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, we measured absorption rates of fluid [J(V)] and HCO3- [J(HCO3-)] 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. Electron microscopy confirmed that each JMPCT was S1. During the 1st 4 wk of postnatal life, J(V) and J(HCO3-) changed relatively little, averaging 0.39 ± 0.03 nl·min-1·mm-1 and 32.4 ± 2.6 pmol·min-1·mm-1, respectively. An increase of more than 100% was observed in J(V) and J(HCO3-)-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 J(HCO3-) is mediated by Na+-H+ exchange and dependent on low cell Na+, we examined the development of another Na+-dependent lumen-to-bath transport system, that for glucose [J(Glc)]. The maturational pattern was similar, with J(Glc) averaging 34.1 ± 2.4 pmol·min-1·mm-1 during the 1st 4 wk of life and then increasing 70% to 56.7 ± 5.3 by 5-6 wk of age. We conclude that the rapid increases in the rate of fluid, HCO3-, and glucose transport follow by approximately 1 wk the maturational changes in the surface areas of the apical, mitochondrial, and basolateral membranes. We hypothesize that 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.
|Original language||English (US)|
|Journal||American Journal of Physiology - Renal Fluid and Electrolyte Physiology|
|State||Published - 1983|
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