Development of solute transport in rabbit proximal tubule. I. HCO-3 and glucose absorption

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Abstract
Infants have a lower renal threshold for HCO3- than adults. To determine whether this phenomenon is due to factors intrinsic to the tubular transporting systems, absorption rates of fluid (Jv) and .**GRAPHIC**. .**GRAPHIC**. were measured in isolated perfused early proximal convoluted tubules (JMPCT) taken from the juxtamedullary renal cortex of developing rabbits. Artificial solutions simulating plasma ultrafiltrate were used to perfuse and bathe the tubule. The bath also contained 6 g/dl albumin. EM confirmed that each JMPCT was S1. During the first 4 wk of postnatal liufe, Jv and .**GRAPHIC**. changed relatively little, averaging 0.39 .+-. 0.03 nl .cntdot. min-1 .cntdot. mm-1 and 32.4 .+-. 2.6 pmol .cntdot. min-1 .cntdot. mm-1, respectively. An increase of > 100% was observed in Jv and .**GRAPHIC**. during the 5th and 6th wk to 0.80 .+-. 0.06 and 67.5 .+-. 8.0, respectively, levels nearly equal to those observed in JMPCT obtained from adult rabbits (1.27 .+-. 0.17 and 89.4 .+-. 18.0, respectively). Because .**GRAPHIC**. is mediated by Na+-H+ exchange and dependent on low cell Na+, the development of another Na+-dependent lumen-to-bath transport system, that for glucose (JGlc) was examined. The maturational pattern was similar, with JGlc averaging 34.1 .+-. 2.4 pmol .cntdot. min-1 .cntdot. mm-1 during the first 4 wk of life and then increasing 70% to 56.7 .+-. 5.3 by 5-6 wk of age. The rapid increases in the rate of fluid, .**GRAPHIC**. and glucose transport follow by .apprx. 1 wk the maturational changes in the surface areas of the apical, mitochondrial and basolateral membranes. The development of either the rate of nephron filtration, the number of luminal Na+ transporters, or the activity of basolateral Na+-extruding pumps may mediate the changes in solute transport observed in the maturing proximal tubule.

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