All of the loop diuretics are available for both oral and parenteral administration. Their onset of action is rapid, usually within 30 minutes after oral and 5 minutes after intravenous administration. They produce peak diuresis in about 2 hours, with a total duration of diuretic action of approximately 6 to 8 hours. Loop diuretics are extensively bound to plasma proteins and are eliminated in the urine by both glomerular filtration and tubular secretion. Approximately a third of an administered dose is excreted by the liver into the bile, from where it may be eliminated in the feces. Only small amounts of these compounds appear to be metabolized by the liver.
The loop diuretics must be present in the tubular fluid before they can become effective. Because of their extensive binding to plasma proteins, filtration across the glomerular capillaries is restricted. Like the thi-azides, however, the loop diuretics are weak organic acids that are substrates for the organic acid secretory system in the proximal tubule. A consequence of this active secretion is that the presence of other organic acids or certain forms of renal disease may impair the therapeutic usefulness of the loop diuretics.
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