Evidence for the O-sulfo derivative of MK-447 as active metabolite of MK-447.

In contrast with furosemide (and other sulfamoylbenzoic and aryloxyacetic acids loop diuretics), MK-447 was unable to inhibit the [Na+,K+,Cl-] cotransport system in human red blood cells. Indeed, this compound was a very poor ion transport inhibitor (inactive on Ca(2+)-sensitive K+ channels, the Ca2+ pump, the Na+:Mg2+ exchange, the Na+:Li+ countertransport system and the [K+,Cl-] cotransport system, and only inhibiting the [Cl-/HCO3-] exchanger and the Na+,K+ pump at high concentrations). Conversely, its urinary metabolite (O-sulfo)-MK-447 was a very potent inhibitor of the [Na+,K+,Cl-] cotransport system (IC50 of 1.6 +/- 0.5 x 10(-6) M; mean +/- S.D. of four experiments). This compound was a much more potent [Na+,K+,Cl-] cotransport inhibitor than furosemide, and almost as active as bumetanide. In addition, (O-sulfo)-MK-447 was a moderate inhibitor of the [Cl-/HCO3-] exchanger (IC50 of 6 +/- 3 x 10(-5) M, n = 3), its potency being intermediate between that of xipamide and that of furosemide. Interestingly, it exhibited some inhibitory activity against Ca(2+)-sensitive K+ channels but only at high concentrations (it had no effect on the [K+,Cl-] cotransport system, the Ca2+ pump or the Na+:Mg2+ exchanger). The results suggest strongly that the O-sulfo derivative of MK-447 is an active natriuretic metabolite of MK-447. This metabolite may be responsible for the salidiuretic action of MK-447.