Maximal Gradients of Na and K across Proximal Tubules of Kidneys of Immature Rats

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Abstract
The magnitude of proximal transtubular ionic gradients produced by mannitol loading has been measured in developing and mature rats using micropuncture techniques. Immature rats were able to establish the same sodium and potassium gradients as did adult animals, providing proximal tubular length is greater than 1,500 μm. No differences in establishment of the gradient determined as a function of tubular length was found between animals obtained from litters left intact or reduced. Animals from reduced litters weighing less than 125 g body weight had higher GFR than did animals from intact litters weighing the same. Although urinary concentrations of sodium and potassium are the same for both populations of immature rats at comparable weights, volume excretion as well as excretion of sodium and potassium is greater in animals from reduced litters.

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