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Abstract

Micropuncture and -perfusion were performed on vasa recta of the exposed renal papilla in Wistar rats in order to abolish uncontrollable changes of sodium and urea concentration within the medullary interstitium. Thereby the collecting tubules could be studied under well defined experimental conditions.
The transport of sodium in the absence of a transtubular concentration difference (Φ _Na) as well as differences in steady state concentration of sodium (ΔC _Na) at zero net flux of water and solutes were measuredin situ in these functionally isolated collecting ducts.
Four groups of animals were studied: adrenalectomized rats, control rats on a normal diet, rats on low sodium diet and normally fed rats with additional aldosterone administration.
The respective Φ _Na were 1.2, 3.1, 4.1 and 4.2·10⁻⁵ μeq×mm⁻²×sec⁻¹, while ΔC _Na were 4, 31, 98 and 93 meq/l.

One may assume that Φ _Na iso is a measure of the transport capacity of the system and that ΔC _Na at a given Φ _Na iso is inversely proportional the leak permeability. Under these conditions our data suggest, that aldosterone causes an increase of intrinsic transport capacity and at the same time a decrease of leak permeability for sodium ions.