Maintenance of Potassium Excretion Despite Reduction of Glomerular Filtration During Sodium Diuresis

It has been clearly established that mammalian renal tubules can secrete potassium. However, it has not been shown in mammals under physiologic conditions what proportion of the urinary potassium is derived from the filtered potassium and what proportion from secreted potassium. In experiments designed to study the various factors which influence potassium excretion it has sometimes been assumed that the reabsorption of the filtered potassium is essentially complete, and that these factors exert their effects on the secretory mechanism. However, evidence for the completeness of reabsorption of filtered potassium is limited and not conclusive. The proposed tubular mechanism for the secretion of potassium is an ion exchange process in the distal tubule (1), where cellular potassium ions are exchanged for sodium ions derived from the glomerular filtrate. It is implicit in this mechanism that potassium secretion will depend in part on the availability of sodium ions for exchange. If the proximal reabsorption of filtered potassium is complete, the rate of excretion of potassium will be independent of the filtered load of potassium, providing an adequate distal load of sodium is maintained. To test this hypothesis, the glomerular filtration rate was reduced in one kidney and the rate of excretion of potassium from this kidney was compared to that of the control kidney, during both high and low rates of sodium excretion.