Sites and mechanisms of action of diuretics in the kidney

Utilization of classical clearance methodology for the determination of diuretic mechanism and site of action is based on four kinds of observations: (1) the effects of diuretic agents on the concentrating and diluting mechanism; (2) an analysis of the pattern of anionic excretion produced by the drug; (3) determination of the action of the agent on acid excretion and on acid-base status; (4) an evaluation of the effects of the diuretic on potassium excretion. Agents (such as acetazolamide) that act in the proximal convoluted tubule cause an enhancement of solute-free water generation (CH2O), induce a phosphaturia and an increase in bicarbonate excretion, alkalinize the urine, and cause a kaliuresis. Those agents which inhibit sodium chloride transport in the loop of Henle (for example, furosemide and ethacrynic acid) reduce both CH2O and the abstraction of tubular water from the collecting duct (TCH2O). They are the most potent natriuretic agents currently available, causing increments in the excretion of sodium in the urine of 15 to 25 per cent of filtered load. Those drugs which act in the early portion of the distal convolution (the thiazides, metolazone) reduce CH2O modestly or not at all and have no effect on TCH2O. They are capable of increasing urinary sodium by 5 to 8 per cent of the filtered load. The special-purpose agents such as triamterene and spironolactone are only mildly effective as natriuretic agents. While they augment fractional sodium excretion by only 2 to 3 per cent, they are useful because of their capacity to reduce urinary potassium excretion, either by a direct renal tubular effect (triamterene) or by competitive inhibition of aldosterone (spironolactone).