Endothelial vasodilator function is depressed in fetal sheep carotid arteries. Postnatal maturation is associated with a moderately increased abundance of endothelial nitric oxide synthase (eNOS). However, when normalized by the luminal surface area or by eNOS concentration, the amount of NO generated in response to the calcium ionophore A23187 or to endothelial shear stress is greater in adult than in fetal arteries. In contrast, NO generation by ADP is not significantly different. Thus, eNOS coupling mechanisms are not fully mature in fetal sheep arteries, and maturation may depend on the nature of the stimulus. Mean±SE, n = 6-8. Source: Adapted, with permission, from White CR, et al., 2005 (451), © 2005, The American Physiological Society.