Endothelial nitric oxide synthase activation and nitric oxide function: new light through old windows

The principle mechanisms operating at the level of endothelial nitric oxide synthase (eNOS) itself to control its activity are phosphorylation, the auto-regulatory properties of the protein itself, and Ca2+/calmodulin binding. It is now clear that activation of eNOS is greatest when phosphorylation of certain serine and threonine residues is accompanied by elevation of cytosolic [Ca2+]i. While eNOS also contains an autoinhibitory loop, Rafikov et al. (2011) present the evidence for a newly identified ‘flexible arm’ that operates in response to redox state. Boeldt et al. (2011) also review the evidence that changes in the nature of endothelial Ca2+ signaling itself in different physiologic states can extend both the amplitude and duration of NO output, and a failure to change these responses in pregnancy is associated with preeclampsia. The change in Ca2+ signaling is mediated through altering capacitative entry mechanisms inherent in the cell, and so many agonist responses using this mechanism are altered. The term ‘adaptive cell signaling’ is also introduced for the first time to describe this phenomenon. Finally NO is classically regarded as a regulator of vascular function, but NO has other actions. One proposed role is regulation of steroid biosynthesis but the physiologic relevance was unclear. Ducsay & Myers (2011) now present new evidence that NO may provide the adrenal with a mechanism to regulate cortisol output according to exposure to hypoxia. One thing all three of these reviews show is that even after several decades of study into NO biosynthesis and function, there are clearly still many things left to discover.