POTASSIUM ACCUMULATION IN THE PROXIMAL CONVOLUTED TUBULES OF THE FROG'S KIDNEY.

1. In a manner similar to that of the sartorius muscle, the isolated kidney of the frog can accumulate K against a gradient to upwards of three times its normal concentration. 2. The K-accumulating region is identified as the proximal tubule, which in the isolated tissue immersed over 24 hours in the cold (2-3 degrees C.) amounts to about 90 per cent of the nephron minus the glomerulus. In the fresh tissue it constitutes about 70 per cent. The cells of the proximal tubule are impermeable to Na, but freely permeable to K and Cl. 3. The distal tubule in the isolated kidney does not accumulate K over the external concentration. The cells are permeable to Na which they actively extrude. This extrusion of Na goes parallel with a loss of osmotically associated water amounting to about 15 per cent of the weight of the fresh kidney, but varying somewhat with the conditions. 4. The accumulation of K in the proximal tubules is in accordance with the equations established for the sartorius muscle, and, as theoretically expected, there is no volume increase (but rather a small decrease) with the large accumulations, when the external Na concentration is maintained throughout. 5. With K accumulation in isotonic mixtures large volume changes occur as K is progressively substituted for Na. Over the range of external K concentration of 10 to 100 mM per litre the weight of the whole kidney changes to 2.5 times and the water of the cells of the proximal tubules increases to over four times. Up to an external K value of 90 mM per litre the mean weight of the kidney shows a linear relation when plotted against the reciprocal of the Na concentration plus the small glucose and Ca concentration. This relation is interpreted theoretically. 6. The effect of cyanide in the isotonic mixtures is to prevent the contraction of the distal tubules and to cause swelling of the same. It does not affect the volume, volume changes, or differential permeability of the proximal tubule. At the same time the membranes of the proximal tubule cells lose their characteristic permeability at a lower level of distension in the presence of cyanide. 7. The mean Na ratio for the kidney after 24 hours' immersion in the cold is 0.26 +/- 0.014 (giving standard deviation of mean). The ratio is defined as See PDF for Equation. For the fresh kidney the mean ratio is 0.39 +/- 0.006. 8. The mean inulin ratio (28 observed in the cold) is 0.23 +/- 0.012 and the same value for 10 observed at room temperature. At room temperature-2 hour immersion-the ratio is increased by cyanide to a mean of 0.32 +/- 0.028, but only a slight increase is caused by cyanide in the cold. 9. The mean hemoglobin ratio after 24 hours' immersion in the cold is 0.17 +/- 0.004 and is unaffected by cyanide.