Agonist-induced tension determines vascular reactivity during anoxia and reoxygenation

To determine the influence of pre-existing pharmacologically-induced tension on vascular reactivity during anoxia and reoxygenation, rat aortic rings were contracted with norepinephrine, epinephrine, endothelin or KCl to 1, 2 or 4 g of tension. The rings were then exposed to anoxia (95% N2) for 10 min followed by reoxygenation (95% O2). The degree of anoxia-mediated contraction varied with the magnitude of tension before anoxia and resembled the length-tension relationship in myocardial fibers. The optimal agonist-induced tension for maximal anoxic contraction was approximately 1 to 2 g. This relationship between tension and anoxic contraction was observed in all but KCl-contracted rings. The agonist- as well as KCl-contracted rings showed normal relaxant response to acetylcholine, suggesting that a decrease in endothelium-derived relaxing factor (EDRF) alone cannot be the basis of anoxic contraction and release of endothelium-derived constricting factors (EDCFs) may relate to anoxic contraction in agonist-preconstricted rings. The relationship between the magnitude of agonist-induced tension and the extent of anoxia-mediated contraction may relate to the ability of endothelium to release EDRF and EDCFs as well as to the degree of phosphorylation in vascular smooth muscle cells. The reoxygenation-mediated contraction was noted to progressively increase in all experiments regardless of the pharmacologic agent used. This increase in reoxygenation-mediated contraction correlated with pre-existing pharmacologic tension, and may relate to calcium influx and restoration of ATP and other mediators in the vascular tissues during reoxygenation.