Protection of vascular wall function in insulin-resistant rats from copper oxidative stress

Endothelial dysfunction and body mass index: is there a role for plasma peroxynitrite?

BACKGROUND

Endothelial function is dependent on the balance between vasoconstrictive and vasodilatory substances. The endothelium ability to produce nitric oxide is one of the most crucial mechanisms in regulating vascular tone. An increase in inducible nitric oxide synthase contributes to endothelial dysfunction in overweight persons, while in underweight persons, oxidative stress contributes to the conversion of nitric oxide to peroxynitrite (measured as nitrotyrosine in vivo). The objective of this study was to elucidate the interaction of body composition and oxidative stress on vascular function and peroxynitrite. This was done through an experimental design with three weight groups (underweight normal weight and overweight), with four treatment arms in each. Plasma nitrotyrosine levels were measured 15-20 hours post lipopolysaccharide (LPS) treatment, as were aortic ring tension changes. Acetylcholine (ACh) and sodium nitroprusside (SNP) challenges were used to observe endothelial dependent and independent vascular relaxation after pre-constriction of aortic rings with phenylephrine.

RESULTS

Nitrotyrosine levels in saline-treated rats were similar among the weight-groups. There was a significant increase in nitrotyrosine levels between saline-treated rats and those treated with the highest lipopolysaccharide doses in each of the weight groups. In response to ACh challenge, Rmax (percentage reduction in aortic tension) was lowest in overweight rats (112%). In response to SNP, there was an insignificantly lower Rmax in the underweight (106%) compared to the normal weight (112%). Overweight rats had a significant decrease in Rmax (83%) in response to SNP, signifying involvement of a more chronic process in tension reduction changes. A lower Rmax accompanied an increase in peroxynitrite after acetylcholine challenge in all weight-groups.

CONCLUSIONS

Endothelial dysfunction, observed as an impairment in the ability to reduce tension, is associated with increased plasma peroxynitrite levels across the spectrum of body mass. In higher-BMI rats, an additional role is played by vascular smooth muscle in the causation of endothelial dysfunction.

Effects of maternal hypoxia or nutrient restriction during pregnancy on endothelial function in adult male rat offspring

Compromised fetal growth impairs vascular function; however, it is unclear whether chronic hypoxia in utero affects adult endothelial function. We hypothesized that maternal hypoxia (H, 12% O2, n= 9) or nutrient restriction (NR, 40% of control, n= 7) imposed from day 15-21 pregnancy in rats would impair endothelial function in adult male offspring (relative to control, C, n= 10). Using a wire myograph, endothelium-dependent relaxation in response to methacholine was assessed in small mesenteric arteries from 4- and 7-month-old (mo) male offspring. Nitric oxide (NO) mediation of endothelium-dependent relaxation was evaluated using N(omega)-nitro-L-arginine methyl ester (L-NAME; NO synthase inhibitor). Observed differences in the NO pathway at 7 months were investigated using exogenous superoxide dismutase (SOD) to reduce NO scavenging, and sodium nitroprusside (SNP; NO donor) to assess smooth muscle sensitivity to NO. Sensitivity to methacholine-induced endothelium-dependent relaxation was reduced in H offspring at 4 months (P < 0.05), but was not different among groups at 7 months. L-NAME reduced methacholine sensitivity in C (P < 0.01), H (P < 0.01) and NR (P < 0.05) offspring at 4 months, but at 7 months L-NAME reduced sensitivity in C (P < 0.05), tended to in NR (P= 0.055) but had no effect in H offspring. SOD did not alter sensitivity to methacholine in C, but increased sensitivity in H offspring (P < 0.01). SNP responses did not differ among groups. In summary, prenatal hypoxia, but not nutrient restriction impaired endothelium-dependent relaxation at 4 months, and reduced NO mediation of endothelial function at 7 months, in part through reduced NO bio-availability. Distinct effects following reduced maternal oxygen versus nutrition suggest that decreased oxygen supply during fetal life may specifically impact adult vascular function.

Low micromolar concentrations of copper augment the impairment of endothelium-dependent relaxation of aortae from diabetic rabbits

Both diabetes mellitus (DM) and elevated plasma copper concentrations are risk factors for cardiovascular disease (CVD). DM is associated with impaired endothelial nitric oxide (NO) and with excess superoxide (O2*-) formation. Copper is also elevated in DM and is also associated with the generation of O2*-. To explore possible interactions between DM and copper, the effect of exogenous copper (CuCl2) on endothelium-dependent relaxation and cyclic guanosine monophosphate (GMP) formation was investigated in aortae from diabetic rabbits. Rabbits were rendered diabetic by intravenous injection of alloxan. Six months after induction of DM, the aortae were excised, cut into rings, and mounted in an organ bath for isometric measurement of acetylcholine (Ach)-evoked relaxation in rings precontracted with phenylephrine (PE). In parallel studies, cyclic (c)GMP formation by aortic rings following stimulation with Ach, calcium ionophore A23187 (A23187) and sodium nitroprusside (SNP) was assessed using radioimmunoassay. The effect of copper on these parameters was then studied using the same methods. Ach-evoked relaxation and Ach- and A23187-evoked cGMP formation were significantly impaired in aortae from diabetic rabbits compared to controls, effects that were reversed with superoxide dismutase (SOD) and catalase (CAT). In contrast, there were no significant differences in SNP-stimulated relaxation or cGMP formation in aortae from diabetic rabbits compared to controls. Copper (1 to 10 micromol/L) promoted a further significant inhibition of Ach-stimulated relaxation in aortae from diabetic but not control rabbits. This reduction by copper was again reversed by SOD and CAT. We conclude that copper augments the reduction of NO bioavailability, which is already impaired in aortae from diabetic rabbits due to excess production of O2*- and H2O2. These results indicate that patients with DM may be susceptible to copper-mediated vasculopathy at much lower concentrations than those that promote vasculopathy in nondiabetic patients.

Cuprous oxidase activity of yeast Fet3p and human ceruloplasmin: implication for function

The Fet3 protein in Saccharomyces cerevisiae and mammalian ceruloplasmin are multicopper oxidases (MCO) that are required for iron homeostasis via their catalysis of the ferroxidase reaction, 4Fe(2+)+O(2)+4H(+)-->4Fe(3+)+2H(2)O. The enzymes may play an essential role in copper homeostasis since they exhibit a strikingly similar kinetic activity towards Cu(1+) as substrate. In contrast, laccase, an MCO that exhibits weak activity towards Fe(2+), exhibits a similarly weak activity towards Cu(1+). Kinetic analyses of the Fet3p reaction demonstrate that the ferroxidase and cuprous oxidase activities are due to the same electron transfer site on the enzyme. These two ferroxidases are fully competent kinetically to play a major role in maintaining the cuprous-cupric redox balance in aerobic organisms.