Renal interstitial volume of the rat kidney

The intravascular plasma volume and the interstitial volume of the rat kidney were determined by an indicator dilution method with a bolus injection of 125-I-labelled human serum albumin and 51-Cr-EDTA into the renal artery and subsequent recording of the indicator-dilution curves in the venous effluent, sampled at intervals of 0.33 s. The curves allowed determination of both the mean transit time of the two indicators and total renal plasma flow. The volumes of distribution were obtained by multiplying these two factors. Under control conditions the plasma volume was 93.0 +/- 11.2 microliters/100 g rat (mean +/- SE), which is 17.9% of the total kidney volume. The interstitial volume was 68.3 +/- 8.6 microliters/100 g, corresponding to 13.1% of the total kidney volume. During expansion with 0.15 M NaCl, 10% of body weight, the plasma and interstitial volumes were not significantly increased. The values for the two volumes were 101.6 +/- 9.7 and 72.8 +/- 6.8 microliters/100 g, respectively. The kidney weight showed, in contrast a clear increase from 539 to 670 mg/100 g, reflecting the expansion of the proximal and distal tubules due to the increased glomerular filtration rate. It is concluded that although the saline load produced a rise in the renal interstitial pressure, the expected expansion of the interstitium became small, due to the parallel expansion of both the vascular and tubular systems which compress the renal interstitium.