CHRONIC EXPOSURE TO DIFFERENT AMBIENT TEMPERATURES AFFECTS b-AR PROFILE AND HEART RESPONSIVENESS TO ISOPROTERENOL AND CALCIUM

Abstract

The purpose of this investigation was to study the effect of thermal acclimation on the myocardium of rock pigeons. We determined the density and affinity of the b -adrenergic receptors in the left ventricle of cold-, heat-, and normothermic-acclimated pigeons by radioligand binding techniques. The mechanical performance of the left ventricle of the heart subjected to either isoproterenol or Ca²⁺ loading was measured using the Langendorff perfusion system. Heat- and cold-acclimated hearts demonstrated a significant downregulation of b - and b2 -adrenoreceptor density accompanied by an increase in their affinity when compared with normothermic-acclimated hearts. In agreement with these results, isoproterenol improved cardiac performance in cold- and heat-acclimated hearts. We also found that the heat-acclimated hearts were capable of functioning successfully when exposed to high Ca²⁺ loads, whereas cold-acclimated hearts were significantly less tolerant of Ca²⁺ loads and less efficient. Thermal acclimation induces cellular alterations in the pigeon heart, thus enabling better coping with hemodynamic and calcium loads, both resulting from thermal stress.