In vivo bioassay of endothelium-derived relaxing factor.

We devised a technique for the in vivo assay of endothelium-derived relaxing factor (EDRF) from cerebral microvessels. We used anesthetized cats equipped with two cranial windows. One window (assay window) was subjected to muscarinic blockade with atropine to inhibit the direct effects of acetylcholine. EDRF production was induced in the donor window by superfusion with acetylcholine. The superfusate was then directed through the assay window with a delay of 6 s where it caused vasodilation equal to that seen in the donor window. The dilation was eliminated by lengthening the transit time from the donor to the assay window to greater than 2 min or by muscarinic blockade with atropine in the donor window but not by indomethacin in the donor window. It was also inhibited by hemoglobin and methylene blue or by selective damage to the endothelium of the vessels in the donor window with topical application of arachidonate or hydrogen peroxide. We conclude that the vasoactivity of the superfusate is due to EDRF released by acetylcholine from cerebral microvessels.