1. Rate of urine formation is very sensitive to potassium concentration.

2. Potassium is concentrated in the urine by a mechanism which is independent of other monovalent cations. Rubidium, caesium and sodium are also capable of maintaining a flow of urine. At low external potassium concentrations, sodium stimulates potassium secretion.

3. Rate of urine secretion is stimulated by low osmotic pressures; the osmotic pressure of urine was slightly hypertonic throughout the range of external osmotic pressure employed. Addition of sucrose depresses rate of urine secretion; the potassium concentration of the urine increased by 1 mM/l. for each 2 mM/l. of sucrose added to the bathing medium.

4. Urine formation is insensitive to sulphanilamide, acetazolamide, ouabain and a wide variation of pH.

5. These observations are discussed in relation to the hypothesis that potassium secretion takes place across both surfaces of the cell. The pump on the basal surface may be a coupled sodium-potassium pump, whereas that on the apical surface may be electrogenic.

6. Microvilli at the apical surface or channels formed by a complex infolding of the basal plasma membrane may represent structural devices by which standing osmotic gradients can be established during solute-linked water transport across the cells of Malpighian tubules.

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