Abstract
Vasa recta blood was sampled from the exposed renal papillae of 11 Munich-Wistar rats (Group I). The mean vasa recta plasma-to-systemic plasma (VR/P) inulin concentration ratio was 5.4±1.2 (SE), which is significantly higher than the mean VR/P protein ratio previously reported (1.74 in descending vasa recta and 1.38 in acending vasa recta). Even if glomerular and vasa recta capillaries are as impermeable to inulin as to protein, VR/P inulin concentration ratios should not exceed VR/P protein concentration ratios, unless inulin is added to vasa recta from an extracapillary source. To determine if inulin can permeate the epithelial lining of the papilla, we bathed exposed papillae with concentrated inulin solutions (5 rad 10 g/ 100 ml). In 7 rats (Group II) the inulin concentration in 23 VR plasma samples ranged from 0 to 492 mg/100 ml, exhibiting large variations within individual rats. A third group of 12 rats, infused intravenously with inulin, exhibited the same pattern of variability. We interpret these data as evidence for occasional contamination of vasa recta blood by urine in adjacent collecting tubules or bathing the papilla.
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