Physiological release of nitric oxide is dependent on the level of vascular tone.

The pressor effect of NG-methyl-L-arginine (NMA) was tested in urethane-anesthetized rats which were untreated (control) or devoid of sympathetic tone. In contrast with controls, the NMA response was attenuated by pithing or ganglionic blockade. In pithed rats, the induction of moderate or intense vasoconstriction with constant phenylephrine infusion restored or augmented, respectively, the NMA pressor response. Our data suggest that vascular tone may physiologically regulate the release of nitric oxide in vivo.