Although platelets have been associated with
angina pectoris,
myocardial infarction, and
sudden death, the platelet's capacity for induction and propagation of cardiac
ischemia remains incompletely defined. We therefore evaluated the effects of platelet activation occurring within the coronary circulation and tested the hypothesis that inhibition of platelet function would prevent platelet-induced cardiac
ischemia. Human platelets were isolated from blood obtained from normal donors by
Sepharose 2B column chromatography, resuspended in
Hepes buffer, and added to the perfusate of a Langendorff rabbit heart (platelet counts greater than 10,000/microliters). Without, and with low dose (10 microM)
prostaglandin E1 (
PGE1), a reversible inhibitor of platelet function, immediate and irreversible global cardiac
ischemia, as monitored by
NADH fluorescent photography, ensued (N = 4) following platelet activation with
thrombin (0.1 to 1 U/ml). Higher concentrations of
PGE1 (0.1 to 1 mM, N = 2) or
aspirin ingestion (1000 mg taken approximately 12, 4, and 1 hr prior to experiment, N = 2) completely prevented this platelet-induced
myocardial ischemia.
Aspirin, unlike
PGE1, was effective despite its inability to block
thrombin-induced platelet aggregation in our in vitro gel-filtered system. We conclude that activation of platelets within the coronary circulation is sufficient for induction of irreversible cardiac
ischemia. The efficacy of
aspirin, a
cyclooxygenase inhibitor, further suggests that the products of arachidonate metabolism (e.g.,
thromboxanes) have a fundamental role in the genesis of platelet-mediated
myocardial ischemia.