Melinda M. Parnell

Impaired l-Arginine Transport and Endothelial Function in Hypertensive and Genetically Predisposed Normotensive Subjects

Background—Impaired endothelium-dependent NO-mediated vasodilation is a key feature of essential hypertension and may precede the increase in blood pressure. We investigated whether transport of the NO precursor l-arginine is related to decreased endothelial function. Methods and Results—Radiotracer kinetics ([3H]l-arginine) were used to measure forearm and peripheral blood mononuclear cell arginine uptake in hypertensive subjects (n=12) and in 2 groups of healthy volunteers with (n=15) and without (n=15) a family history of hypertension. In conjunction, forearm blood flow responses to acetylcholine and sodium nitroprusside were measured before and after a supplemental intra-arterial infusion of l-arginine. In vivo and in vitro measures of l-arginine transport were substantially reduced in the essential hypertension and positive family history groups compared with the negative family history group; however, no difference was detected in peripheral blood mononuclear cell mRNA or protein expression levels for the cationic amino acid transporter CAT-1. Plasma concentrations of l-arginine and NG,NG′-dimethylarginine (ADMA) did not differ between groups. l-Arginine supplementation improved the response to acetylcholine only in subjects with essential hypertension and positive family history. Conclusions—Similar to their hypertensive counterparts, normotensive individuals at high risk for the development of hypertension are characterized by impaired l-arginine transport, which may represent the link between a defective l-arginine/NO pathway and the onset of essential hypertension. The observed transport defect is not due to apparent alterations in CAT-1 expression or elevated endogenous ADMA.

Reduced Myocardial and Systemic l-Arginine Uptake in Heart Failure

Abstract— Altered nitric oxide (NO) bioavailability has been ascribed an important role in the pathophysiology of congestive heart failure (CHF). In the peripheral vasculature, we recently demonstrated a depression of l-arginine transport in association with pharmacological evidence of reduced endothelial function. In contrast, increased myocardial NO generation has been proposed to account for a component of the reduced myocardial contractility in CHF, although this remains controversial. We determined the whole body clearance rate and cardiac fractional extraction of l-arginine during a steady-state intravenous infusion of [3H]l-arginine (300 nCi/min) in 9 healthy control subjects and 7 patients with moderate to severe CHF. In patients with CHF, there was a 30% reduction in the transcardiac extraction of [3H]l-arginine compared with controls (P <0.05), which was accompanied by a trend toward reduced [3H]l-citrulline release (P =0.06). In conjunction, the systemic clearance rate of [3H]l-arginine was significantly lower in patients with CHF (778±148 versus 1278±144 mL/min, P <0.05). In association with these biochemical indices, we observed a 38% reduction (P <0.05) in the mRNA expression of the cationic amino acid transporter CAT-1 in ventricular myocardial samples from patients with CHF compared with healthy unused donor myocardium, whereas myocardial NOS enzymatic activity and NOS protein were unchanged. These data indicate the presence of a significant reduction in the myocardial uptake of l-arginine in patients with CHF. Furthermore, this abnormality seems to be part of a systemic downregulation of l-arginine transport.

An age-related decline in endothelial function is not associated with alterations in L-arginine transport in humans.

Objectives Endothelial dysfunction is established in aged individuals; however, the mechanism(s) are not fully elucidated. We have previously identified l-arginine transport as a potential rate-limiting factor in nitric oxide (NO) production in heart-failure patients, characterized with endothelial dysfunction. We therefore aimed to investigate whether the age-related decline in endothelial function is due to reduced transport of the NO precursor, l-arginine. Methods Thirty-seven healthy males aged between 19 and 69 were recruited. Throughout 40 min of intra-arterial (i.a.) infusion of [3H]l-arginine (100 nCi/min), venous blood samples were withdrawn for the determination of l-arginine clearance. Venous occlusion plethysmography was then used to record the forearm blood flow responses to i.a. infusions of acetylcholine (ACh; 9.25 and 37 μg/min) and sodium nitroprusside (SNP; 2 and 8 μg/min). Results While ACh-induced vasodilation decreased with age (37 μg/min; young 15.87 ± 1.30, middle-aged 9.59 ± 1.33, older 10.42 ± 1.12 ml/min per 100 ml tissue; P = 0.001), there was no change in forearm [3H]l-arginine clearance (young 126.08 ± 19.05, middle-aged 122.47 ± 20.96, older 126.56 ± 19.56 ml/min; NS). Further [3H]l-arginine uptake studies in isolated peripheral blood mononuclear cells supported our in vivo findings, demonstrating no difference in [3H]l-arginine transport across the age spectrum. Conclusions The present study excludes the hypothesis of impaired l-arginine transport as a potential mechanism for the age-related decline in endothelial function.

beta-Adrenoceptor-mediated, nitric-oxide-dependent vasodilatation is abnormal in early hypertension: restoration by L-arginine.

Background It is unknown whether β-adrenoceptor-mediated vasodilatation is altered in early hypertension and whether it can be modulated by l-arginine. Methods and design We measured changes in forearm blood flow by plethysmography in response to acetylcholine (9 and 37 μg/min), sodium nitroprusside (200 and 800 ng/min) and the β-receptor agonist, isoproterenol (50 and 200 ng/min) in 12 patients with essential hypertension (group EH) and in healthy volunteers with (group PFH; n = 14) and without (group NFH; n = 14) a family history of essential hypertension, before and during concomitant infusion of l-arginine (10 μmol/min). In five individuals from each group, infusion of acetylcholine and isoproterenol was repeated during the concurrent blockade of nitric oxide synthesis by NG-monomethyl-l-arginine (l-NMMA; 4 μmol/min). Results The response to acetylcholine was reduced in groups EH and PFH compared with group NFH (both P < 0.05), whereas the vasodilator effects of isoproterenol and sodium nitroprusside were similar in all three groups. Acetylcholine- and isoproterenol-induced vasodilatation did not change during infusion of the nitric oxide precursor, l-arginine, in group NFH, but were significantly enhanced by l-arginine in groups PFH and EH [forearm blood flow before and after isoproterenol 200 ng/min: group PFH 11.8 ± 1.02 and 13.3 ± 1.08 ml/min, respectively (P < 0.05); group EH: 11.3 ± 1.57 and 14.9 ± 1.91 ml/min, respectively (P < 0.01)]. Co-infusion of l-NMMA blunted the response to acetylcholine and isoproterenol in group NFH (P < 0.05), but did not significantly modify vasodilatation in groups PFH and EH. Conclusions Although β-adrenergic vasodilatation seemed to be unaltered in early hypertension, l-arginine enhanced the response to isoproterenol, whereas concomitant inhibition of nitric oxide synthase by l-NMMA had no significant effect. These findings suggest that the nitric oxide component of isoproterenol-mediated vasodilatation is impaired in early hypertension and possibly compensated by increased β-adrenoceptor responsiveness of smooth muscle cells. In this setting, supplementation of the nitric oxide precursor, l-arginine, enhances the vasodilator response to β-adrenergic stimulation.