Time-dependent effects of aldosterone on sodium transport and cell membrane resistances in rabbit distal colon

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Abstract
Aldosterone stimulates Na+-absorption in rabbit distal colon. Due to circadian variations in plasma aldosterone level, Na+-transport varies in this epithelium. In vitro measurements (Ussing-chambers) yielded a transepithelial voltage (V t ) of 13±1.6 mV for low-transporting epithelia (LT) and 25.7±2 mV for high-transporting epithelia (HT). However, the comparison of transepithelial conductance (G t ) in LT epithelia (2.73±0.21 mS/cm2) and HT epithelia (2.96±0.41 mS/cm2) revealed no difference. Colons from both groups were stimulated by exogenous aldosterone (4 h prior to experiment). The transepithelial values changed as follows: LT epithelia showed a significantly increased V t (26.1±4 mV) and G t (3.74±0.23 mS/cm2), whereas in HT epithelia both parameters remained unchanged. Transepithelial amiloride-sensitive conductance was higher in HT than in LT. However, only in LT epithelia aldosterone increased this conductance. To get a more detailed view of the action of aldosterone, we used intracellular microelectrodes to calculate the resistances of apical (R a ), basolateral (R bl ) and paracellular (R p ) pathway. The calculation of the resistances was based on a lumped equivalent circuit model and changes in R a were induced by 50 μM/l mucosal amiloride. Comparison of the control tissues revealed a lower R bl only in HT tissues. In both groups stimulation by exogenous aldosterone led to a marked decrease of R a . Furthermore R bl was reduced to the same value as in HT control tissues. A leak resistance (R l) was found, which was modulated by aldosterone in LT- and in HT epithelia. Differences in amiloride-sensitive transepithelial conductance between both epithelia groups could be explained by a regulation of r l . Along with the regulation of the R p the results indicate that the effects of exogenous aldosterone depended on the transport state of the rabbit distal colon. In LT epithelia aldosterone only influenced the resistances of the transcellular pathway. In HT epithelia aldosterone decreased cellular resistances and increased the paracellular resistance. Possible reasons of the augmented R p are discussed.

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