ANTIKALIURETIC ACTION OF TRIMETHOPRIM IS MINIMIZED BY RAISING URINE PH

This study was designed to test the hypothesis that the antikaliuresis caused by trimethoprim could be diminished by alkalinizing the lumina l fluid in the CCD, thereby converting trimethoprim from its cationic, active form to an electroneutral, inactive form. Timethoprim-induced inhibition of transepithelial Na+ transport was examined in A6 distal nephron cells by analysis of short circuit current. The voltage-depend ence of the trimethoprim-induced block of Na+ channels was examined wi th patch clamp recordings of A6 cells. The antikaliuretic effect of tr imethoprim was examined in vivo in rats pretreated with desoxycorticos terone and with NH4Cl to lower urine pH, and in rats also receiving ac etazoiamide to raise urine pH. We found that the concentration of trim ethoprim required to inhibit the amiloride sensitive component of shor t circuit current by 50% (IC50) was 340 mu M (at pH 8.2) and 50 mu M ( at pH 6.3). The IC(50)s of protonated trimethoprim were similar (34 mu M at PH 8.2 and 45 mu M at pH 6.3). The mean time open for the high s electivity, Na+ channel was reduced from 1679 +/- 387 msec to 502 +/- 98 msec with addition of 10(-5) M trimethoprim to patch pipette soluti on at the resting membrane potential (-V-pipette = 0 mV). Further decr eases in mean time open were observed as -V-pipette was reduced (that is, apical membrane hyperpolarization) to -40 mV (mean time open = 217 +/- 85 msec) and to -80 mV (mean time open = 69 +/- 13 msec). In vivo , trimethoprim caused a > 50% reduction in potassium (K+) excretion du e primarily to a fall in the [K+] in the lumen of the terminal CCD. Th is effect of trimethoprim was markedly attenuated in an alkaline urine induced by acetazolamide. We conclude that it is the charged, protona ted species of trimethoprim which blocks epithelial Na+ channels. Incr easing urinary pH decreases the concentration of the charged species o f trimethoprim and minimizes its antikaliuretic effect.