Cardioprotective actions of specific thromboxane receptor antagonist in acute myocardial ischemia.

Thromboxane A2 (TxA2) has been implicated as a potential mediator of myocardial damage during acute ischemia. A potent and specific TxA2 receptor antagonist, SQ-29,548 (2 mg/kg bolus + 2 mg/kg/h) was tested in a cat acute coronary ligation model of myocardial ischemia over a 5-h observation period. Those cats given the TxA2 receptor antagonist had a significant reduction in elevated S-T segment from 0.32 to 0.17 mV (p less than 0.01) in contrast to cats given only vehicle which showed a progressive increase in S-T segment elevation over the 5-h course of the experiment. Furthermore, the rise in plasma creatine kinase (CK) activity during myocardial ischemia was significantly attenuated after SQ-29,548 administration (p less than 0.05). This was confirmed by direct myocardial biopsies which demonstrated a reduction in the loss of myocardial CK and nitrogenous compounds from the ischemic region. Because heart rate (HR), mean arterial blood pressure (MABP), and the pressure rate index (PRI) were unaffected by SQ-29,548 administration, its mechanism of protection probably does not occur through reduction of myocardial oxygen demand. Furthermore, specificity of SQ-29,548 for thromboxane/endoperoxide receptors was demonstrated in the isolated cat coronary arteries. These data suggest that SQ-29,548 reduces the damage associated with myocardial ischemia through direct TxA2 receptor antagonism. The data are also consistent with an important role of TxA2 in the pathophysiology of myocardial ischemia.