The short-term action of aldosterone in physiological concentration on net fluxes of Na, K, Cl, HCO3, osmolytes, and water was examined in the proximal colon and rectal colon of adrenalectomized (ADX) rats in vivo. The measuring time was 12 h, divided in eight periods of 90 min. (a) Aldosterone alone (6 nmol h-1kg-1) did not stimulate transport in ADX rats. In these experiments plasma [K] increased to fatal values. A basal glucocorticoid substitution of 24 nmol h-1kg-1 corticosterone caused plasma K to stay constant throughout the experiment, so that epithelial transport was not handicapped by non-specific effects of ADX, but this also did not restore the decreased transport of ADX rats to control values. Under these conditions (absence of aldosterone) in the rectal colon Na and H2O transport was zero, whereas in the proximal colon flux rates were depressed by between 30% and 50%. In contrast, basal glucocorticoid substitution of 18 nmol h-1kg-1 corticoster-one plus infusion of 6 nmol h-1kg-1 aldosterone caused transport stimulation to values not significantly different from those of non-ADX controls. We conclude that after ADX, aldosterone at physiological concentrations increases transport if, as a prerequisite, a basal glucocorticoid substitution is provided. Transport of Na, K, and H2O is under the total control of aldosterone in the rectal colon but is only moderately altered in the proximal colon. (b) In the proximal colon aldosterone is effective on electroneutral Na transport consistent with the Na/H, Cl/HCO3 double exchanger, while in the rectal colon aldosterone controls the amiloride-sensitive Na channel, which is the sole apical Na transporter in that segment. In this respect the rectal colon is functionally distinct from the distal colon of other studies in the rat, where under unstimulated conditions the double exchanger is present. Regarding Na transport mechanisms, the rat large intestine thus consists of three distinct segments, the proximal, distal, and rectal colon. (c) In the rectal, but not the proximal colon, active net K absorption against the electrochemical gradient took place in the absence of aldosterone, but was suppressed in its presence. (d) The flux ratio of osmolytes over H2O was constantly hypertonic as compared to the plasma (387±1 mosmol l-1), independent of absolute flux size, the colonic segment examined, or the steroid concentration used. This effect may be due to a hypertonic unstirred layer within the lamina propria. © 1990 Springer-Verlag.
