Influence of haemoglobin and erythrocytes on the effects of EDRF, a smooth muscle inhibitory factor, and nitric oxide on vascular and non‐vascular smooth muscle

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Abstract
The relaxant action of endothelium‐derived relaxing factor (EDRF), the smooth muscle inhibitory factor (IF) isolated from the bovine retractor penis (BRP), nitric oxide (NO) and sodium nitroprusside (NaNP) on four vascular and non‐vascular smooth muscle preparations has been examined. Their sensitivity to EDRF, the IF and NO was the same, suggesting all might be NO. Sodium nitroprusside produced complete relaxation of the rat anococcygeus at low doses, suggesting an action additional to the intracellular release of NO. Haemoglobin added to solutions of EDRF, activated IF or NO completely removed their relaxant properties, consistent with all three acting by virtue of NO. Suspensions of red blood cells with a haemoglobin concentration equivalent to that used in the previous experiments were as effective as haemoglobin in abolishing the relaxant effect of EDRF or NO but were ineffective against the activated IF. The similarity in sensitivity of a series of smooth muscles and the binding by haemoglobin are consistent with NO being the active principle of both EDRF and the acid activated IF. The abolition of the effect of EDRF by red blood cells (RBCs) is further confirmation for this hypothesis, but the ineffectiveness of RBCs against acid‐activated IF suggests that either the latter is not NO or that it is bound in a way which makes it unable to diffuse through cell membranes.