Conductive properties of the proximal tubule in Necturus kidney.

Abstract

The electrical properties of the proximal tubule of the in vivoNecturus kidney were investigated by injecting current (as rectangular waves) into the lumen or into the epithelium of single tubules and by studying the resulting changes of transepithelial (VL) and/or cell membrane potential (V c) at various distances from the source. In some experiments paired measurements of VL and Vc were performed at two abscissas x and x'. The luminal length constant of about 1,030 pm was shown to provide a good estimate of the transepithelial resistance, specific resistance (Ra'E = 420 ~. cm 2) and/or per unit length (ram = 1.3 X 104 ~.cm). The apparent intraepithelial length constant was subject to distortions arising from concomitant current spread in the lumen. The resistances of luminal membrane (rL), basolateral membrane (rs), and shunt pathway (rs) were estimated by two independent methods at 3.5 • 104, 1.2 • 104, and 1.7 X 104 ~.cm, respectively. The corresponding specific resistances were close to 1,200, 600, and 600 ~.cm 2. There are two main conclusions of this study. (a) The resistances of cell membranes and shunt pathway are of the same order of magnitude. The figure of the shunt resistance is at variance with the notion that the proximal tubule of Necturus is a leaky epithelium. (b) A rigorous assessment of the conductive properties of concentric cylindrical double cables (such as renal tubules) requires that electrical interactions arising from one cable to another be taken into account. Appropriate equations were developed to deal with this problem.

Cellular and paracellular resistances of theNecturus proximal tubule

“…Although subject to similar variation in the physiological status of the kidney and in the experimental technique, reported values of 2 C in free-flow tubules are less variable than reported values of 2 t. For example, Windhager et al [22] and Anagnostopolous and Velu [2] reported values for 2~r of 200 ~tm and Anagnostopolous et al [1] values of 2 c from 214 to 250~tm for the Necturus proximal tubule and a value of 248 gm is reported in the present study.…”

“…Spatial decay of current in one cable is unaffected by the other cable. Recently, Anagnostopolous et al [1] have suggested that the measurements of cellular cable properties in the Necturus proximal tubule by means of intracellular current injection as shown in method d of this paper may be inaccurate because of a significant cable-to-cable interaction or "cross talk. "…”

Resistance properties of the diluting segment ofAmphiuma kidney: Influence of potassium adaptation

“…Therefore, special care was taken to reduce the influence of "crosstalk" on the obtained resistance values. As demonstrated in the mammalian cortical thick ascending limb of Henle's loop [10] some length dependence of the apparent cell membrane resistance ratio has been observed resembling "crosstalk" as also discussed recently for the amphibian proximal tubule [1,13,16]. Furthermore, it is known that the evaluation of the length constant of the cell cable at distances of more than 700/~m from the current source Assuming a K+-selective luminal cell membrane, Elu and extraceUular K + are used to estimate intracellular potassium.…”

Electrical properties of chloride transport across theNecturus proximal tubule

“…cm 2 [14]. However, they are less than those of Anagnostopoulos et al [4] who report values of 420 ft. cm 2 for Rte. The Rte of Anagnostopoulos et al is in the range of the resistances found across the late proximal tubule of Necturus which range from 429 [11] to 641 fl-cm 2 [14].…”