Leukotriene end organ antagonists increase pulmonary blood flow in fetal lambs.

The factors responsible for maintaining the normally low pulmonary blood flow and high pulmonary vascular resistance in the fetus are not well understood. Since leukotrienes are potent pulmonary vasoconstrictors in many adult animal species, we determined whether leukotrienes were perhaps involved in the control of the fetal pulmonary circulation by studying the effects of putative leukotriene end organ antagonists in two groups of fetal lambs. In six fetal lambs studied at 130-134 days gestation, FPL 55712 increased pulmonary blood flow by 61% (P less than 0.05) and reduced pulmonary vascular resistance by 45% (P less than 0.05). There was a small increase in heart rate but no changes in pulmonary and systemic arterial pressures and systemic arterial blood gases. In six other fetal lambs studied at 130-140 days gestation, FPL 57231 increased pulmonary blood flow by 580% (P less than 0.05) and decreased pulmonary vascular resistance by 87% (P less than 0.05). Pulmonary and systemic arterial pressures decreased (P less than 0.05), and heart rate increased (P less than 0.05). Leukotriene end organ antagonism significantly increases fetal pulmonary blood flow and decreases pulmonary vascular resistance. Leukotrienes may play a role in the physiological control of the fetal pulmonary circulation.