L‐Arginase induces Vascular Dysfunction in Old Spontaneously Hypertensive Rats

Abstract

BackgroundAging is a major non‐modifiable risk factor for hypertension. Structural, functional and mechanical changes occur with aging. These changes are similar to those seen in hypertension in the vasculature. Also, aging increases the vascular dysfunction that occurs in hypertension. L‐arginase is a ureohydrolase enzyme that converts L‐arginine to L‐ornithine and urea. Arginase action reduces substrate (L‐arginine) availability for the formation of nitric oxide (NO). This reduces the level of NO and leads to reduced vasodilation and ultimately, vascular dysfunction.HypothesisThis study tests the hypothesis that the age dependent vascular dysfunction in SHRs is mediated by arginase.MethodsYoung (12–14 weeks) and old (11–12 months) male Wistar and spontaneously hypertensive rats (SHR) were used in this study. Mean arterial pressure (MAP) was measured in the rats. They were then euthanized and 5ml of blood was drawn from the heart to obtain serum for the assay of arginase activity. Mesenteric resistance arteries (MRAs) and thoracic aortae were excised and placed in ice‐cold physiological salt solution (PSS). Arterial segments were either snap‐frozen in liquid nitrogen and stored for immunoblotting studies or cut into 2mm rings for reactivity studies. Cumulative concentration‐response curves to phenylephrine (PE; 10‐9‐3×10‐5M), acetylcholine (Ach; 10‐9 – 3×10‐5M) and sodium nitroprusside (SNP; 10‐12 – 3×10‐5 M) were performed in the absence or presence (30‐minute exposure) of various agents: L‐arginase, 0.05U/ML (MRA) or 0.5U/ML (aorta); Rho kinase inhibitor, Y27632 − 10‐5mmol/L; or 0.5U/ML L‐arginase + 10‐5mmol/L Y27632 (aorta). All chemicals were obtained from Sigma‐Aldrich MO, USA. Data are expressed as mean + S.E.M. of 6 rats per group. Statistical differences were calculated using Student’s t‐test and two‐way ANOVA with repeated measures followed by Bonferroni post hoc test. Significance was set at p<0.05.ResultsMAP increased during aging in the SHRs (132.2±10.2 vs 166.2±1.9mmHg p<0.05.) but not in the Wistar rats (104.4±3.0 vs 92.0±1.5mmHg). Arginase impaired the endothelium‐dependent relaxation responses of thoracic aortic and MRA arterial rings to Ach in the old Wistars (Emax aorta: 68.66248±1.96% vs 56.46116±3.42%) and SHRs (Emax aorta: 29.41753±2.19% vs 7.943045±1.86%). Arginase also impaired endothelium‐independent relaxation response to SNP in the old SHRs only (Emax aorta: 88.62396±4.10% vs 31.45081±10.61%). Correspondingly, arginase potentiated contractile responses of thoracic aortic and MRA arterial rings to PE in the old SHRs (Emax MRA: 25.24835±1.93mN vs 36.43248±2.99mN) which was attenuated by the Rho Kinase inhibitor (Y26732) in the presence or absence of arginase. We also observed no differences in the serum arginase activity in the four groups of rats. On the contrary, arginase activity in the thoracic aortae of young Wistar rats was reduced compared to the other groups.ConclusionsThe results of this study suggest that arginase impairs both endothelium‐dependent and –independent vasorelaxation responses, through the NO signaling pathway.Support or Funding InformationNIH