Renal responses to diuretic drugs in freshwater catfish Ictalurus punctatus.

Renal tubules of freshwater teleosts consist of proximal, intermediate, distal, and collecting segments. Diuretic drugs were injected into freshwater channel catfish to define the mode of natriuresis and diuresis. Although the catfish lack a loop of Henle, ethacrynic acid (1 mg/kg), furosemide (1 mg/kg) and, to a smaller extent, hydrochlorothiazide (2 mg/kg) produced marked chloruresis and natriuresis. The amount of sodium and chloride reabsorbed decreased, and the decrease in fractional reabsorption of chloride was greater than that of sodium. All three drugs, however, elicited only mild diuresis. This finding suggests that these drugs inhibit sodium and chloride reabsorption in tubules which are highly impermeable to water, most likely along distal segments of the nephron. Ethacrynic acid and furosemide appear to inhibit both reabsorption and secretion of potassium. Acetazolamide (50 mg/kg) induced moderate diuresis which seemed to reflect increased glomerular filtration rate. Smaller doses of acetazolamide increased urinary bicarbonate excretion without the increase in filtration rate, suggesting a possible inhibition of carbonic anhydrase in the renal tubules.