OBJECTIVES: To determine whether pentamidine directly affects the transport of renal ions and thus provides a mechanism for hyperkalemia, which develops in as many as 100% of patients with the acquired immunodeficiency syndrome (AIDS) who receive pentamidine for more than 6 days. DESIGN: Transepithelial and single-channel electrical measurements were made on two models of distal-nephron ion transport: an amphibian distal-nephron cell line (A6) and primary cultures of rabbit cortical collecting tubules. RESULTS: Luminal bath application of pentamidine to A6 monolayers inhibited the amiloride-sensitive, short-circuit current with a 50% inhibitory concentration of 700 microM (five experiments). In the principal cell apical membranes of cortical collecting tubule primary cultures, amiloride-sensitive, 4-picosiemen Na+ channels in cell-attached patches were also identified. When the luminal membrane was directly exposed to 1.0 microM of pentamidine in the patch pipette solution, channel activity decreased by 40% (11 experiments). Channel inhibition rapidly reversed with washout of intrapipette pentamidine (four experiments). In contrast, replacement of either the luminal bath outside the patch pipette (four experiments) or the serosal bath (five experiments) with pentamidine did not significantly affect Na+ channel activity in the patches. CONCLUSIONS: Luminal or "urinary" pentamidine inhibits distal nephron reabsorption of Na+ by blocking apical Na+ channels in a manner similar to "potassium-sparing" diuretics (for example, amiloride and triamterene). This results in a decrease in the electrochemical gradients that drive secretion of distal nephron K+. Because pentamidine is eliminated through urinary excretion, this renal tubular effect provides a mechanism for pentamidine-induced hyperkalemia.