Thromboxane mimics the pulmonary but not systemic vascular responses to bolus HCl injections in broiler chickens

Bolus i.v. injections of 1.2 N HCl elicit a rapid but transient pulmonary vasoconstriction in broiler chickens. In mammals, the pulmonary vasoconstrictive response to bolus acid injection depends on increased synthesis of thromboxane A(2); however, the vascular responsiveness of domestic fowl to thromboxane previously had not been evaluated. In the present study, we tested the hypothesis that, if HCl triggers pulmonary vasoconstriction by stimulating thromboxane A(2) synthesis in broilers, then bolus i.v. injections of the potent thromboxane A(2) mimetic U44069 (9,11-dideoxy-9alpha,11alpha-epoxy-methanoprostaglandin F(2alpha); 1 micromol/mL; 0.5 mL injected volume) should trigger hemodynamic responses similar to those elicited by HCl (1.2 N; 1.5 mL injected volume). Both HCl and the thromboxane mimetic elicited twofold or greater increases in pulmonary vascular resistance, which in turn increased pulmonary arterial pressure by 50% despite concurrent reductions in cardiac output. The reductions in cardiac output were associated with reductions in stroke volume but not heart rate. The thromboxane mimetic also increased the total peripheral resistance, which minimized the reduction in mean systemic arterial pressure associated with the decrease in cardiac output. In contrast, HCl injections did not increase total peripheral resistance; consequently, the reduction in cardiac output caused the mean systemic arterial pressure to decrease by 30 mm Hg. Mannitol (2.5%; 1.5 mL) was injected i.v. as a volume control, and had no influence on any of the variables. This study provides the first direct evidence that thromboxane is a potent pulmonary vasoconstrictor in broilers, and provides support for the hypothesis that thromboxane mediates the pulmonary vasoconstrictive response to bolus i.v. injections of HCl. The differential response of the systemic vasculature to the thromboxane mimetic and HCl may indicate that cardiopulmonary responses to HCl injections are not mediated solely via thromboxane production. Alternatively, a direct dilatory effect of elevated hydrogen ion concentrations on the systemic vasculature may have counteracted any tendency for simultaneously evolved endogenous thromboxane to elicit systemic vasoconstriction.