Nitric oxide and vasodilation in human limbs

Both the skeletal muscle and skin of humans possess remarkable abilities to vasodilate. Marked vasodilation can be seen in these vascular beds in response to a variety of common physiological stimuli. These stimuli include reactive hyperemia (skin and muscle), exercise hyperemia (muscle), mental stress (muscle), and whole body heating (skin). The physiological mechanisms that cause vasodilation in response to these stimuli are poorly understood, and the substance(s) responsible for it remain unclear. In this context, recent attention has been focused on the possible contribution of nitric oxide (NO) to the regulation of hyperemic responses in human skin and skeletal muscle. The emerging picture is that NO is not an essential component of the dilator response seen during reactive hyperemia. However, it does appear that NO may play a modest role in exercise hyperemia. NO appears to play a major role in the skeletal muscle vasodilation seen in response to mental stress in humans. Preliminary evidence also indicates that NO is not essential for the normal dilator responses observed in the cutaneous circulation during body heating in humans, but this issue needs further study. There are a number of possible mechanisms that might mediate NO release in humans, and the role of these mechanisms in the various hyperemic responses is also poorly understood. The role of altered NO-mediated vasodilation in some disease states is also discussed. Whereas NO is a potent vasodilating substance, the actions of NO alone do not explain a variety of poorly understood vasodilator mechanisms in conscious humans. Much work remains for those interested in the role of NO in the regulation of blood flow to the skin and skeletal muscle of humans.