Micropuncture and microanalytical techniques were used to study the effect of antidiuresis and water diuresis on osmolality, bicarbonate concentration, and water reabsorption in the proximal tubule of the dog nephron. Samples collected during antidiuresis and water diuresis remained isotonic to plasma throughout the first 50% of the proximal convoluted tubule. Mean bicarbonate concentrations of 16 mEq/liter and 17 mEq/liter were found in the middle third of the tubule during antidiuresis and water diuresis, respectively. These values were slightly less than the plasma concentration of 22 mEq/liter. Proximal tubular fluid samples for inulin concentration were collected during antidiuresis, water diuresis, and during vasopressin infusion in water-loaded dogs. A mean tubular fluid to plasma (TF/P) inulin ratio of 2.3 was found in the middle third of the proximal tubule during antidiuresis. This value is significantly different ( P < 0.01) from a mean of 1.6 in the same portion of the tubule during water diuresis. Vasopressin administration to hydrated dogs returned the TF/P inulin ratio in the middle third of the proximal tubule to 2.0. These results suggest that vasopressin stimulated Na reabsorption in the proximal tubule of the dog nephron.
Samples of fluid from the proximal tubule were collected for the measurement of pH and bicarbonate concentration before and after the administration of acetazolamide (Diamox). Samples collected before acetazolamide were consistently more acid than plasma with the most acid samples coming from the more distal portion of the proximal tubule. After the intravenous administration of acetazolamide, the pH and bicarbonate concentration were consistently higher than in plasma. Bicarbonate concentrations as high as 2.8 times that in plasma were observed. The rise in proximal tubular fluid bicarbonate concentration after acetazolamide is presumably due to a reduction in the rate of bicarbonate reabsorption out of proportion to any impairment in proximal tubular fluid volume reduction.
A number of investigators (1-9) have recently studied the potassium concentration in the proximal tubule of rat and Necturus. Unfortunately, there are significant differences in the results obtained by different observers. The general conclusion, however, that potassium is reabsorbed against an electrochemical gradient in the proximal tubule of the rat seems justified. Furthermore, if one compares the ratio of the concentration of potassium in tubule fluid to that in plasma (TF/P K) to the corresponding ratio for inulin, potassium appears to be extensively reabsorbed from the proximal tubule.The study reported in this paper was undertaken in an attempt to define the mechanism by which potassium is handled in the proximal tubule of the dog nephron. It was made possible by the development of a sensitive ultramicromethod for the determination of potassium in biological fluids and the adaptation of micropuncture techniques to the study of renal function in dog (10). The concentrations of inulin and potassium were determined in the same samples of proximal tubule fluid. In separate sets of experiments, proximal transtubular potential differences were also determined. The concentrations of inulin and potassium in tubule fluid were compared with the respective concentrations in plasma in order to evaluate net potassium reabsorption, and the TF/P K ratio was compared with equilibrium ratios calculated from the transtubular potential by the Nernst equation.The results of this study differ quantitatively, although not qualitatively, from those reported by others using rat and Necturus as experimental * Submitted for publication July 12, 1963; accepted November 26, 1963. Presented in part at the 55th Annual Meeting of the American Society for Clinical Investigation, April 28, 1963. t Special Public Health Fellow. animals. To determine whether this difference was related to species differences or to methodology, a limited number of proximal TF/P K ratios were also determined in rats and Necturus. MethodsStudies were performed on male and female mongrel dogs weighing from 9 to 15 kg. The dogs were anesthetized by rapid injection of 200 mg of sodium pentothal administered intravenously as a 2.5% solution in distilled water; small additional doses were administered from time to time to maintain the desired level of anesthesia. The left kidney was exposed through a flank incision, and indwelling catheters were inserted into the ureter of the experimental kidney, the foreleg vein, and the femoral vein. Methods of exposing and supporting the kidney for micropuncture as well as the techniques of tubule fluid collection and identification have previously been reported (10). The samples of tubule fluid were generally 0.03 to 0.05 IAI in volume and were collected in 5 minutes or less.In all dog experiments, 15-minute urine collection periods for the determination of inulin and potassium clearances were obtained while tubule fluid samples were being collected. After appropriate priming and sustaining infusions of inulin ha...
Previous micropuncture studies ( 1,2) have demonstrated that potassium is extensively reabsorbed in the proximal tubule under conditions in which the net excretion of potassium in th-e urine varies, supporting the hypothesis that major adjustments in the rate of potassium excretion occur in distal parts of the nephron (3). To further evaluate the reabsorptive process in the dog nephron, fractional reabsorption of potassium from the proximal tubule was studied by micropuncture techniques during reductions in glomerular filtration rate (GFR) produced by renal arterial constriction, during proximal tubular inhibition of sodium reabsorption produced by isotonic saline infusion (4, 5), and during stimulation of potassium secretion by potassium loading and acetazolamide administration. MethodsMale and female mongrel dogs weighing between 9 and 18 kg were used as experimental animals. The dogs were anesthetized by the rapid injection of 200 mg of thiopental sodium administered intravenously as a 2.5% solution in distilled water; small additional doses were administered from time to time to maintain the desired level of anesthesia. Indwelling catheters were inserted into the ureters through a suprapubic midline incision. Polyethylene catheters were also inserted into the foreleg veins and the femoral vein or artery for the administration of fluids and collection of blood. The left kidney (experimental) was exposed through a flank incision and prepared for micropuncture as previously reported (6). Methods of tubular fluid collection and identification were also similar to those previously reported (6).In all experiments, 15-minute urine collection periods for the determination of inulin and potassium clearances * Submitted for publication December 9, 1965; accepted May 18, 1966. Supported by grant AM-07352-03 from the National were obtained while tubular fluid samples were being collected. After appropriate priming and sustaining infusions of inulin had been given, 30 minutes was allowed to elapse before collections were begun to insure stable concentrations of inulin in plasma. Venous or arterial blood was drawn at the midpoint of each period, and urine was collected through the ureteral catheters.Four groups of dogs were studied: 1) normal hydropenia, 2) hydropenia with renal arterial constriction, 3) potassium loaded with acetazolamide administration, and 4) normal dogs undergoing isotonic saline diuresis. In the hydropenic group with renal arterial constriction, an adjustable rubber clamp was placed around the renal artery of the experimental kidney before its bifurcation. The renal artery was gradually constricted until there was an observable decrease in kidney tissue turgor, and urine flow was reduced by at least 50% as compared with the control kidney. Only those tubular fluid samples were used that were collected during clearance periods in which the GFR of the experimental kidney was 75%o or less of the control kidney.Potassium-loaded animals were fed a diet containing 20 g of KCI daily for 1 week before the ...
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