Rhodamine 123 has been shown to be a substrate for P-glycoprotein in multidrug resistant cells, In the present investigation the disposition of rhodamine 123 was studied in the isolated perfused rat kidney, After exposing the kidneys to perfusate concentrations ranging from 10 to 1000 n g/m l, the renal clearance was 4-1 times the clearance by glomerular filtration, respectively, indicating active and saturable secretion of rhodamine 123. The rate-limiting step in secretion was found to be membrane passage from cell to tubular lumen. Suprisingly, renal clearance was not influenced by the P-glycoprotein inhibitors cyclosporin A or digoxin. However, pretreatment o f the kidneys with verapamil and quinidine (inhibitors of both P-glycoprotein and organic cation transport) or cimetidine (organic cation transport inhibitor) resulted in a significantly reduced rhodamine 123 clearance, indicating that the renal organic cation carrier may be involved in active secretion. Rhodamine 123 accumulated extensively in the isolated perfused rat kidney; tissue concentrations o f 2 7 0 -3 6 0 times the perfusate concentration were determined. Similar accumulation ratios at different perfusate concentrations were found, suggesting that the compound enters the tubular cells by (facilitated) diffusion. In conclusion, rhodamine 123 accumulated extensively in the isolated perfused rat kidney and active renal secretion appears to be pi*eferentially mediated by the organic cation carrier and not by P-glycoprotein.

No abstract

1 Sulphonylurea drugs have been shown to protect against hypoxic damage in isolated proximal tubules of the kidney. In the present study we investigated whether these drugs can protect against hypoxic damage in a whole kidney preparation. 2 Tolbutamide (200 mM) and glibenclamide (10 mM) were applied to the isolated perfused rat kidney prior to changing the gassing from oxygen to nitrogen for 30 min. 3 Hypoxic perfusions resulted in an increased fractional excretion of glucose (FE % glucose 14.3+1.5 for hypoxic perfusions vs 4.9+1.6 for normoxic perfusions, mean+s.e.mean, P50.05), which could be completely restored by 200 mM tolbutamide (5.7+0.4 for tolbutamide vs 14.3+1.5 for untreated hypoxic kidneys, P50.01). Furthermore, tolbutamide reduced the total amount of LDH excreted in the urine (220+100 mU for tolbutamide vs 1220+160 mU for untreated hypoxic kidneys, P50.01). Comparable results were obtained with glibenclamide (10 mM). 4 In agreement with the e ect on functional parameters, ultrastructural analysis of proximal tubules showed increased brush border preservation in tolbutamide treated kidneys compared to untreated hypoxic kidneys. 5 We conclude that glibenclamide and tolbutamide are both able to reduce hypoxic damage to proximal tubules in the isolated perfused rat kidney when applied in the appropriate concentrations.

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 fig inL~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 iH ) -3 -0 db 2-8 fig min" , Michaelis-Menten constant for tubular transport for the high affinity site (KTfH ) = 0-5 ± 0-8 fig mL , maximum transport velocity for the low affinity site ^, 0 -2503:36 ¿¿g_min~ , and Michaelis-Menten constant for tubular transport for the low affinity site ( K T(l ) = 62 ±17 fig 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 f i g g~\ affinity constant for renal accumulation at the high affinity site (KaiH ) = 0*2±04 ¿¿gmL" 1, theoretical maximum capacity of the low affinity site (RM<) = 1640± 1100 jug g and affinity constant for renal accumulation at the low affinity site (K a ,l) = 60± 58 fig mL-1 .TThe 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.

No abstract