Renal transport kinetics of furosemide in the isolated perfused rat kidney

Application of the isolated perfused rat kidney model to assess gender effects on drug excretion

PURPOSE

To study the effect of gender on the renal disposition of two organic anions, p-aminohippuric acid (PAH) and furosemide (FSM) in the isolated perfused rat kidney (IPK).

METHODS

IPK experiments (3-4 per treatment group) were conducted using kidneys from male and female Sprague Dawley rats. PAH was administered as a continuous infusion (with loading dose, targeted steady-state concentration 10 ug/mL). FSM was added as a bolus dose (2.65 mg, targeted concentration 33 ug/mL). Urine was collected in 10-min. intervals and perfusate was sampled at the midpoint of each collection period. Control (drug naïve) perfusions were performed for both genders. PAH and FSM were measured by HPLC. Kidney viability (GFR [estimated using inulin clearance], sodium reabsorption, glucose reabsorption) was monitored continuously during each perfusion experiment (2-h duration).

RESULTS

Good kidney function was maintained across all study groups, and lower GFR estimates in female kidneys were due to differences in kidney weight. For PAH, kidney weight corrected renal clearance (0.88 +/- 0.37 mL/min/g vs. 0.59 +/- 0.19 mL/min/g) and excretion ratio (3.8 +/- 1.7 vs. 2.2 +/- 0.72) were significantly higher in male kidneys. For FSM, renal clearance was significantly lower in female (0.10 +/- 0.05 mL/min/g) compared to male kidneys (0.15 +/- 0.07 mL/min/g). Mass balance analysis showed that FSM cumulative urinary excretion was significantly higher and kidney accumulation was significantly lower in experiments with male kidneys.

CONCLUSIONS

The study demonstrates that the IPK is a useful model to assess gender effects on renal drug disposition. The renal excretion of organic anions is reduced in female rats, possibly due to gender differences in expression and/or activity of membrane transporters (both basolateral and luminal) in the kidney.

Interactions of human organic anion transporters with diuretics

The tubular secretion of diuretics in the proximal tubule has been shown to be critical for the action of drugs. To elucidate the molecular mechanisms for the tubular excretion of diuretics, we have elucidated the interactions of human organic anion transporters (hOATs) with diuretics using cells stably expressing hOATs. Diuretics tested were thiazides, including chlorothiazide, cyclothiazide, hydrochlorothiazide, and trichlormethiazide; loop diuretics, including bumetanide, ethacrynic acid, and furosemide; and carbonic anhydrase inhibitors, including acetazolamide and methazolamide. These diuretics inhibited organic anion uptake mediated by hOAT1, hOAT2, hOAT3, and hOAT4 in a competitive manner. hOAT1 exhibited the highest affinity interactions for thiazides, whereas hOAT3 did those for loop diuretics. hOAT1, hOAT3, and hOAT4 but not hOAT2, mediated the uptake of bumetanide. hOAT3 and hOAT4, but not hOAT1 mediated the efflux of bumetanide. hOAT1 and hOAT3, but not hOAT2 and hOAT4 mediated the uptake of furosemide. In conclusion, it was suggested that hOAT1 may play an important role in the basolateral uptake of thiazides, and hOAT3 in the uptake of loop diuretics. In addition, it was also suggested that bumetanide taken up by hOAT3 and/or hOAT1 is excreted into the urine by hOAT4.

Alternative high-performance liquid chromatographic assay for p-aminohippuric acid (PAH): effect of aging on PAH excretion in the isolated perfused rat kidney

Para-aminohippuric acid (PAH), an indicator of renal plasma flow, is a commonly used marker of organic anion transport by the kidney. An analytical method for PAH using HPLC was developed. The method is simple, fast and requires a minimum amount of organic solvent. Sample preparation involved protein precipitation with zinc sulfate. Para-amino benzoic acid was utilized as an internal standard (IS). Chromatography was performed using a reversed-phase phenyl column with UV detection at a wavelength of 254 nm. Mobile phase consisted of 0.1 M acetic acid and acetonitrile (99:1) at a flow rate of 1 ml/min. The assay was validated over a standard concentration range from 1 to 25 microg/ml. Accuracy, precision, reproducibility and specificity of the method was established with coefficients of variation <10%. The method was sensitive and showed linear response in peak height ratio (analyte:IS) over the concentration range studied (r(2)>0.99). The assay was used to study the effect of aging on PAH excretion in the isolated perfused rat kidney model. Experiments were conducted in kidneys from young (2-3 months, n=6), adult (6-9 months, n=5) and aged (12-16 months, n=3) male Sprague-Dawley rats at an initial drug concentration of 20 microg/ml. Significant differences in kidney function (e.g. glomerular filtration rate and glucose reabsorption) were observed in aged kidneys. Despite a 5-fold reduction in glomerular filtration rate, PAH renal clearance (kidney weight-corrected) decreased by only 2-fold in aged (2.2+/-0.42 ml/min per gram) compared to young (4.6+/-0.70 ml/min per gram, P<0.05) rats. Furthermore, renal excretion ratio was significantly higher in aged rats (27+/-8.0 vs. 15+/-5.0, P<0.05). These preliminary findings challenge the 'Whole Nephron Hypothesis' that assumes parallel reductions in renal filtration and secretory capacity secondary to disease or aging.

Renal excretion and accumulation kinetics of 2-methylbenzoylglycine in the isolated perfused rat kidney

The effect of protein binding on kidney function has been studied by investigating the renal accumulation and secretion of the hippurate analogue 2-methylbenzoylglycine in the isolated perfused rat kidney in the absence and presence of bovine serum albumin (BSA). Experiments were performed with either 2.5% pluronic or a combination of 2.2% pluronic and 2% BSA as oncotic agents; a wide concentration range (1-190 micrograms mL-1) of 2-methylbenzoylglycine was studied. Tubular secretion appeared to be a function of the amount of unbound drug in the perfusate and was best described by a model consisting of a high and low affinity Michaelis-Menten term. Parameters obtained after the analysis of renal excretion data were maximum transport velocity for the high affinity site (TM,H) = 3.0 +/- 2.8 micrograms min-1, Michaelis-Menten constant for tubular transport for the high affinity site (KT.H) = 0.5 +/- 0.8 microgram mL-1, maximum transport velocity for the low affinity site (TM,L) = 250 +/- 36 micrograms min-1, and Michaelis-Menten constant for tubular transport for the low affinity site (KT,L) = 62 +/- 17 micrograms mL-1. The compound accumulated extensively in kidney tissue, ratios up to 175 times the perfusate concentration were reached. Accumulation data were best analysed by a two-site model similar to the model used to describe renal excretion. Calculated parameters were theoretical maximum capacity of the high affinity site (RM,H) = 26 +/- 23 micrograms g-1, affinity constant for renal accumulation at the high affinity site (KA,H) = 0.2 +/- 0.4 microgram mL-1, theoretical maximum capacity of the low affinity site (RM,L) = 1640 +/- 1100 micrograms g-1 and affinity constant for renal accumulation at the low affinity site (KA,L) = 60 +/- 58 micrograms mL-1. The very high accumulation in kidney tissue could be explained by active tubular uptake, mediated by the secretory mechanisms involved, and dependent on the amount of free drug in the perfusate. This study shows that anionic drugs, subject to active secretion, may reach high concentrations in tubular cells even at low plasma concentrations.

Changes in renal clearance of furosemide due to changes in renal blood flow and plasma albumin concentration

We have shown that, within therapeutic plasma concentrations, the unbound fraction of furosemide changes in direct proportion to the reciprocal of the plasma albumin concentration (correlation coefficient 0.99). Changes in the albumin concentration were produced by ultrafiltration of human plasma using a haemofiltration filter. Thus, we propose that, when studying changes in the pharmacokinetics of a highly protein bound drug, calculated changes in the unbound fraction offer an alternative to actual measurement of the unbound concentration, which is often difficult. Nine healthy volunteers receiving a continuous furosemide infusion were studied in normovolaemia and after dehydration (-1.4 kg), with and without pretreatment with an angiotensin converting enzyme inhibitor (captopril) or an a1-adrenoceptor blocking agent (prazosin). Significantly larger changes in the renal clearance of furosemide were found that could be explained by changes in the unbound fraction. Following dehydration, the unbound fraction of furosemide was decreased by about 5%, while its renal clearance fell by 27%, 33% and 13% after pretreatment with placebo, captopril and prazosin, respectively. The secretory clearance of the unbound furosemide changed substantially and in parallel with changes in the renal blood flow. It is suggested that changes in the renal clearance and excretion of furosemide and its t1/2 are much more dependent on changes in renal blood flow than on changes in its unbound fraction.

Influence of the unbound concentration of cefonicid on its renal elimination in isolated perfused rat kidneys

The effect of variations in plasma protein binding on the renal excretion of cefonicid was assessed by using isolated perfused rat kidneys. Cefonicid exhibits preferential binding ex vivo to human serum albumin (HSA), as opposed to bovine serum albumin (BSA), and is eliminated mainly by tubular secretion, a process that was reported to be dependent on the total drug concentration. This contradicts previous studies with antimicrobial compounds and other drugs of low renal extraction in which the unbound drug concentration was shown to be the driving force for carrier-mediated tubular transport. To clarify this discrepancy, we performed perfusion studies by using 6% BSA at initial concentrations of 200 micrograms/ml (n = 6) and 20 micrograms/ml (n = 9) and in a combination of 4% BSA plus 2% HSA at initial concentrations of 200 micrograms/ml (n = 4). The excretion ratio [ER = CLR/(fu x GFR)] of cefonicid decreased with increasing unbound concentrations, whereas no apparent relationship with the total concentration was evident. At similar total concentrations of cefonicid, the renal clearance remained unchanged; the secretion clearance increased significantly in the 4% BSA-2% HSA experiments, reflecting the reduced unbound fraction and unbound drug concentration of cefonicid. The excretion ratio data were compatible with a model in which Michaelis-Menten kinetics were required to describe active transport and secretion was dependent on the unbound cefonicid concentration. As a result, changes in plasma protein binding as a result of drug interactions or disease states could significantly influence the tubular transport capability of compounds with low renal extraction.

Isolated perfused rat kidney as a tool in the investigation of renal handling and effects of nonsteroidal antiinflammatory drugs

An isolated perfused rat kidney (IPK) preparation is described in which renal perfusion flow, perfusion pressure, urinary flow, urinary pH, and glomerular filtration rate (GFR) are recorded continuously during the perfusion experiment. The usefulness of this IPK system in studying the renal handling and the effects of non-steroidal antiinflammatory drugs (NSAIDs) is shown using salicyluric acid (SU), salicylic acid (SA), and naproxen (NA). Excretion of SU involves glomerular filtration, active secretion, and passive reabsorption. The excretion rates of SA and NA were both much lower than their filtration rate, indicating extensive reabsorption. All three drugs accumulate in the IPK but at different levels. SU accumulates much more than either SA or NA. The effects on renal function were different for the three drugs studied. SU had no effect on kidney function. SA perfusate concentrations greater than 100 micrograms/mL caused diuresis and natriuresis, while SA concentrations less than 100 micrograms/mL did not influence kidney function. NA perfusate concentrations ranging from 0.16 to 25 micrograms/mL caused a decrease in urinary flow and sodium excretion. Very high NA concentrations (greater than 500 micrograms/mL) caused an increase in urinary flow and sodium excretion. We conclude that the IPK is a suitable preparation for characterizing and comparing renal handling and effects of NSAIDs.

Plasma protein binding of frusemide in liver disease: effect of hypoalbuminaemia and hyperbilirubinaemia

The binding of frusemide was studied in the plasma of 20 healthy subjects and 45 patients with liver disease. The unbound percentage (mean +/- s.d.) of frusemide was 1.64 +/- 0.21 healthy subjects) and 2.24 +/- 0.79 (patients) (P less than 0.01). By grouping the patients on the basis of plasma albuminaemia and bilirubinaemia four clusters namely: 'normal concentrations of albumin and bilirubin' (A), 'hyperbilirubinaemia and normal albumin concentration' (B), 'hypoalbuminaemia and normal bilirubin concentration' (C) and 'hypoalbuminaemia and hyperbilirubinaemia' (D) were defined. The unbound percentage of frusemide was 1.80 +/- 0.36 in (A); 2.44 +/- 1.05 in (B); 2.23 +/- 0.38 in (C); 2.76 +/- 0.77 in (D). The figure for healthy volunteers was not different from A, whereas it was significantly lower than those for B and D (P less than 0.01) and for C (P less than 0.05). A lowered binding of frusemide was associated with hypoalbuminaemia or hyperbilirubinaemia.

Indomethacin secretion in the isolated perfused proximal straight rabbit tubule. Evidence for two parallel transport mechanisms

We studied indomethacin as a probe of anion transport across the isolated perfused proximal straight tubule of the rabbit and discovered that a substantial component of transport may occur by anion exchange at the basolateral membrane. Various perturbations involving direct or indirect dissipation of the cellular sodium gradient (ouabain, sodium- or potassium-free solutions, cooling to 18 degrees C) resulted in only a 50% inhibition of indomethacin transport, which raised the question of a co-existent alternative pathway for secretion. Similarly, the anion exchange inhibitor, 4,4'-diisothiocyanostilbene (DIDS), diminished indomethacin secretion by only 50%. Cooling followed by DIDS or the reverse sequence resulted in additive inhibition such that the combination abolished active secretion of indomethacin. We conclude that active secretion of indomethacin by the proximal straight tubule appears to be in part sodium gradient dependent; the remainder may be driven by an anion exchanger on the basolateral membrane.