title: Effect of volume expansion on the paracellular flux of lanthanum in the proximal tubule.
abstract: Although studies of volume expansion (VE) in the Necturus suggest a major role for paracellular flux in reabsorption by the proximal tubule, results from morphologic or electrophysiologic studies of the effect of VE on the rat proximal tubule suggest only a minor role for paracellular transport. In the present study, during in vivo microperfusion, lanthanum was used as an extracellular marker to determine bidirectional paracellular flux in the rat proximal tubule before and during 10% VE. Lanthanum itself did not affect proximal tubule reabsorption (delta 0.7 +/- 3.3 nl/min, LaCl3 versus saline infusion, n = 7). When lanthanum was added to the luminal perfusate, paracellular lanthanum efflux from the lumen to the interstitium was 28.9 +/- 6.6 pg/min per mm, n = 7. Subsequent VE significantly decreased the paracellular lumen-to-interstitium efflux to 12.8 +/- 8.3 pg/min per mm concomitant with a 49% decrease in proximal fluid reabsorption (delta -2.6 +/- 0.9 nl/min per mm, P < 0.05). When lanthanum was infused interstitially, by means of a chronically implanted matrix, there was significant paracellular lanthanum influx from the interstitium into the lumen (143.9 +/- 18.6 pg/min per mm, n = 4). Subsequent VE significantly increased this interstitium-to-lumen influx to 212.1 +/- 29.2 pg/min per mm as proximal reabsorption was significantly decreased by 58% (delta -2.8 +/- 0.8 nl/min per mm, P < 0.05). Thus, VE affects bidirectional paracellular flux in a manner that would decrease proximal reabsorption; paracellular efflux from the lumen to the interstitium was decreased, whereas paracellular influx from the interstitium to the lumen was increased.
authors.affiliation: Department of Internal Medicine, Mayo Clinic, Rochester, Minnesota, USA.
authors.forename: C R
authors.initials: CR
authors.lastname: Ramsey
authors.forename: T J
authors.initials: TJ
authors.lastname: Berndt
authors.forename: F G
authors.initials: FG
authors.lastname: Knox
mesh_terms: D000042:Absorption; D000818:Animals; D001692:Biological Transport; D015415:Biomarkers / Q000032:analysis; D002463:Cell Membrane Permeability / Q000502:physiology*; D004195:Disease Models, Animal; D005110:Extracellular Space / Q000378:metabolism*; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D007811:Lanthanum / Q000008:administration & dosage / Q000032:analysis* / Q000493:pharmacokinetics; D008297:Male; D010953:Plasma Volume / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012016:Reference Values; D019108:Tight Junctions / Q000378:metabolism
chemical_list: D015415:Biomarkers; D007811:Lanthanum