Regulation of amiloride-sensitive electrogenic sodium transport in the rat colon by steroid hormones

The role of steroids in the regulation of colonic sodium transport was examined by infusing steroids into adrenalectomized (ADX) rats and evaluating the short-circuit current (ISC) in vitro. Amiloride-sensitive ISC was induced by aldosterone and corticosterone with half-maximal doses (ED50) of 2 and 260 micrograms X kg-1 X h-1), respectively. Synthetic glucocorticoids such as methylprednisolone (33 mg/kg) and dexamethasone (ED50 = 30 micrograms X kg-1 X h-1) were also effective. Supramaximal doses of aldosterone (7.5 times ED50) for 24 h increased the total ISC (7-fold), the amiloride-sensitive ISC (366-fold), and the conductance (2-fold), as well as the potassium-stimulated phosphatase activity (2-fold) (reported previously). Compared with aldosterone, supramaximal doses of dexamethasone (4 times ED50) produced greater increases in the total ISC (15-fold) and the amiloride-sensitive ISC (674-fold). In contrast to aldosterone, dexamethasone also increased the amiloride-insensitive ISC (3-fold). Glucocorticoid action was not mediated by insulin since the ISC from diabetic ADX rats was increased by dexamethasone to a similar extent (11-fold) as in nondiabetic rats. Estradiol, progesterone, and testosterone did not stimulate the colonic ISC of ADX rats. The ED50 values of corticosterone and aldosterone, measured in terms of amiloride-sensitive sodium transport, produced serum levels that were slightly above those of unstressed, adrenal-intact animals and thus must be considered physiological. It is concluded that at physiological levels both steroids may mediate amiloride-sensitive sodium transport in the rat colon. However, as judged from changes in serum steroid levels, aldosterone is the physiological regulator of elevated sodium absorption in sodium deficiency.