Placental vascular responses to leukotriene receptor antagonist FPL 55712

We have previously shown that FPL 55712, a specific leukotriene receptor antagonist, potentiates estrogen induced uterine hyperemia in nonpregnant rabbits. We therefore chose to investigate the vascular responses of pregnant rabbits to leukotriene blockade. Nine unanesthetized animals carrying 46 viable fetuses were used in this study. Regional blood flows were measured by the radioactive microsphere technique. In 5 rabbits control blood flows were measured after vehicle administration and FPL 55712, 1 mg/kg bolus, followed by infusion of 100 micrograms/kg/min was given via the jugular vein. Regional blood flows were measured again after 10 minutes of infusion. The procedural order was reversed in the remaining 4 animals. Resistance was calculated as mean arterial pressure divided by total flow to an organ. FPL 55712 decreased the blood pressure from 83 +/- 2 to 76 +/- 3 mmHg (P less than .001). Uterine resistance was not significantly changed (387 +/- 44 to 362 +/- 42 mmHg X ml-1 X min X gm), but renal resistance fell from 18.5 +/- 1.1 to 15.1 +/- 1.2 mmHg X ml-1 X min X gm (P less than .01). FPL 55712 induced maternal placental vasodilatation with resistance decreasing from 291 +/- 33 to 261 +/- 31 mmHg X ml-1 X min X gm (P less than .04). Vehicle administration did not cause dilation in any vascular bed. FPL 55712 appears to be a placental vasodilator whose action is most likely due to receptor blockade of the vasoconstrictive endogenous leukotrienes.