Adenosine coronary vasodilation and sympathetic adrenergic vasoconstriction during altered PCO2

We used the open-chest, anesthetized dog to investigate the possible influence of blood PCO2 on alpha-adrenergic constriction (ansa subclavia stimulation) of previously dilated (adenosine infusion into the left anterior descendens LAD) coronary vessels. During hypercapnia, LAD flow was increased to a significantly greater degree by adenosine than during normocapnia. Adenosine infusion during hypocapnia was least effective in dilating the coronary vasculature. Ansa stimulation at the peak of the adenosine response attenuated LAD flow by 7 and 33 percent respectively during hypo- and hypercapnia. Although there was a significant effect of PCO2 on the vascular response to adenosine, the ability of the adrenergic nerves to attenuate this response in the presence of an altered PCO2 seemed to relate to this pre-existing level of coronary tone.