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

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Abstract
The influence of aldosterone upon water and Na transport properties of the distal colon was studied in long-term adrenalectomy (11-29 days). Six groups of rats were used: I, normal (control); II, adrenalectomized; III, adrenalectomized, acutely substituted with aldosterone (200 .mu.g/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 .times. 75 .mu.g/kg day); VI, adrenalectomized, substituted chronically with dexamethasone (120 .mu.g/kg day). 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. In adrenalectomy (group II), net water absorption (Jv) was significantly decreased from (normal) 54.4 .mu.l/h cm2 .+-. 10.5 (n = 9) to 41.2 .mu.l/h cm2 .+-. 7.3 (n = 4), and net Na+ absorption (JNa) was decreased from 13.6 .mu.mol/h cm2 + 3.5 to 8.5 .mu.mol/h cm2 .+-. 0.9 (isotonic perfusate). Similarly, Jv was decreased from 54.0 .mu.l/h cm2 .+-. 8.3 (n = 4) to 37.3 .mu.l/h cm2 .+-. 4.2 (n = 7), and JNa from 8.6 .mu.mol/h cm2 .+-. 2.1 to 4.2 .mu.mol/h cm2 .+-. 2.1 (hypotonic perfusate). Acute aldosterone substitution in adrenalectomy (III) had no effect upon Jv (37.1 .mu.l/h cm2 .+-. 10.3; n = 5) but increased JNa to 10.3 .mu.mol/h cm2 .+-. 0.3. The luminal Na+ steady-steady 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. 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. In group V, Jv (54.9 .mu.l/h cm2 .+-. 10.9; n = 7) and JNa (11.9 .mu.mol/h cm2 .+-. 1.7; n = 6) were not significantly different from normal. In group VI, Jv (37.3 .mu.l/h cm2 .+-. 6.0; n = 5) and JNa (8.0 .mu.mol/h cm2 .+-. 1.4) were not significantly different from group II. The mineralocorticoid effects of aldosterone in long-term adrenalectomy appear to represent the principal determining factors of colonic Jv and Jna.