Effects of aldosterone and dexamethasone on apical membrane properties and Na-transport of rabbit distal colon in vitro

The effects of pre-treatment in vivo with aldosterone and dexamethasone were investigated on rabbit distal colon. Apical Na-permeability and net sodium transport were measured in vitro. In this epithelium, Natransport is entirely electrogenic. It can therefore be measured electrically as the fraction of short circuit current which is blockable by amiloride. The epithelia were studied in an Ussing chamber and the electrical values recorded by a computerized digital voltage clamp. Transepithelial parameters, and the transapical membrane parameters (in preparations depolarized from the serosal side) were investigated after treatment with the two hormones.Under transepithelial conditions, aldosterone and dexamethasone stimulated the short circuit current (I_sc) from control (17.4 A/cm²) to a similar degree (86.6 and 93.8 A/cm²). However, whereas aldosterone did not alter the transepithelial resistance (R_T) significantly, dexamethasone reducedR_T from 357 to 167 · cm². The stimulation of the potential difference (V_T) under control condition (6.6 mV) was therefore significantly different between aldosterone (28.7 mV) and dexamethasone (16 mV). Mucosal amiloride (0.1 mM) inhibitedI_sc andV_T completely under all conditions.Steady state current-voltage relations were obtained by voltage clamping the tissues in staircase increments before and after mucosal treatment with amiloride. As measured by the difference between these two states, Na-currents were calculated both for the transepithelial and the transapical condition. Intracellular Na-activity and apical Na-permeability were then calculated by the Nernst and Goldman-Hodgkin-Katz equations. These values were found to be increased after treatment with both hormones. Dexamethasone was a more potent stimulator of both values. The result suggest different effects on other cellular properties in addition to the effect on the presumably common pathway across the apical membrane.