Aldosterone on sodium transport of rat distal colon in long-term adrenalectomy during acute and chronic substitution.

1. The influence of aldosterone upon water and sodium transport properties of the distal colon was studied in long-term adrenalectomy (11-29 days).2. Six groups of rats were used: I, normal (control); II, adrenalectomized; III, adrenalectomized, acutely substituted with aldosterone (200 mug/kg 4 h); IV, adrenalectomized rats receiving aldosterone simultaneously with the specific inhibitor spironolactone (40 mg/kg within 4 h); V, adrenalectomized, substituted chronically with aldosterone (2 x 75 mug/kg day); VI, adrenalectomized, substituted chronically with dexamethasone (120 mug/kg day).3. Distal colon segments were perfused in vivo with isotonic Ringer solution. In addition, a hypotonic electrolyte solution (Na(+) 111 mM) was used in groups I and II.4. In adrenalectomy (group II), net water absorption (J(v)) was significantly decreased from (normal) 54.4 mul/h cm(2)+/-10.5 (n = 9) to 41.2 mul/h cm(2)+/-7.3 (n = 4), and net Na(+) absorption (J(Na)) was decreased from 13.6 mumol/h cm(2)+3.5 to 8.5 mumol/h cm(2)+/-0.9 (isotonic perfusate). Similarly, J(v) was decreased from 54.0 mul/h cm(2)+/-8.3 (n = 4) to 37.3 mul/h cm(2)+/-4.2 (n = 7), and J(Na) from 8.6 mumol/h cm(2)+/-2.1 to 4.2 mumol/h cm(2)+/-2.1 (hypotonic perfusate).5. Acute aldosterone substitution in adrenalectomy (III) had no effect upon J(v) (37.1 mul/h cm(2)+/-10.3; n = 5) but increased J(Na) to 10.3 mumol/h cm(2)+/-0.3.6. The luminal Na(+) steady-state concentration was higher in group II (11.2 mmol l(-1)+/-3.6; n = 6) than in group I (3.3 mmol l(-1)+/-1.4; n = 29). Acute aldosterone substitution restored this value to normal (3.0 mmol l(-1)+/-1.2; n = 4). The aldosterone effect was partly blocked by spironolactone: the Na(+) steady-state concentration was 6.4 mmol/l+/-0.6 (n = 3) in group IV.7. At the steady-state luminal Na(+) concentration, the osmotically driven net water fluxes were not different in groups I and II, indicating that the hydraulic permeability coefficient is not altered in adrenalectomy.8. In group V, J(v) (54.9 mul/h cm(2)+/-10.9; n = 7) and J(Na) (11.9 mumol/h cm(2)+/-1.7; n = 6) were not significantly different from normal.9. In group VI, J(v) (37.3 mul/h cm(2)+/-6.0; n = 5) and J(Na) (8.0 mumol/h cm(2)+/-1.4) were not significantly different from group II.10. The mineralocorticoid effects of aldosterone in long-term adrenalectomy appear to represent the principal determining factors of colonic J(v) and J(Na).