On the range of alpha-adrenergic regulation of coronary vascular resistance

In order to obtain an estimate of the range of alpha-adrenergic resistance regulation in the coronary vascular system, the following studies were performed: in 15 anesthetized dogs the circumflex coronary artery was cannulated and perfused with above-normal constant pressure to that coronary venous oxygen tension never fell below 40 mmHg. Thus, it was possible to eliminate the influence of the metabolic factor regulating coronary resistance. Furthermore, 15 isolated isovolumetrically working guinea-pig hearts were perfused according to the Langendorff method. Stimulation, resp. blockade, of alpha-receptors was achieved by administering xylometazoline, resp. phentolamine. Xylometazoline increased coronary resistance in both the isolated and the in-situ heart. Administering maximum doses to the anesthetized dog led to an increase in resistance to about 200%. This is equivalent to a reduction of conductance to about 50%. Phentolamine produced no significant effects in the isolated heart. Maximum dosage administered to the heart in situ led to a resistance decrease to about 60%, equivalent to an elevation of conductance to about 170%. If we let control values of coronary resistance and conductance be equal to 100%, our experiments showed alpha-adrenergic regulation of coronary resistance to range from about 60% to 200% and conductance to range from about 50% to 170%.