Schreiber M, Schlanger L E, Chen C B, Lessan-Pezeshki M, Halperin M L, Patnaik A, Ling B N, Kleyman T R
Department of Medicine, St. Michael's Hospital, University of Toronto, Canada.
Kidney Int. 1996 Jan;49(1):82-7. doi: 10.1038/ki.1996.11.
This study was designed to test the hypothesis that the antikaliuresis caused by trimethoprim could be diminished by alkalinizing the luminal fluid in the CCD, thereby converting trimethoprim from its cationic, active form to an electroneutral, inactive, form. Trimethoprim-induced inhibition of transepithelial Na+ transport was examined in A6 distal nephron cells by analysis of short circuit current. The voltage-dependence of the trimethoprim-induced block of Na+ channels was examined with patch clamp recordings of A6 cells. The antikaliuretic effect of trimethoprim was examined in vivo in rats pretreated with deoxycorticosterone and with NH4Cl to lower urine pH, and in rats also receiving acetazolamide to raise urine pH. We found that the concentration of trimethoprim required to inhibit the amiloride sensitive component of short circuit current by 50% (IC50) was 340 microM (at pH 8.2) and 50 microM (at pH 6.3). The IC50S of protonated trimethoprim were similar (34 microM at pH 8.2 and 45 microM at pH 6.3). The mean time open for the high selectivity, Na+ channel was reduced from 1679 +/- 387 msec to 502 +/- 98 msec with addition of 10-5 M trimethoprim to patch pipette solution at the resting membrane potential (-Vpipette = 0 mV). further decreases in mean time open were observed as -Vpipette 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 due primarily to a fall in the [K+] in the lumen of the terminal CCD. This effect of trimethoprim was markedly attenuated in an alkaline urine induced by acetazolamide. We conclude that it is the charged, protonated species of trimethoprim which blocks epithelial Na+ channels. Increasing urinary pH decreases the concentration of the charged species of trimethoprim and minimizes its antikaliuretic effect.
通过碱化集合管腔液，可减少甲氧苄啶引起的抗利尿作用，从而使甲氧苄啶从其阳离子活性形式转变为电中性的非活性形式。通过分析短路电流，在A6远端肾单位细胞中检测了甲氧苄啶诱导的跨上皮Na⁺转运抑制作用。利用A6细胞的膜片钳记录检测了甲氧苄啶诱导的Na⁺通道阻断的电压依赖性。在预先用脱氧皮质酮和NH₄Cl预处理以降低尿液pH值的大鼠以及同时接受乙酰唑胺以提高尿液pH值的大鼠体内，检测了甲氧苄啶的抗利尿作用。我们发现，将短路电流的氨氯地平敏感成分抑制50%（IC₅₀）所需的甲氧苄啶浓度在pH 8.2时为340 μM，在pH 6.3时为50 μM。质子化甲氧苄啶的IC₅₀相似（pH 8.2时为34 μM，pH 6.3时为45 μM）。在静息膜电位（-Vpipette = 0 mV）下，向膜片钳微管溶液中添加10⁻⁵ M甲氧苄啶后，高选择性Na⁺通道的平均开放时间从1679 ± 387毫秒减少至502 ± 98毫秒。随着-Vpipette降低（即顶端膜超极化）至-40 mV（平均开放时间 = 217 ± 85毫秒）和-80 mV（平均开放时间 = 69 ± 13毫秒），平均开放时间进一步减少。在体内，甲氧苄啶导致钾（K⁺）排泄减少> 50%，这主要是由于终末集合管腔中[K⁺]下降所致。在乙酰唑胺诱导的碱性尿液中，甲氧苄啶的这种作用明显减弱。我们得出结论，是带电荷的质子化甲氧苄啶物种阻断了上皮Na⁺通道。提高尿液pH值可降低带电荷的甲氧苄啶物种的浓度，并使其抗利尿作用最小化。
