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 α₁- and α₂-receptor-mediated stimulation with precapillary α₁- and α₂-receptors and postcapillary α₂-receptors interacting with precapillary and postcapillary β₂-receptors. In addition, epinephrine appears to have a greater effect on β₂-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 β₂-receptor system and an attenuation of the α-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 tone is elevated. As a result, these altered pulmonary vascular responses may affect pulmonary capillary pressure, a major determinant of lung fluid balance.