Effect of exogenous nitric oxide on baroreflex function in humans

Nitric oxide (NO) donors inhibit sympathetic neurotransmission and barorece ptor activity and can directly stimulate heart rate (HR) in vitro. To asses s whether exogenous NO affects cardiovascular autonomic control in humans, we tested the baroreceptor-cardiac reflex [baroreflex sensitivity (BRS)I an d the arterial blood pressure (BP) and HR variability during an infusion of the NO donor sodium nitroprusside (SNP, 2 mu g . kg(-1) . min(-1)) or 5% g lucose in 16 healthy subjects. The hypotensive action of SNP was prevented by phenylephrine (PE, 0.9 +/- 0.15 mu g . kg(-1) . min(-1)). The SNP + PE i nfusion did not affect BRS or HR variability, but it caused a significant r eduction in the diastolic and systolic BP low-frequency power. In addition, SNP + PE caused a sustained 12% increase in HR in the absence of changes i n brachial and aortic BP. In conclusion, SNP had no effect on the cardiac-v agal limb of the baroreflex in humans but caused a substantial reduction in BP low-frequency power consistent with a decreased baroreflex/sympathetic control of peripheral resistance. The increase in HR in the absence of baro receptor downloading confirms our previous finding of a direct positive chr onotropic effect of NO donors.