Changes in cardiovascular beta-adrenoceptor responses during hypothermia

The purpose of this study was to determine cardiovascular β-adrenergic responses during hypothermia. In the present study, we used isoproterenol (Iso), a nonselective, potent β-adrenoceptor agonist, well known for its positive chronotropic and inotropic pharmacologic actions at normothermia. Rats were instrumented to measure mean arterial pressure (MAP) and left ventricular (LV) pressure–volume changes using a Millar pressure–volume conductance catheter. Core temperature was manipulated from 37 (normothermia) to 24 °C (hypothermia) and back to 37 °C (rewarming) using both internal and external heat exchangers. During cooling at each temperature (33, 30, 27, and 24 °C), central hemodynamic variables and MAP were measured while intravenously infusing Iso (doses of 1.7, 5, 10, and 20 ng/min). Seven animals underwent all phases of the protocol. At normothermia Iso infusion resulted in a significant, dose-dependent increase in heart rate (HR), stroke volume (SV), cardiac output (CO), LV dP/dt_max (left ventricular maximum derivative of systolic pressure over time) but no change in MAP. During cooling Iso infusion caused no dose-dependent change in any of the hemodynamic variables. After rewarming, baseline HR and LV dP/dt_max were increased, whereas SV was significantly reduced when compared with their pre-hypothermic baseline values. This study shows that physiological cardiovascular responses mediated by the β-adrenoceptor are significantly diminished during core hypothermia.