Nitric oxide (NO) is an important biological mediator involved in control of blood vessel tone, neurotransmission, and immune function (1 –3 ). NO has physicochemical properties that make it ideal to function as an intercell communicator, and these include an ability to travel rapidly between cells (4 ), a short half-life, and a rapid oxidation to inactive anions (2 ). Unfortunately, some of these same characteristics make quantification of NO difficult. Most studies both in vivo and in vitro have used indirect indices of NO production, such as breakdown products of NO metabolism, second messengers, or biological events in effector systems. However, recent developments in probe technology have allowed the direct detection of NO in certain defined situations.
