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Transepithelial electrochemical gradients in the proximal convoluted tubule during potassium depletion in the rat

D. G. Shirley, S. J. Walter, E. J. Folkerd, R. J. Unwin * and M. A. Bailey

In order to examine the electrochemical gradient for potassium reabsorption across the S₂ segment of the proximal convoluted tubule, transepithelial potential differences and transepithelial potassium concentrations were measured in anaesthetized potassium-replete and potassium-depleted rats.

Potassium-depleted rats were markedly hypokalaemic (plasma potassium, 1·4 ± 0·1 vs. 4·1 ± 0·1 mmol l^-1 in potassium-replete rats) and had a significantly reduced muscle potassium content. In confirmation of previous reports, glomerular filtration rate was slightly reduced, while fractional reabsorption in the proximal convoluted tubule was enhanced.

In potassium-replete animals, the transepithelial potential difference (PD) at the late proximal convoluted tubule was +2·1 ± 0·3 mV (lumen positive) and the tubular fluid to plasma ultrafiltrate concentration ratio for potassium (TF_K/UF_K) at the same site was 1·03 ± 0·01. In potassium-depleted rats, there was a striking reversal of the transepithelial PD (to -4·0 ± 0·4 mV), while the TF_K/UF_K was increased to 1·19 ± 0·03.

The data from both potassium-replete and potassium-depleted animals are consistent with accumulating evidence that potassium reabsorption in the proximal convoluted tubule is passive in nature and depends partly on diffusion down an electrochemical gradient.

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