K-SPARING DIURETIC ACTIONS OF TRIMETHOPRIM - INHIBITION OF NA+ CHANNELS IN A6 DISTAL NEPHRON CELLS()

Hyperkalemia complicates trimethoprim-sulfamethoxazole (TMP-SMX) thera py in over 20% of HIV-infected patients. TMP is a heterocyclic weak ba se, similar to amiloride, a ''K+-sparing'' diuretic and Na+ channel bl ocker. Apical TMP is known to inhibit amiloride-sensitive short circui t current in A6 cells, a tissue culture model for mammalian cortical c ollecting tubule principal cells [1]. We used cell-attached patch clam p techniques to investigate the effect of TMP on the 4 pS, highly sele ctive Na+ channel in the apical membrane of A6 cells grown on permeabl e supports in the presence of 1.5 mu M aldosterone. Baseline channel a ctivity at resting membrane potential, measured as NPo (N of channels x open probability), was 1.09 +/- 0.50 (N = 18). NPo (0.92 +/- 0.38; N = 9) was unchanged when 10(-3) M TMP was added to the basolateral bat h for 30 minutes. However, apical exposure with pipettes containing 10 (-3) or 10(-5) M TMP reduced NPo approximate to tenfold (0.12 +/- 0.08 ; N = 7 and 0.18 +/- 0.14; N = 12, respectively). Kinetic analysis rev ealed the appearance of a new closed state after apical TMP treatment. Another group of A6 cells were pretreated with 10(-3) M apical TMP fo r 30 minutes prior to patching with pipettes filled with TMP-free sali ne. NP, progressively rose from 0.07 +/- 0.09 to 0.87 +/- 0.23 (N = 5) as the residual TMP was diluted within the pipette. Apical or basolat eral pretreatment (30 min) with 10(-3) M SMX did not change Na+ channe l activity. In conclusion, in A6 distal nephron cells: (1) TMP reversi bly blocks highly selective Na+ channels; (2) direct interaction with the outer channel pore is required since inhibition was observed with apical, but not basolateral TMP; (3) the SMX component of TMP-SMX prep arations has no direct effect on Na+ channel activity; (4) This K+-spa ring diuretic effect likely contributes to the hyperkalemia associated with TMP therapy in HIV-infected patients.