Sodium excretion and renal precession of sodium over inulin.

Renal excretory patterns for inulin and 22Na were measured in rats after simultaneous injection into a renal artery. In normal rats, the excretion of 22Na was much faster than inulin. This is referred to as "precession of sodium" and showed that some of the excreted 22Na had bypassed the intratubular route. During water diuresis, 65% of excreted 22Na was excreted before inulin. The quantity of 22Na preceding inulin (0.10 +/- 0.03% of the amount injected) increased eightfold during infusion of atrial natriuretic peptide (ANP). In saline diuresis, 45% of excreted 22Na (1.47 +/- 0.39% of the amount injected) preceded inulin, but precession was not affected by ANP. In rats with a nephrotic syndrome, 13% of excreted 22Na (0.34 +/- 0.12% of the amount injected) preceded inulin during saline diuresis and increased to 1.66 +/- 0.53% of the amount injected during the infusion of ANP. Precession of sodium was completely abolished in kidneys with papillary necrosis, suggesting that 22Na entered the tubular lumen in the inner medulla. These experiments demonstrate that, in rats, a large fraction of excreted sodium enters the tubules by a route other than filtration. The quantity of sodium entering the tubules by this route shows a 10- to 15-fold variation under conditions that modify the rate of sodium excretion.