Micropuncture Study of Potassium Concentration in Proximal Tubule of Dog, Rat, and Necturus*

Abstract

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...

Micropuncture study of nephron function in the rhesus monkey

“…The conclusion that potassium reabsorption in the proximal convoluted tubule is passive is at variance with the conclusions of several previous studies in which TF/P potassium concentration ratios significantly above and below one were found (and in which a significant transtubular electrical potential was assumed) (31)(32)(33)(34)(35). We are unable to explain the difference between the concentration ratios found in this study and previous studies.…”

“…A recent study of the electrical potential in the rat nephron by Frdmter and Hegel led them to the conclusion that there is no electrical potential difference between the proximal tubule lumen and the surrounding interstitial fluid (30). Their primary observations were essentially identical with those obtained by other workers who have measured the electrical potentials in the proximal convoluted tubules of rats (2,31,32) and dogs (33). When the tubule is first punctured a negative deflection is encountered; in a large number of studies, these negative deflections have always varied rather widely but have generally averaged close to 20 mv, tip of the exploring electrode negative.…”

Etude par microponction de I’élaboration de I’urine

“…Une valeur moyenne du (F/P)k voisine de 1 dans le tubule proximal, chez le rat non diurétique, a été rapportée par L it c h f ie l d et B ott [26] ; par contre, pour la plupart des auteurs ce rapport est inférieur à l'unité : le (F/P)k vaut 0,69 pour W ir z et B ott chez le rat en diurèse osmoti que [51], 0,79 pour M a rsh et al [29], 0,92 pour B loomer et al [2], 0,80 pour M a l n ic et al [27], 0,73 pour K h u r i et al [22] chez le rat non diurétique; cependant, des valeurs moyennes supérieures à 1 sont rapportées par Watson et al [47] : 1,30 chez le rat non diurétique et 1,09 chez des rats en diurèse saline. Toujours est-il que l'on peut estimer la quantité de potassium réabsorbée à la fin du tubule proximal comme égale environ aux 2/3 de la quantité de potassium qui a été fil trée.…”

Potassium reabsorption in the proximal tubule of the dog nephron.

“…Inulin and potassium were determined in duplicate from the same sample of tubular fluid by methods previously described (1). Inulin in plasma and urine was determined by the macroanthrone method of Fuhr, Kaczmarczyk, and Kruittgen (7).…”

“…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.…”