Assessment of real-time and quantitative changes in renal hemodynamics in healthy overweight males: Contrast-enhanced ultrasonography vs para-aminohippuric acid clearance.

OBJECTIVE: To determine the ability of renal contrast-enhanced ultrasonography (CEUS) to detect acute drug-induced changes in renal perfusion (using the glucagon-like peptide (GLP)-1 receptor agonist exenatide and nitric oxide [NO]-synthase inhibitor L-NG -monomethyl arginine [l-NMMA]), and assess its correlation with gold standard-measured effective renal plasma flow in humans. METHODS: In this prespecified exploratory analysis of a placebo-controlled cross-over study, renal hemodynamics was assessed in 10 healthy overweight males (aged 20-27 years; BMI 26-31 kg/m2 ) over two separate testing days; during placebo (isotonic saline) and subsequent exenatide infusion (Day-A), and during l-NMMA, and subsequent exenatide plus l-NMMA infusion (Day-B). Renal cortical microvascular blood flow was estimated following microbubble infusion and CEUS destruction-refilling-sequences. Renal cortical microvascular blood flow was compared with simultaneously measured effective renal plasma flow in humans, derived from para-aminohippuric acid-clearance methodology. RESULTS: On Day-A, effective renal plasma flow increased by 68 [26-197] mL/min/1.73 m2 during exenatide vs placebo infusion (+17%; P = .015). In parallel, exenatide increased renal cortical microvascular blood flow, from 2.42 × 10-4 [6.54 × 10-5 -4.66 × 10-4 ] AU to 4.65 × 10-4 [2.96 × 10-4 -7.74 × 10-4 ] AU (+92%; P = .027). On Day-B, effective renal plasma flow and renal cortical microvascular blood flow were reduced by l-NMMA, with no significant effect of concomitant exenatide on renal hemodynamic-indices assessed by either technique. Effective renal plasma flow correlated with renal cortical microvascular blood flow on Day-A (r = .533; P = .027); no correlation was found on Day-B. CONCLUSIONS: Contrast-enhanced ultrasonography can detect acute drug-induced changes human renal hemodynamics. CEUS-assessed renal cortical microvascular blood flow moderately associates with effective renal plasma flow, particularly when perfusion is in normal-to-high range. Renal CEUS cannot replace effective renal plasma flow measurements, but may be a complementary tool to characterize regional kidney perfusion.

Assessment of endothelial function of the renal vasculature in human subjects.

BACKGROUND: L-Arginine, the substrate of nitric oxide (NO) synthase, and N(G)-monomethyl-L-arginine (L-NMMA), a competitive inhibitor of endothelial NO synthase, are used to analyze endothelial function of the renal vasculature. However, little is known about the appropriate dose of L-arginine to be used and the duration of action of L-arginine and L-NMMA. METHODS: Twenty-nine healthy male subjects (age, 27+/-1 years) were examined. In protocol 1 (N = 17), L-arginine at low (100 mg/kg) and high dose (250 mg/kg), and high-dose L-arginine combined either with L-NMMA (total dose, 4.25 mg/kg; N = 9) or placebo (N = 8) were given. In protocol 2 (N = 12), L-NMMA was given before L-arginine infusion (100 mg/kg). Glomerular filtration rate (GFR) and renal plasma flow (RPF) were measured at rest and at the end of each infusion step. RESULTS: In protocol 1, L-arginine dose dependently increased RPF and GFR (RPF: 599+/-19 v 630+/-18 v 690+/-24 mL/min, P <.05; GFR: 111+/-3 v 115+/-3 v 121+/-3 mL/min, P <.01; for baseline, L-arginine 100 mg/kg and 250 mg/kg, respectively). However, these changes could not be antagonized by coinfusion of L-NMMA to L-arginine 250 mg/kg: RPF and GFR remained unchanged in both the placebo and the L-NMMA group. In protocol 2, L-NMMA decreased RPF (492+/-18 v 567+/-27 mL/min, P <.01) and increased GFR (122+/-4 v 118+/-3 mL/min, P <.05). These changes could only be partially reversed by subsequent infusion of L-arginine (RPF: 533+/-15 mL/min; GFR: 121+/-4 mL/min; both parameters P = NS v L-NMMA and v baseline). CONCLUSIONS: L-arginine at a dose of 100 mg/kg is sufficient to analyze endothelial function of the renal vasculature. The prolonged effect of L-NMMA and L-arginine must be taken into account in study protocols using both substances. Thus, stimulation and blockade of NO synthase cannot be examined in the same protocol.

Assessment of nitric oxide production by measurement of [15N]citrulline enrichment in human plasma using high-performance liquid chromatography-mass spectrometry.

Nitric oxide (NO) is formed by a class of NO synthases (NOS), which convert arginine into citrulline. A decreased in vivo NO availability can be the result of an increased NO inactivation or a decreased NO production. The latter can be assessed by measurement of isotopic enrichment of plasma citrulline during infusion of isotopically labeled arginine. The potential of high-performance liquid chromatography (HPLC) coupled to mass spectrometry (MS) to determine enrichments of [15N2]arginine and [15N]-citrulline in plasma during infusion of [15N2]arginine in humans was investigated. Two types of MS instruments were evaluated: a sector-type mass spectrometer equipped with a frit fast-atom bombardment (FAB) interface and a quadrupole instrument with electrospray ionization (ESI). FAB-MS appeared to be unsuitable for determination of isotope ratios, because background ions influenced the observed isotope ratio in an unpredictable way. In combination with either off- or on-line reversed-phase HPLC, ESI-MS proved to be a more reliable technique. However, the amount of material that is introduced in the mass spectrometer is critical and should be carefully controlled. During infusion of [15N2]arginine in 14 healthy subjects, a mean arginine-to-citrulline conversion rate of 0.22 +/- 0.07 (SD) mumol.kg-1.h-1 was found. In 4 subjects who received an intravenous infusion with the NOS antagonist L-NMMA, the conversion rate decreased from 0.30 +/- 0.14 to 0.10 +/- 0.06 mumol.kg-1.h-1. It is concluded that ESI-MS in combination with HPLC can be successfully applied for determination of arginine and citrulline enrichments in plasma, thus providing a useful tool for assessment of in vivo NO production.