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

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 M/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.Abbreviations V_t transepithelial potential difference (mV) - R_r transepithelial resistance ( · cm²) - G_t transepithelial conductance (mS/cm²) - I_sc calculated short circuit current (A/cm²) - V_a apical membrane potential difference (mV) - V_bl basolateral membrane potential difference (mV) - voltage divider ratio ( = R_a/R_bl) - voltage divider ratio ( = R_a/R_bl) after amiloride - R_a apical membrane resistance ( · cm²) R_a = R_l · R_a^/Na/(R_l + R_a^/Na) - R_a^/Na apical membrane resistance to Na, ( · cm²) - R_bl basolateral membrane resistance ( · cm²) - R_c cellular resistance ( of apical and basolateral resistance) ( · cm²) - R_l leak resistance located in the apical membrane ( · cm²) - R_p resistance of the paracellular pathway ( · cm²) - G_a apical membrane conductance (mS/cm²) - G_bl basolateral membrane conductance (mS/cm²) - G_l apical leak conductance (compare R_l) (mS/cm²) - G_p paracellular conductance (mS/cm²) - G_t transepithelial conductance (mS/cm²) - HF hard feces period - SF soft feces period - HT_contr high transporting control epithelia - LT_contr low transporting control epithelia - HT_aldo aldosterone pretreated high transporting epithelia - LT_aldo aldosterone pretreated low transporting epithelia; the prime designates data obtained after addition of amiloride