Lack of effect of cicletanine and its sulfoconjugated metabolite on thiazide receptor expressed in Xenopus oocytes

Although the renal receptor at which cicletanine acts is unknown, cicletani ne was assumed to act like thiazide diuretics. Here we tested cicletanine a nd its natriuretic metabolite, cicletanine-sulfate, for inhibitory activity against the thiazide-sensitive NaCl cotransporter expressed in Xenopus ooc ytes. The renal thiazide-sensitive Na-CI cotransporter was expressed in Xen opus laevis oocytes injected with rat cRNA TSCr (TSCr : thiazide-sensitive cotransporter from rat kidney) and both, racemic (+/-) cicletanine and its sulfoconjugated metabolite were tested for inhibitory activity against oocy te Na-22(+) uptake catalyzed by this cotransporter. Polythiazide was used a s reference thiazide. Polythiazide fully inhibited NaCl cotransporter funct ion with IC50 approximate to 1.2 x 10(-7) M. Conversely, neither cicletanin e, nor cicletanine sulfate were able to inhibit such cotransporter, i.e.: a minimum concentration of 10(-4) M of cicletanine was necessary to induce a slight cotransporter inhibition (29.5 +/- 18.2 %). Cicletanine sulfate was inactive, even at 10(-4) M. In conclusion : (i) the natriuretic metabolite of cicletanine (cicletanine sulfate) is unable to inhibit thiazide-sensiti ve NaCl cotransporter and (ii) inhibition of such cotransporter by cicletan ine required concentrations equal or higher than 10(-4) M - concentrations much more higher than urinary therapeutic ones in humans (approximate to 10 (-6) M). These results clearly demonstrate that cicletanine does not act li ke thiazide diuretics.