Influence of thromboxane inhibition on the severity of myocardial ischemia in cats

Abstract

The effects of thromboxane (Tx) inhibition or arachidonic acid (AA) infusion were studied in anesthetized cats during acute myocardial ischemia (MI). AA (7.2 mg kg⁻¹ h⁻¹) or imidazole (25 mg kg⁻¹ h⁻¹) infusions were initiated 30 min after occlusion of the left anterior descending coronary artery. Assessment of the degree of protection of the ischemic myocardium was made by measurement of S-T segment elevation, plasma and myocardial creatine phosphokinase (CPK) activities, and myocardial amino-nitrogen content. Assessment of Tx inhibition was performed by radioimmunoassay. Administration of imidazole inhibited the sevenfold increase in plasma thromboxane B₂ (TxB₂) levels occurring in MI (p < 0.001 at 2–5 h), markedly decreased S-T segment elevations at 2–5 h (p < 0.025), significantly prevented the elevation in plasma CPK (p < 0.05, at 4 and 5 h), and significantly reduced the loss in myocardial CPK activity and myocardial amino-nitrogen content. AA infusion also reduced the increase in TxB₂ post-MI, significantly decreased (p < 0.025) S-T segment elevations at 2 5 h, caused a decrease in plasma CPK levels (p < 0.05 at 5 h), but did not prevent loss of myocardial CPK or amino-nitrogen. In summary, the administration of imidazole resulted in significant protection of the myocardium in all indices of ischemic damage measured, while AA infusion resulted in only a partial protection. The mechanism of the imidazole protection of ischemic myocardial tissue appears to be via inhibition of Tx synthesis although we cannot exclude a hemodynamic or cytoprotective mechanism. These results suggest that specific inhibition of Tx formation is beneficial during acute MI.