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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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323changed by addition of choline. In one of 4 experiments betaine caused a significant acceleration of abscission but in the other experiments only a small effect was observed.These results with betaine and choline additions are not unexpected since most enzyme systems are quite specific in their action. The slight effect of betaine can be explained by the greater molecular similarity of betaine to methionine. In addition, betaine, like methionine, occurs naturally in plants. Additional experiments concerning enzymes in abscission are needed to characterize an overall mechanism of the phenomenon of abscission.Summary. Methionine (presumably as a methyl donor) accelerates floral abscission in tobacco. IAA retards abscission. When the two factors are added to developing ovaries in combination, in general the effect is one of diminishing the effect of either when added alone. However, high concentrations of IAA completely overcome the accelerating effect of methionine.

The antidiuretic hormone regulates the ability of the kidney to concentrate urine. There are two theories as to the nature of the renal concentrating mechanism: 1) the existence of an active water pump in a distal area which removes free water from the urine and 2) the existence of a countercurrent multiplier system in the renal pyramid which allows for passive movement of water out of the collecting ducts in response to an osmotic gradient. During stop flow, the kidneys are unable to alter appreciably the total osmotic pressure of urine. Measurement of Na concentrations along the length of the renal pyramid may be used as an index of the magnitude of the countercurrent gradient. Such determinations were made before and during mannitol diuresis in dogs. An osmotic gradient along the pyramids was evident in control kidneys and disappeared upon establishment of osmotic diuresis. These data suggest that high rates of urine flow through the loop of Henle may wash out the pre-existing countercurrent gradient. This accounts for the inability of the kidneys to concentrate stop flow urine. It also demonstrates that osmotic diuresis, which is generally thought of as only a proximal event, must also result from diminished distal water reabsorption due to the washout of the countercurrent gradient.