Nitric oxide (NO) is produced by a group of enzymes called nitric oxide synthases (NOS). These enzymes catalyze the production of NO and L-citrulline from L-arginine, O2, and NADPH-derived electrons. The cell types that express eNOS include vascular endothelial cells, cardiomyocytes and others. In blood vessels, NO produced by the eNOS in endothelial cells functions as a vasodilator thereby regulating blood flow and pressure. Mutant eNOS knockout mice have blood pressure that is 30% higher than wild-type littermates. Within cardiomyocytes, eNOS affects Ca2 currents and contractility. Expression of eNOS is usually reported to be constitutive, though modest degrees of regulation occur in response to factors such as shear stress, exercise, chronic hypoxia, and heart failure.