Effect of organic cations on the renal tubular secretion of pseudoephedrine in the rat

Isolated rat kidney perfused with dextran and bovine serum albumin: A stable model for investigating renal drug handling

INTRODUCTION

The rat isolated perfused kidney (IPK) preparation is a very useful model for pharmacokinetic and pharmacologic studies. Bovine serum albumin (BSA) is the oncotic agent used most commonly in IPK models, but the protein is expensive and varies significantly in quality. The present study evaluated the use of dextran to replace a large proportion of BSA as the oncotic agent, to establish a more reliable and economic IPK model for pharmacokinetic studies.

METHODS

The right kidneys of male Sprague-Dawley rats were isolated and perfused in recirculating mode with Krebs-Henseleit pH 7.4 buffer containing amino acids, glucose and 65 g/l BSA (BSA group, n=11) or 6.5 g/l BSA plus 36 g/l dextran (dextran/BSA group, n=6). (14)C-Inulin was added to the perfusate to permit estimation of glomerular filtration rate (GFR). An antiviral guanosine analogue, AM188, was administered to the IPK perfusate to investigate its renal disposition. During the 130-min experimental period, urine was collected in 10-min intervals and perfusate was collected at the midpoint of these intervals.

RESULTS

The kidney functional parameters were generally better and more stable in the dextran/BSA IPKs when compared to the BSA group. At a similar perfusate flow rate, the IPKs in the dextran/BSA group exhibited lower renal artery perfusion pressure, a higher GFR, and more extensive tubular reabsorption of water, glucose, and sodium. These functional parameters were acceptable and stable throughout the whole experimental period in the dextran/BSA group. The renal clearance of AM188 was higher in the dextran/BSA group compared with that in the BSA group.

DISCUSSION

Using a large proportion of dextran and a small proportion of BSA as oncotic agent in perfusate provides an improved IPK preparation. This offers a reliable and economic rat IPK model for pharmacokinetic studies.

Renal excretion mechanisms of the antiviral nucleoside analogue AM 188 IN the rat isolated perfused kidney

1. AM 188 is an antiviral guanosine analogue that undergoes extensive renal excretion in humans. The present study was designed to investigate the disposition of AM 188 over a range of concentrations in the rat isolated perfused kidney (IPK) to explore the mechanisms involved in its renal handling. 2. Right kidneys of male Sprague-Dawley rats (n = 23) were isolated and perfused in recirculating mode with Krebs'-Henseleit (pH 7.4) buffer containing 0.65% bovine serum albumin, 3.6% dextran, amino acids and glucose. [14C]-Inulin was added to the perfusate reservoir to permit estimation of glomerular filtration rate (GFR). [3H]-AM 188 and unlabelled AM 188 were added to the perfusate as a bolus initially, followed by a constant rate of infusion at 5, 25, 125, 500 or 1000 microg/min to achieve initial target perfusate concentrations of 1, 5, 25, 100 or 200 microg/mL, respectively. During the 130 min over which AM 188 was infused, urine was collected in 10 min intervals (commencing 10 min after the bolus dose) and perfusate was collected at the mid-point of these intervals to permit calculation of the renal clearance (CLR) of AM 188. Binding of AM 188 in perfusate, measured using ultrafiltration, was negligible. 3. The bolus dose and infusion regimen produced relatively stable AM 188 concentrations in perfusate in the 5, 25 and 125 micro g/min groups and progressively increasing concentrations in the 500 and 1000 microg/min groups. High-pressure liquid chromatography analysis of IPK perfusate and urine suggested that there was no or negligible metabolism of AM 188 in the kidney. The CLR/GFR ratio for AM 188 (mean+/-SD) was 5.76 +/- 1.57, 5.99 +/- 0.52, 6.02 +/- 1.47, 3.38 +/- 0.26 and 1.08 +/- 0.42 in the 5, 25, 125, 500 and 1000 microg/min groups, respectively, showing significant reductions at the two highest infusion rates (P < 0.05). Although there was no difference between the five groups in the distribution of AM 188 between kidney tissue and perfusate (KT/P), at the end of perfusion the corresponding urine-to-tissue concentration ratio declined significantly in the 1000 microg/min group. 4. AM 188 undergoes substantial net renal secretion over a wide range of perfusate concentrations. A reduction in renal clearance at perfusate concentrations above 25 microg/mL could be due to saturation of carrier-mediated transport at the brush border membrane and/or a solubility limitation leading to precipitation of AM 188 in tubular cells and/or tubular urine.