Conductive properties of the proximal tubule in Necturus kidney.

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Abstract
The electrical properties of the proximal tubule of the in vivo Necturus 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 (VC) 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 1030 .mu.m provided a good estimate of the transepithelial resistance, specific resistance (RTE = 420 .OMEGA. .cntdot. cm2) and/or per unit length (rTE = 1.3 .times. 104 .OMEGA. .cntdot. 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 (rB), and shunt pathway (rS) were estimated by 2 independent methods at 3.5 .times. 104, 1.2 .times. 104, and 1.7 .times. 104 .OMEGA. .cntdot. cm, respectively. The corresponding specific resistances were close to 1200, 600, and 600 .OMEGA. .cntdot. cm2. 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. 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.