Localization of Nephron Transport by Stop Flow Analysis

¹ From the Department of Physiology, School of Medicine, University of Michigan, Ann Arbor, Michigan

Localization of transtubular transport of substances along the nephrons of a pentobarbitalized dog is characterized by allowing concentration patterns to develop during stopped tubular flow (ureteral occlusion with stopped filtration). Previous administration of an osmotic diuretic mannitol provides a watery menstruum against which characteristic concentration patterns are developed point to point along the nephrons. On release of occlusion the diuretic flush washes the concentration pattern along each nephron. Serial urine samples, delivered from a polyethylene catheter, segment this pattern into an ordered array. Plots against time show distortion of the concentration pattern due to variable flow rate. The filtration indicator inulin is injected late after occlusion to reach the filter surfaces and to signal new entry of filtrate into the urine samples after the occlusion is released. The rising concentration pattern of this inulin delineates the randomizing effect of variable lengths among nephrons. Para-aminohippurate (PAH) injected after occlusion signals entry sites for secretion. These appear ahead of inulin. If glucosuria is developed before occlusion such that the glucose reabsorptive transfer maximum is exceeded, the occlusion now allows further time for lowering of glucose concentration along the tubule. The lowered glucose concentration pattern overlaps that of rising PAH, the PAH here being continuously infused beginning before occlusion. PAH secretion is proximal in the mammal. Very distal fluid is relatively sodium free; Na concentrations in proximal areas are not lowered below that accomplished during free diuretic flow. Phosphate shows only reabsorption and a proximal area overlapping glucose. Total measured blood flow is unimpaired during 8-minute occlusions. Plasma PAH extraction ratios are unchanged. Postocclusively injected K⁴² and Na²⁴ reach all stop flow samples. Evidently peritubular blood flow continues to reach all points along the nephron.

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