Comparative effects of thromboxane B2 on the canine and feline pulmonary vascular bed

The pulmonary vascular effects of thromboxane (TX) B2 were investigated in the intact chest dog and cat. In both species, TXB2 increased lobar arterial pressure in a dose-related fashion without affecting left atrial pressure. Since blood flow to the lobe was maintained constant, the increases in lobar arterial pressure reflect increases in pulmonary lobar vascular resistance. In the dog, the increases in lobar arterial pressure were associated with increases in small vein pressure suggesting that TXB2 increases lobar vascular resistance by constricting pulmonary veins and upstream segments. In both the cat and dog, TXB2 and prostaglandin E2 had comparable pressor activity, whereas both substances were less potent than prostaglandin F2 chi. Responses to TXB2 were similar in experiments in which blood flow was controlled or when pulmonary flow varied naturally. Pulmonary vasoconstrictor responses to TXB2 were similar when the lung was ventilated, during bronchial occlusion in the dog and during an apneic interval in the cat. The increases in vascular resistance in both species were not altered by doses of indomethacin that blocked responses to arachidonic acid. The present studies demonstrate that TXB2 has significant vasoconstrictor activity in the feline and canine pulmonary vascular bed and suggest that responses to TXB2 are not dependent on alterations in bronchomotor tone or enhanced prostaglandin synthesis in the lung.