Drug-induced changes in renal hippurate clearance as a measure of renal blood flow

Modulation of the renal response to ACE inhibition by ACE insertion/deletion polymorphism during hyperglycemia in normotensive, normoalbuminuric type 1 diabetic patients

ACE inhibition protects kidney function, but ACE insertion/deletion (I/D) polymorphism affects renal prognosis in type 1 diabetic patients. ACE genotype may influence the renal benefits of ACE inhibition. We studied the impact of ACE I/D polymorphism on the renal hemodynamic changes induced by ACE inhibition in type 1 diabetes. We studied renal hemodynamics (glomerular filtration rate [GFR], effective renal plasma flow [ERPF], filtration fraction [GFR/ERPF], mean arterial pressure [MAP], and total renal resistances [MAP/ERPF]) repeatedly during normoglycemia and then hyperglycemia in 12 normotensive, normoalbuminuric type 1 diabetes and the II genotype (associated with nephroprotection) versus 22 age- and sex-matched subjects with the ACE D allele after three randomly allocated 2- to 6-week periods on placebo, 1.25 mg/day ramipril, and 5 mg/day ramipril in a double-blind, cross-over study. During normoglycemia, the hemodynamic changes induced by ramipril were similar in both genotypes. During hyperglycemia, the changes induced by ramipril were accentuated in the II genotype group and attenuated dose dependently in the D allele group (treatment-genotype interaction P values for ERPF, 0.018; MAP, 0.018; and total renal resistances, 0.055). These results provide a basis to different renal responses to ACE inhibition according to ACE genotype in type 1 diabetes.

Dynamic renal blood flow measurement by positron emission tomography in patients with CRF

BACKGROUND

Positron emission tomography (PET) is a functional imaging device that allows dynamic regional blood flow measurements. We performed a study to test whether PET could detect acute changes in renal blood flow (RBF) in patients with chronic renal failure (CRF).

METHODS

RBF was measured by means of PET (PET-RBF) using oxygen 15-labeled water (H2(15)O) in eight men with hypertension and moderate CRF before and 5, 40, 80, and 120 minutes after the injection of quinaprilat (10 mg intravenously). Effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) were measured simultaneously by para-aminohippuric acid (PAH-ERPF) and inulin clearances before and 20, 60, 100, and 140 minutes after quinaprilat injection.

RESULTS

Baseline RBF and ERPF were decreased in all patients (221 +/- 20 mL/min/100 g and 225 +/- 38 mL/min/1.73 m2, respectively). PET-RBF increased significantly after quinaprilat injection (+15%, +26%, +19%, and +23% versus baseline; P < 0.003). PAH-ERPF did not increase significantly (-6%, +12%, +20%, and +15% versus baseline; P = 0.15). GFR (50.1 +/- 8.9 mL/min/1.73 m2 at baseline) did not change significantly after quinaprilat injection; however, filtration fraction (GFR-ERPF ratio) decreased significantly from 0.23% +/- 0.02% to 0.20% +/- 0.02% (P = 0.0004). Mean arterial pressure decreased significantly after quinaprilat injection (P < 0.005).

CONCLUSION

This study dynamically measured RBF by means of PET in patients with CRF for the first time. It showed that RBF rapidly increased after quinaprilat injection. PET using H2(15)O is a powerful method for the noninvasive measurement of dynamic changes in RBF that remain undetected by PAH clearance.

Continuous measurements of renal perfusion in pigs by means of intravascular Doppler

BACKGROUND

Changes in renal blood flow are considered to play a significant role in the induction and maintenance of kidney failure, but are difficult to monitor with currently available techniques. The objective was to validate renal flow measurements with Doppler guidewires and to apply this technique to assess dose and time dependency of the renal vascular effects of norepinephrine (NE).

METHODS

In 10 anesthetized pigs, flow velocity in renal arteries (FVart) and veins (FVvein) and volumetric renal blood flow (VBF) were measured before and after intravenous bolus application of incremental doses of NE (2 to 200 microg).

RESULTS

FVart curves exactly reflected the changes in VBF. Beat-to-beat analysis revealed a strong linear correlation over a mean VBF range of less than 0.05 to 0.35 L/min (median correlation coefficient with FVart, r = 0.998), and significant but less close relationships were also found between VBF and FVvein. Ten seconds after the administration of 200 microg NE, FVart dropped from 71 to 6 cm/sec and was 90% reversible after 48 seconds. Similarly, the renal vascular resistance temporarily rose from 988 to 13711 mm Hg. min/L. In contrast, NE-induced increases in systemic vascular resistance were on average a maximum of 1.5-fold but persisted for more than 60 seconds.

CONCLUSIONS

Doppler flow measurements in the renal artery provide an excellent surrogate of volumetric blood flow, which may be useful for continuous monitoring of renal hemodynamics. The renal vasculature is more sensitive when compared with the systemic vasculature, but also appears to evoke more efficient counter-regulatory mechanisms in response to NE.

Renal blood flow measurement by positron emission tomography using 15O-labeled water

BACKGROUND

Only few noninvasive methods have the potential to quantitate renal blood flow (RBF) in humans. Positron emission tomography (PET) is a clinical imaging method that can be used to measure the tissue blood flow noninvasively. The purpose of this study was to validate PET measurement of RBF using 15O-labeled water (H215O), a tracer that allows repeated measurements at short time intervals.

METHODS

RBF was measured in six pigs by PET and by radioactive microspheres (MS). Three measurements were performed in each pig at baseline (BL), during vascular expansion and dopamine infusion (DA; 20 microg. kg-1. min-1 intravenously), and during angiotensin II (Ang II) infusion (50 ng. kg-1. min-1 intravenously). RBF was estimated from aortic and renal tracer kinetics using a model adapted from the blood flow model described by Kety and Smith.

RESULTS

PET and MS values correlated strongly (y = 0.79x + 42, r = 0.93, P < 0.0001) over the RBF range from 100 to 500 mL. min-1. 100 g-1. Pharmacologically induced changes were significant and were measured equally well by PET and MS: 38 and 39%, respectively, below BL (P < 0.005 and P < 0.05) under Ang II, and 47 and 48%, respectively, above BL (P < 0.005 and P < 0.01) under DA. A Bland and Altman representation showed a low average difference of -17 +/- 45 mL. min-1. 100 g-1 (mean +/- SD).

CONCLUSION

To our knowledge, this study provides the first validation of RBF measurement by PET using H215O over a large range of RBF values (100 to 500 mL. min-1. 100 g-1), which correspond to RBF values in both healthy subjects and in patients suffering from chronic renal failure.

Renal changes on hyperglycemia and angiotensin-converting enzyme in type 1 diabetes

Hyperglycemia causes capillary vasodilation and high glomerular capillary hydraulic pressure, which lead to glomerulosclerosis and hypertension in type 1 diabetic subjects. The insertion/deletion (I/D) polymorphism of the angiotensin I-converting enzyme (ACE) gene can modulate risk of nephropathy due to hyperglycemia, and the II genotype (producing low plasma ACE concentrations and probably reduced renal angiotensin II generation and kinin inactivation) may protect against diabetic nephropathy. We tested the possible interaction between ACE I/D polymorphism and uncontrolled type 1 diabetes by measuring glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) during normoglycemia ( approximately 5 mmol/L) and hyperglycemia ( approximately 15 mmol/L) in 9 normoalbuminuric, normotensive type 1 diabetic subjects with the II genotype and 18 matched controls with the ID or DD genotype. Baseline GFR (145+/-22 mL/min per 1.73 m2) and ERPF (636+/-69 mL/min per 1.73 m2) of II subjects declined by 8+/-10% and 10+/-9%, respectively, during hyperglycemia; whereas baseline GFR (138+/-16 mL/min per 1.73 m2) and ERPF (607+/-93 mL/min per 1.73 m2) increased by 4+/-7% and 6+/-11%, respectively, in ID and DD subjects (II versus ID or DD subjects: P=0.0007 and P=0.0005, for GFR and ERPF, respectively). The changes in renal hemodynamics of subjects carrying 1 or 2 D alleles were compatible, with a mainly preglomerular vasodilation induced by hyperglycemia, proportional to plasma ACE concentration (P=0.024); this was not observed in subjects with the II genotype. Thus, type 1 diabetic individuals with the II genotype are resistant to glomerular changes induced by hyperglycemia, providing a basis for their reduced risk of nephropathy.

Effects of low-dose dopamine on renal and systemic hemodynamics during incremental norepinephrine infusion in healthy volunteers

OBJECTIVES

To assess the effects of low-dose dopamine on norepinephrine-induced renal and systemic vasoconstriction in normotensive healthy subjects.

DESIGN

On separate days, either a low-dose dopamine (4 microg/kg/min) or a placebo (5% glucose) infusion was added in a single, blinded, randomized order to incremental norepinephrine infusions of 40, 80, and 150 ng/kg/min over a 60-min period each.

SETTING

Outpatient clinic of a university-affiliated hospital.

SUBJECTS

Normotensive healthy volunteers.

INTERVENTIONS

Infusions of norepinephrine and dopamine.

MEASUREMENTS AND MAIN RESULTS

Blood pressure and heart rate were measured with a semiautomated device, and glomerular filtration rate and effective renal plasma flow were determined with constant infusions of 125I-iothalamate and 131I-hippurate, respectively. Norepinephrine alone progressively increased mean arterial pressure to pressor levels, whereas this effect was attenuated by the addition of dopamine (p < .05 vs. norepinephrine alone). Glomerular filtration rate increased during lower norepinephrine doses and did not decrease at the highest norepinephrine dose. Addition of dopamine further increased glomerular filtration rate. Effective renal plasma flow decreased with each norepinephrine alone infusion step, but this decrease was completely prevented by concomitant dopamine infusion (p < .01 vs. norepinephrine). Sodium excretion tended to decrease with norepinephrine, but increased two- to three-fold after addition of dopamine (p < .01 vs. norepinephrine alone).

CONCLUSIONS

In healthy man, norepinephrine causes a large decrease in renal plasma flow but not in glomerular filtration rate. Concomitant dopamine administration prevents this decrease in renal plasma flow, increases sodium excretion, and also attenuates the norepinephrine-induced systemic blood pressure increase. These findings warrant further clinical evaluation of the effect of concomitant low-dose dopamine and norepinephrine administration in critically ill patients.