EFFECT OF VOLUME EXPANSION ON THE PARACELLULAR FLUX OF LANTHANUM IN THE PROXIMAL TUBULE

Although studies of volume expansion (VE) in the Necturus suggest a ma jor 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 paracel lular transport. In the present study, during in vivo microperfusion, lanthanum was used as an extracellular marker to determine bidirection al paracellular flux in the rat proximal tubule before and during 10% VE. Lanthanum itself did not affect proximal tubule reabsorption (Delt a 0.7 +/- 3.3 nl/min, LaCl3 versus saline infusion, n = 7). When lanth anum 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-i nterstitium 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 pe r mm, P < 0.05). When lanthanum was infused interstitially, by means o f a chronically implanted matrix, there was significant paracellular l anthanum influx from the interstitium into the lumen (143.9 +/- 18.6 p g/min per mm, n = 4). Subsequent VE significantly increased this inter stitium-to-lumen influx to 212.1 +/- 29.2 pg/min per mm as proximal re absorption was significantly decreased by 58% (Delta -2.8 +/- 0.8 nl/m in per mm, P < 0.05). Thus, VE affects bidirectional paracellular flux in a manner that would decrease proximal reabsorption; paracellular e fflux from the lumen to the interstitium was decreased, whereas parace llular influx from the interstitium to the lumen was increased.