A comparison of the action of the endothelium‐derived relaxant factor and the inhibitory factor from the bovine retractor penis on rabbit aortic smooth muscle

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Abstract
The dependence of relaxation of rabbit aortic strips by carbachol and by the inhibitory factor from the bovine retractor penis (BRP) on the presence of endothelium has been compared. Carbachol‐induced relaxation is abolished by removing the endothelium, inhibitory factor‐induced relaxation is unimpaired. The inhibitory factor, therefore, does not act by releasing an endothelium‐derived relaxing factor (EDRF). The effect of inhibitors of eicosanoid metabolism on relaxation was examined. Quinacrine and nordihydroguaiaretic acid abolished the relaxant effect of carbachol and flurbiprofen had no effect. The relaxation produced by the inhibitory factor was unaffected by quinacrine and flurbiprofen while nordihydroguaiaretic acid potentiated the response. No eicosanoid appears, therefore, to be involved in the relaxant effect of the inhibitory factor from the BRP. Methylene blue, a drug reported to inhibit guanylate cyclase, in a concentration of 10 μM selectively abolished the relaxation produced by carbachol. However, at the higher concentration of 30 μM it abolished almost completely the response to inhibitory factor from the BRP and reduced inhibition by sodium nitroprusside. It is not possible from these results to exclude the possibility that the EDRF and the inhibitory factor from the BRP are chemically related.