title: Diurnal and Long-term Variation in Plasma Concentrations and Renal Clearances of Circulating Markers of Kidney Proximal Tubular Secretion.
abstract: The renal proximal tubule is essential for removing organic solutes and exogenous medications from the circulation. We evaluated diurnal, prandial, and long-term biological variation of 4 candidate endogenous markers of proximal tubular secretion.
  
  
  
  We used LC-MS to measure plasma and urine concentrations of hippurate (HA), cinnamoylglycine (CMG), indoxyl sulfate (IS), andp-cresol sulfate (PCS) in 25 healthy adults. We measured plasma concentrations of secreted solutes at 13 time points over a 24-h period, and again after 2 weeks and 14 weeks of follow-up. We further measured 24-h renal clearances of secreted solutes at baseline, 2 weeks, and 14 weeks.
  
  
  
  Plasma concentrations of secreted solutes varied over the 24-h baseline period. Diurnal variation was greatest for HA, followed by CMG, IS, and PCS. Plasma concentrations of HA (P= 0.002) and IS (P= 0.02), but not CMG and PCS, increased significantly following meals. Long-term intraindividual biological variation (CVI) in plasma concentrations of secreted solutes over 14 weeks varied from 21.8% for IS to 67.3% for PCS, and exceeded that for plasma creatinine (CVI, 7.1%). Variation in 24-h renal clearances was similar among the secreted solutes [intraindividual variation (CVA+I), 33.6%-47.3%] and was lower using pooled plasma samples from each study visit.
  
  
  
  Plasma concentrations of HA, CMG, IS, and PCS fluctuate within individuals throughout the day and over weeks. Renal clearances of these secreted solutes, which serve as estimates of renal proximal tubule secretion, are also subject to intraindividual biological variation that can be improved by additional plasma measurements.
authors.affiliation: Kidney Research Institute, Seattle, WA.|Division of Nephrology, Department of Medicine, University of Washington, Seattle, WA.
authors.forename: Matthew B
authors.initials: MB
authors.lastname: Rivara
authors.affiliation: Kidney Research Institute, Seattle, WA.|Division of Nephrology, Department of Medicine, University of Washington, Seattle, WA.
authors.forename: Leila R
authors.initials: LR
authors.lastname: Zelnick
authors.affiliation: Kidney Research Institute, Seattle, WA.|Division of Clinical Chemistry, Department of Laboratory Medicine, University of Washington, Seattle, WA.
authors.forename: Andrew N
authors.initials: AN
authors.lastname: Hoofnagle
authors.affiliation: Kidney Research Institute, Seattle, WA.
authors.forename: Rick
authors.initials: R
authors.lastname: Newitt
authors.affiliation: Kidney Research Institute, Seattle, WA.|Departments of Pathology and Biochemistry, University of Vermont College of Medicine, Burlington, VT.
authors.forename: Russell P
authors.initials: RP
authors.lastname: Tracy
authors.affiliation: Cancer Prevention Program, Division of Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA.|Department of Epidemiology, University of Washington, Seattle, WA.|Division of Metabolism, Endocrinology, and Nutrition, Department of Medicine, University of Washington, Seattle, WA.
authors.forename: Mario
authors.initials: M
authors.lastname: Kratz
authors.affiliation: Division of Metabolism, Endocrinology, and Nutrition, Department of Medicine, University of Washington, Seattle, WA.
authors.forename: David S
authors.initials: DS
authors.lastname: Weigle
authors.affiliation: Kidney Research Institute, Seattle, WA; brk@uw.edu.|Division of Nephrology, Department of Medicine, University of Washington, Seattle, WA.|Cancer Prevention Program, Division of Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA.
authors.forename: Bryan R
authors.initials: BR
authors.lastname: Kestenbaum
mesh_terms: D000328:Adult; D015415:Biomarkers / Q000097:blood; D002853:Chromatography, Liquid; D003408:Cresols / Q000097:blood*; D005260:Female; D005998:Glycine / Q000031:analogs & derivatives* / Q000097:blood; D006626:Hippurates / Q000097:blood*; D006801:Humans; D007200:Indican / Q000097:blood*; D007687:Kidney Tubules, Proximal / Q000737:chemistry / Q000378:metabolism*; D008297:Male; D013463:Sulfuric Acid Esters / Q000097:blood*; D053719:Tandem Mass Spectrometry
chemical_list: D015415:Biomarkers; D003408:Cresols; D006626:Hippurates; D013463:Sulfuric Acid Esters; C090448:cinnamoylglycine; C408690:4-cresol sulfate; D007200:Indican; C030514:hippuric acid; D005998:Glycine