Background-The platelet products thromboxane A 2 and serotonin have been shown to cause constriction of welldeveloped coronary collateral vessels. This study was performed to determine whether intravascular platelet activation produced with platelet activating factor (PAF) can cause a decrease in coronary collateral blood flow. Methods and Results-Collateral vessel growth was induced by embolization of a hollow stainless steel plug into the left anterior descending coronary artery (LAD) of

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... dogs. The animals were returned to the laboratory 3 to 6 weeks later for surgical instrumentation and measurement of collateral blood flow. Collateral flow was assessed by measuring retrograde blood flow from the cannulated collateral-dependent artery. PAF (10 nmol) was injected into the left main coronary artery to allow products of platelet activation to reach collateral vessels arising from the left coronary system. PAF caused a vasoconstrictor response, which became maximal 3 minutes after injection and resulted in a 40.3Ϯ7.4% decrease in retrograde blood flow (32.1Ϯ2.1 to 19.6Ϯ3.2 mL/min; PϽ0.05). By 15 minutes after the PAF injection, both retrograde blood flow and transcollateral resistance had returned to normal. After pretreatment with the thromboxane A 2 receptor antagonist SQ30,741, the vasoconstrictor response to PAF was abolished and, in contrast to the decrease in retrograde blood flow from PAF alone, a weak vasodilator effect was unmasked. Conclusions-PAF caused a decrease in coronary collateral blood flow. This vasoconstrictor response required the participation of thromboxane A 2 . (Circulation. 1998;98:1431-1437.)