Albumin multiplier in kidney vasa recta analyzed by microspectrophotometry of T-1824

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Abstract
Proof of counter-current multiplication requires demonstration of an albumin gradient along entire course of vase recta where not accessible to micro-puncture. Evans blue (T-1824) was measured 5 min. after iv injection into hamsters, in uniform 7-[mu] tissue sections cut from frozen-dried papilla. A cross section of a vas rectum, dyed blue, is centered in the diaphragm of the microscope as the photometric field. Color is measured under a dual-beam, microspectrophotometer using the 2-wave length method (650 and 677 m[mu]). Dye concentration rises along descending vasa where they occur in isolated bundles in the outer medulla. Opposite the lower end of the thick limbs of Henle, the concentration is 2 times that in other tissue capillaries used as standards. Here lateral transudation exceeds the normal standard. Below this level albumin rises further to a peak 3 mm above the papilla tip. Here vasa fan out, descending not often touching ascending. Interstitial albumin, as opposing vector, must enable further transudation as it forms with parallel ascending vasa a return limb for countercurrent. Only here do we see interstitial Evans blue. Massive lateral transudation reduces plasma flow correspondingly; hence the rate of entry lower into the papilla of indicators commonly used to measure circulation time and blood flow.