Loop diuretic infusion increases thiazide-sensitive Na+/Cl--cotransporter abundance: Role of aldosterone

Chronic infusion of loop diuretics into animals induces structural and func tional changes in the distal nephron. These changes include increases in th e activity of the thiazide-sensitive Na+/Cl--cotransporter (NCC). The NCC w as recently demonstrated to be an aldosterone-induced protein. These experi ments were designed to test the hypotheses that chronic loop diuretic infus ion, with replacement of NaCl losses, increases NCC protein abundance and t hat this effect results, in part, from stimulation by aldosterone. Sprague- Dawley rats received vehicle (group 1), furosemide (22 mg/ 100 g body wt pe r d) (group 2), or furosemide plus spironolactone (22 and 20 mg/100 g body wt per d, respectively) (group 3). Urine output was higher for groups 2 and 3 than for group 1 (151 +/- 32, 149 +/- 24, and 12 +/- 4 mi, respectively; P < 0.0001). Immunoblot analysis of NCC protein demonstrated that loop diu retics increased NCC protein abundance by nearly 100% (from 2562 +/- 30 to 5248 +/- 151 arbitrary units, P < 0.01). Spironolactone decreased NCC prote in abundance by (66% (to 3532 +/- 113 units), compared with the furosemide- treated group (P < 0.005). Northern blot analysis of NCC mRNA demonstrated no significant effect of furosemide (NCC/glyceraldehyde-3-phosphate dehydro genase ratios: group 1, 0.6 +/- 0.12; group 2, 0.5 +/- 0.05; P > 0.05, NS) These results indicate that increased NCC activity during chronic loop diur etic infusion is associated with increases in NCC protein abundance. A port ion of the furosemide effect can be prevented by blockade of mineralocortic oid receptors.