproterenol had a pronounced positive inotropic effect at 30°C, the contraction force increased from 1.2 ± 0.1 mN in the control to 2.4 ± 0.4 mN after the addition of the agonist and there was a significant acceleration of the time parameters of contraction, that is, the time to reach the maximum contraction decreased from 101 ± 6 ms to 85 ± 4 ms; the relaxation time decreased by 50% from 55 ± 3 ms to 36 ± 1 ms. Under hypothermic conditions, isoproterenol caused a powerful negative inotropic effect, reducing the contraction force from 2.2 ± 0.4 mN to 1.2 ± 0.4 mN. The tendency to accelerate contraction persisted as at 30°C: the time to reach the maximum contraction decreased from 717 ± 52 ms to 624 ± 50 ms, and the relaxation time decreased by 50% from 667 ± 86 ms to 450 ± 40 ms. Thus, under conditions of deep hypothermia at 10°C, the direction of the inotropic response to isoproterenol changed from positive to negative, while the lusitropic effect remained negative.