Hypoxic augmentation: The tale of a strange contraction.

Almost fifty years ago, experiments on isolated veins showed that acute hypoxia augments venoconstrictor responses in vitro and that such facilitation relied on anaerobic glycolysis. Over the years, this phenomenon was extended to a number of arterial preparations of different species and revisited, from a mechanistic point of view, with the successive demonstration that it depends on calcium handling in the vascular smooth muscle cells, is endothelium-dependent and requires the production of nitric oxide (NO) by endothelial nitric oxide synthase (eNOS) and the activation of soluble guanylyl cyclase (sGC). However, rather than the vasodilator cyclic nucleotide 3',5'-cyclic guanosine monophosphate (cGMP), its canonical product, the latter enzyme produces 3',5'-cyclic inosine monophosphate (cIMP) instead during acute hypoxia; this non-canonical cyclic nucleotide facilitates the contractile process in the vascular smooth muscle cells. This 'biased' activity of soluble guanylyl cyclase appears to involve stimulation of NAD(P)H:quinone oxidoreductase 1 (NQO-1). The exact interactions between hypoxia, anaerobic metabolism and NQO-1 leading to biased activity of soluble guanylyl cyclase remain to be established.