Abstract
Glucocorticoid administration decreases renal sodium/phosphate cotransport in the proximal tubule due to a down-regulation of the sodium/phosphate cotransporter but has no effect on the sodium-dependent transport of glucose or proline. The objectives of the present investigation were to determine the effects of the glucocorticoid methylprednisolone (MPL) on 1) inorganic sulfate renal clearance in rats in vivo, 2) sodium/sulfate cotransport in kidney cortex membrane vesicles, and 3) the cellular mechanism of the MPL-induced alterations in sulfate renal transport. Male adrenalectomized Wistar rats received an i.v. dose of 50 mg/kg MPL or the vehicle. Urine samples were collected for 12 h after the administration of MPL, and blood samples were collected at the midpoint of the urine collection. Other animals were sacrificed at 4, 6, and 12 h after MPL administration, and the kidney cortex was removed for RNA or membrane preparations. Kidney cortex sodium/sulfate cotransporter (NaSi-1) mRNA levels were determined by reverse transcription-polymerase chain reaction and NaSi-1 protein levels were determined by enzyme-linked immunosorbent assay. The urinary excretion rate and renal clearance of sulfate were significantly increased in MPL-treated animals (144.0 ± 27.0 versus 65.3 ± 21.3 μmol/12 h/kg and 0.208 ± 0.038 versus 0.078 ± 0.025 ml/min/kg, mean ± S.E., n = 9–12 in treated versus control). The Vmax value for sodium-dependent sulfate transport in brush border membrane vesicles (representing reabsorption in the proximal tubules) was significantly decreased in MPL-treated animals compared with controls (0.68 ± 0.07 versus 0.88 ± 0.05 nmol/mg of protein/10 s, mean ± S.E.). There was no change in the Km value for sodium/sulfate cotransport in brush-border membrane and no change in sulfate/anion exchange in basolateral membrane vesicles. Membrane fluidity in brush border membrane and basolateral membrane vesicles, determined by the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene was unaltered by MPL treatment. NaSi-1 mRNA levels were significantly decreased at 4 and 6 h, but not 12 h, after MPL administration, whereas NaSi-1 protein expression was significantly decreased at 4, 6, and 12 h. Therefore, MPL treatment increases the renal clearance of inorganic sulfate, at least in part, due to down-regulation of NaSi-1 mRNA and protein expression in the kidney.
Footnotes
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Send reprint requests to: Marilyn E. Morris, Ph.D., 527 Hochstetter Hall, Department of Pharmaceutics, State University of New York at Buffalo, Amherst, NY 14260. E-mail:memorris{at}acsu.buffalo.edu
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↵1 This work was supported by a POWRE grant from the National Science Foundation (IBN 9973499) and grants from the Western New York Kidney Foundation/Upstate NY Transplant Services. The work was presented as an abstract at the American Association of Pharmaceutical Scientists Annual Meeting, November 1999.
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↵2 Present address: Central Research Division, Pfizer Inc., Groton, CT 06340.
- Abbreviations:
- BBM
- brush-border membrane
- NaSi-1
- sodium sulfate cotransporter
- sat-1
- sulfate anion transporter
- MPL
- methylprednisolone
- BLM
- basolateral membrane
- DPH
- 1,6-diphenyl-1,3,5-hexatriene
- bp
- base pairs
- RT-PCR
- reverse transcription-polymerase chain reaction
- Received February 2, 2000.
- Accepted May 1, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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