The inversion of the inotropic effect of isoproterenol in the rat myocardium during deep hypothermia

Abstract

The present study examined the effects of the в-adrenergic receptor agonist, isoproterenol, on contractile activity of right ventricle papillary muscles in the rat heart at near-physiological temperature (30°C) and under conditions of deep hypothermia (10°C). Isoproterenol exerts a pronounced positive inotropic effect at 30°C. After agonist addition, the force of contraction increases from 1.2 ± 0.1 mN in control to 2.4 ± 0.4 mN that leads to a reliable acceleration of time parameters of contraction. So, time-to-maximum contraction decreased from 101 ± 6 ms to 85 ± 4 ms; time-to-50% relaxation declined from 55 ± 3 ms to 36 ± 1 ms. Under hypothermic conditions, isoproterenol produced a powerful negative inotropic effect, reducing the force of contraction from 2.2 ± 0.4 mN to 1.2 ± 0.4 mN. Similarly as at 30°C, there was a tendency for increase in contraction speed, so time-to-maximum contraction decreased from 717 ± 52 ms to 624 ± 50 ms, and time-to-50% relaxation was shortened from 667 ± 86 ms to 450 ± 40 ms. Thus, under conditions of deep hypothermia at 10°C, the isoproterenol-induced inotropy changes from positive to negative, while negative lusitropic effect remains clear.