EFFECT OF CATECHOLAMINES ON PULMONARY CIRCULATION AT ELEVATED VASCULAR TONE

The effect of catecholamine stimulation on the longitudinal resistance and compliance distribution in the canine pulmonary vasculature was evaluated under control vascular tone and after vascular tone was elevated using the thromboxane analogue U-46619. The arterial-, venous-, and double-occlusion techniques were used to measure the segmental resistances and compliances in isolated dog lung blood perfused at constant flow. The results of this study indicate that at control vascular tone the catecholamines norepinephrine and epinephrine increase pulmonary vascular resistance and decrease pulmonary vascular compliance through alpha(1)- and alpha(2)-receptor-mediated stimulation with precapillary alpha(1)- and alpha(2)-receptors and postcapillary alpha(2)-receptors interacting with precapillary and postcapillary beta(2)-receptors. In addition, epinephrine appears to have a greater effect on beta(2)-receptors than norepinephrine. When vascular tone was elevated, the effect of norepinephrine and epinephrine on pulmonary vascular resistance was not present, which may be due to the appearance of a more pronounced vasodilatory beta(2)-receptor system and an attenuation of the alpha-mediated vasoconstrictor responses. In addition, neither catecholamine had any significant effect on pulmonary vascular compliance when vascular tone was raised. These data suggest that the adrenergic-receptor systems modulating pulmonary vascular resistance and compliance in the canine pulmonary circulation are altered when vascular t;one is elevated. As a result, these altered pulmonary vascular responses may affect pulmonary capillary pressure, a major determinant of lung fluid balance.