Between 1987 and 1988 our research group made a couple of interesting findings. First, we discovered that vascular endothelial cells are able to generate nitric oxide gas (NO) 1, thus explaining the actions of the'endothelium-derived relaxing factor'that had been described some seven years earlier2. And subsequently we found that NO was synthesized directly from the aminoacid L-arginine3. Here I present an overview of the new understanding that NO has brought to physiology and pathophysiology, with particular emphasis on the therapeutic possibilities that have arisen as a consequence.

We started to call the biochemical pathway the L-arginine: NO pathway, and the enzyme responsible for this conversion NO synthase3. To demonstrate the formation of NO from L-arginine we cultured vascular endothelial cells on microcarrier beads and perfused them in vitro with two types of arginine, either uniformly labelled, ie with 15N-labelled nitrogen in all the nitrogen atoms of the molecule, or guanidino labelled, ie labelled in two guanidino nitrogen atoms. The effluent was connected directly to a mass spectrometer and we measured the formation of 15N-labelled NO. Every time we stimulated the cells with bradykinin, we could see the release of 15N-labelled NO and, since the amounts that were generated from both types of arginine were very similar, we concluded that NO was specifically made from the guanidino nitrogen atoms of the aminoacid4. Shortly after this it was discovered that, in the process of making NO, molecular oxygen was introduced in the formation of N-hydroxy L-arginine, which is an intermediate in the synthesis and later on in the formation of L-citrulline, which is the co-product of the generation of NO. The NO produced in this way is usually transferred from a generator cell to an effector cell where it exerts a biological function. In the case of the vasculature, it goes from the endothelial to the smooth muscle cell to activate the soluble guanylate cyclase and by doing so it produces vascular relaxation5.