CARDIOVASCULAR AND PULMONARY EFFECTS OF THROMBOXANE-B2 IN THE DOG

The hemodynamic properties of thromboxane B2 (TxB2), a product of prostaglandin endoperoxide metabolism, have not been thoroughly described. TxB2 is a bronchoconstrictor, but its effects on the systemic circulation and circulating platelets are unknown. Its precursor, thromboxane A2 (TxA2), is a potent vasoconstrictor as well as a platelet-aggregating agent. Using intact anesthetized dogs, we investigated the effects of TxB2 on pulmonary artery pressure (PAP), airway pressure (AP), systemic arterial pressure (SAP), and myocardial contractility (MC). Vascular responses were evaluated in relation to changes in platelet population and aggregability. Intravenous TxB2 (25 and 50 μg/kg) increased AP (mean 62% and 69%) and PAP (50% and 86%), respectively, whereas SAP and MC responses were inconsistent. Left ventricular injections (25 μg/kg) also increased AP (36%) and PAP (36%). Intraventricular administration of TxB2 produced a consistent elevation of SAP (10%) with a concomitant fall in MC (11%). These vascular responses were not consistent with alterations in platelet number or aggregability. A tachyphylactic response to TxB2 developed in AP and PAP at both dose levels and with both routes of administration. Intravenous and intraventricular TXB2 (25 μg/kg) produced a parallel decreasing response in PAP, suggesting the possible saturation of TxB2 binding sites or the depletion of a catabolic enzyme in the lung.