Platelets are suggested to exacerbate
ischemia-induced myocardial injury, which has led to the study of various antiplatelet
therapies including
thromboxane synthetase inhibitors (TXSI). Two such agents, benzylimidazole and
OKY-046, reduce
infarct size commensurate with a diminution in serum
thromboxane B2 formation in anesthetized dogs subjected to 90 minutes of coronary artery occlusion followed by 5 hours of reperfusion. In contrast, platelet depletion with specific antiserum does not reduce
infarct size but prevents the cardioprotection afforded by the TXSI. Platelet-derived
prostaglandin endoperoxides (
PGG2 and
PGH2), which cannot be converted to
thromboxane A2 in the inhibited platelet, can be transformed to
PGE2 and
PGD2 in plasma and to PGI2 by the blood vessel wall. These
prostaglandins are considered "cardioprotective." Consequently, a low dose of
aspirin (3-5 mg/kg) given 24 hours before
coronary occlusion was used to selectively block the platelet
cyclooxygenase enzyme.
Aspirin, by itself, does not reduce
infarct size, but it suppresses the myocardial salvage induced by
OKY-046. Thus, TXSI reduce
infarct size by platelet-dependent,
aspirin-sensitive mechanism that depends on the redirection of platelet-derived
PGG2 and
PGH2 to protective metabolites, rather than inhibition of
thromboxane A2 per se. Moreover, myocardial salvage induced by the TXSI is accompanied by a reduction in neutrophil accumulation in the myocardium, as indicated by the levels of the neutrophil-specific
myeloperoxidase enzyme. Platelet depletion or pretreatment with
aspirin prevents the TXSI-induced suppression of neutrophil accumulation. Consequently, it is proposed that the
prostaglandin-mediated protective effects of TXSI can be resolved, at least in part, in terms of a braking action on neutrophil activation to prevent leukocyte-dependent tissue injury.