Background: The effect of alpha-adrenergic receptor activation on regional contractile function and transmural myocardial blood flow is controversial. Accordingly, the effects of selective alpha 1-(methoxamine) and alpha 2-(BHT 933) receptor stimulation on regional contractile function and transmural myocardial blood flow distribution were studied in 15 anesthetized open-chest dogs.
Methods and results: The alpha-adrenergic agonists were separately infused into the cannulated left circumflex coronary artery during control and ischemic conditions in the same animal. Mean coronary perfusion pressure was held constant by a servocontrolled pump in an extracorporeal circuit. Ischemia was created by reducing coronary perfusion pressure to the level at which percent systolic wall thickening (%WT) decreased by 54%. Contractile function during control conditions was unchanged, whereas under ischemic conditions a further significant decrease in %WT of 27% occurred with either alpha 1- or alpha 2-receptor stimulation without any change in the anterior (control) wall function. Both alpha 1- and alpha 2-receptor stimulations during control conditions resulted in a relatively uniform transmural decrease in blood flow with no change in the subendocardial-to-subepicardial blood flow ratio. With alpha 1-stimulation during ischemia (n = 13), there was a tendency toward decreased subepicardial blood flow with no change in subendocardial flow, resulting in an increased subendocardial-to-subepicardial blood flow ratio (0.61 +/- 0.23 to 0.82 +/- 0.40, p less than 0.05). alpha 2-Receptor stimulation during ischemia (n = 12) produced a significant decrease in subepicardial blood flow (0.45 +/- 0.20 to 0.35 +/- 0.12 ml/min/g, p less than 0.01) with no change in subendocardial blood flow, also resulting in an increased subendocardial-to-subepicardial blood flow ratio.
Conclusions: These results indicate the selective vasoconstriction in outer wall layers during ischemia mediated by either alpha 1- or alpha 2-receptors can cause a decrease in regional contractile function despite unchanged subendocardial blood flow and improved subendocardial-to-subepicardial flow ratio. This suggests an adverse effect of alpha-adrenergic vasoconstriction during ischemia in this coronary perfusion pressure-controlled canine model.