Aldosterone regulates paracellular pathway resistance in rabbit distal colon

Summary Regulation of the paracellular pathway in rabbit distal colon by the hormone aldosterone was investigated in vitro in Ussing chambers by means of transepithelial and microelectrode techniques. To evaluate the cellular and paracellular resistances an equivalent circuit analysis was used. For the analysis the apical membrane resistance was altered using the antibiotic nystatin. Under control conditions two groups of epithelia were found, each clearly dependent on the light: dark regime. Low-transporting epithelia (LT) were observed in the morning and high-transporting epithelia (HT) in the afternoon. Na⁺ transport was about 3-fold higher in HT than in LT epithelia. Incubating epithelia of both groups with 0.1 mol·1^-1 aldosterone on the serosal side nearly doubled in LT epithelia the short circuit current and transepithelial voltage but the transepithelial resistance was not influenced. Maximal values were reached after 4–5 h of aldosterone treatment. In HT epithelia due to the effect of aldosterone all three transepithelial parameters remained constant over time. Evaluation of the paracellular resistance revealed a significant increase after aldosterone stimulation in both epithelial groups. This increase suggests that tight junctions might have been regulated by aldosterone. The hormonal effect on electrolyte transport was also dependent on the physiological state of the rabbit colon. Since net Na⁺ absorption in distal colon is, in addition to transcellular absorption capacity, also dependent on the permeability of the paracellular pathway, the regulation of tight junctions by aldosterone may be a potent mechanism for improving Na⁺ absorption during hormone-stimulated ion transport.Abbreviations V_t transepithelial potential difference (mV) - R_t transepithelial resistance (·cm²) - G_t transepithelial conductance (mS·cm^-2) - I_sc calculated short circuit current (A·cm^-2) - V_a apical membrane potential difference (mV) - V_bl basolateral membrane potential difference (mV) - voltage divider ratio - R_a apical membrane resistance (·cm²) - R_bl basolateral membrane resistance (·cm²) - R_c cellular resistance ( of apical and basolateral resistance) (·cm²) - R_p resistance of the paracellular pathway (·cm²) - G_a apical membrane conductance (mS·cm^-2) - G_bl basolateral membrane conductance (mS·cm^-2) - G_p paracellular conductance (mS·cm^-2) - G_t transepithelial conductance (mS·cm^-2) - HT_contr high transporting control epithelia - LT_contr low transporting control epithelia - HT_aldo aldosterone incubated high transporting epithelia - LT_aldo aldosterone incubated low transporting epithelia