Vasoconstriction of canine coronary collateral vessels with vasopressin limits blood flow to collateral-dependent myocardium during exercise.

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Abstract
This study was performed to test the hypothesis that active constriction of coronary collateral vessels can worsen hypoperfusion of collateral-dependent myocardium during exercise. Studies were performed in seven adult mongrel dogs in which intermittent followed by permanent occlusion of the left circumflex coronary artery produced an area of collateral-dependent myocardium without gross evidence of infarct. Myocardial blood flow was determined with microspheres while measurement of aortic and distal coronary pressures allowed calculation of collateral and small vessel resistance at rest and during treadmill exercise. The ability of collateral vessel constriction to limit blood flow was assessed by infusion of vasopressin during exercise. During control conditions, blood flow in the collateral zone underwent a subnormal increase during exercise in comparison with the normal zone (1.74 +/- 0.27 versus 2.50 +/- 0.40 ml/min/g, respectively, p less than 0.05). Infusion of vasopressin in a dose that caused no change in normal zone flow (0.01 microgram/kg/min i.v.) produced a 30 +/- 5% further decrease in flow to the collateral zone (p less than 0.01). This decrease in collateral zone flow resulted from a 48 +/- 14% increase in transcollateral resistance in response to vasopressin infusion (p less than 0.01), as well as a 40 +/- 9% increase in small vessel resistance in the collateral zone (p less than 0.01). These data demonstrate that active constriction of both collateral vessels and coronary resistance vessels can contribute to hypoperfusion of collateral-dependent myocardium during exercise.