Mouse models of nitric oxide synthase deficiency.

Knockout mice for each of the three nitric oxide (NO) synthase (NOS) genes have been generated. Their phenotypes reflect the roles of each NOS isoform in physiologic and pathologic processes. This article reviews how neuronal NOS (nNOS) and endothelial NOS (eNOS) knockout mice have contributed to our knowledge of the roles of NO in cerebral ischemia, cardiovascular processes, and the autonomic nervous system. In some instances, the effects of NO produced by one isoform antagonize the effects of NO produced by another isoform. For example, after cerebral ischemia, the nNOS isoform is involved in tissue injury, whereas the eNOS isoform is important in maintaining blood flow. All three isoforms are expressed in the respiratory tract, but only the nNOS isoform appears to be involved in modulating airway responsiveness and only the inducible NOS isoform appears to respond to antigen stimulation. In the cardiovascular system, endothelial NO is important for vascular tone, systolic and diastolic cardiac function, vascular proliferative responses to injury, platelet aggregation, and hemostasis.