The biochemistry laboratory identifies metabolites of nitric oxide bioavailability and reactive oxygen species applied to endothelial and vascular health.
Nitric oxide (NO) is a simple diatomic molecule and a diffusible gas. It is unstable and has a half-life of seconds. It has been shown to play a role in the regulation of almost every major organ system.
In the vasculature, NO is synthesized by the endothelium and can also be carried by red blood cells. A simplified map of possible NO pathways in the vasculature is shown below.
NO has several anti-atherogenic properties, including:
- Inhibition of platelet aggregation (anti-thrombotic)
- Inhibition of leucocyte adhesion to the vessel wall (anti-inflammatory)
- Inhibition of smooth muscle growth into the lumen (anti-proliferative)
- Inhibition of vasoconstrictors (vasodilation-indirect)
- Vasodilation (via shear and/or receptor mediators)
NO in the Plasma
We have investigated the effects of vascular health on NO metabolites. Plasma nitrite (NO2-) is the main oxidation product of NO and has been shown to reflect changes in eNOS activity. It therefore appears to be a good marker of recent NO bioavailability.
Example of a Recent Study
- RF = greater than two traditional risk factors, no clinically diagnosed CVD
- DM = type 2 diabetes no clinically diagnosed CVD
- PAD = intermittent claudication > three months and ABI<0.9 at rest.
- (In Press, Nitric Oxide: Biology and Chemistry, 2009)
We have shown plasma NO2- can be upregulated following occlusion (five minutes) and reactive hyperemia in the arm. In 15 apparently healthy subjects (34.1+7.3yrs) mean plasma nitrite concentrations were 415+64.0nM at baseline, and 634+57.1nM following reactive hyperemia. This represents a 52.5 percent increase in plasma nitrite (p=0.015). Perhaps, just as significant as the increase in the mean plasma nitrite concentration is the observation that of the 15 individuals who performed the test, the plasma nitrite level fell in only two (and in one of those by less than 5 percent). There was no change in plasma NOx (predominantly NO3-) levels.
Additionally, we have measured plasma NO2- in several different populations with varying severity of vascular disease or risk. We use acute exercise stress as a stimulus to test if individuals can upregulate NO.
There were no differences in resting plasma nitrite values between groups. However, following the GXT, the RF group had significantly greater plasma nitrite concentrations than the other groups (after adjustment for both age and VO2peak). Furthermore, within group (pre-post) t-tests revealed a significant increase in plasma nitrite in the RF group (+39.3 percent), no change in the DM (-15.51 percent), and a significant decrease in the PAD (-44.20 percent) and PAD+DM (-39.95 percent). There were no significant changes in plasma nitrate from baseline for any of the groups following treadmill exercise.
Exercise training has been shown to improve endothelial function, so we are currently examining its effects on nitrite response to an acute exercise stress (nitrite flux) in subjects with various degrees of vascular disease. In particular we are looking at PAD and diabetic subjects. The improvement in "nitrite flux" is shown in the figure below. This was related to changes in physiological endothelial function and exercise performance.
(This work is supported in part by NIHLBI grant 5RO1 HL-075752-05.)
NO in the Red Blood Cell
James group in Cardiff (Wales) have previously shown there may be defects in NO offloading in glycated RBC's (found in Diabetes). We have similar findings, and the pilot data opposite suggests that glycation (HbA1c11.3 percent) moves the free O2 binding curve to the left in comparison to normal Hb (A1c 5.4 percent), indicating a change from R to T state at lower pO2 and “later” O2 offloading.
We also measured the physiological responses of this glycation effect. Below is data from a single rabbit aortic ring suspension trial (three rings per condition). In the presence of glutathione (GSH), normally glycated Hb (A1c5.4 percent) dilated hypoxic vessel rings both prior to (9 percent) and following SNO loading (18.5 percent). Under identical conditions there was no response from highly glycated Hb (A1c11.8 percent). Taken together, these findings suggest that in glycated cells, NO may be "stuck" on the cell, which may effect delivery at the micro-vessels.
The loss of this “RBC-NO carrier” system may be very pertinent for vascular disease prevention in diabetic subjects.
This ongoing work is funded by the 7-08-IN-01 American Diabetes Association Innovative Award.
Thomas Stabler is responsible for the day-to-day running of the lab.
Office: Building GSRB1, Room 1033, 595 LaSalle Street, Durham, NC, 27710