The effects of acetylcholine analogues, serotonin and catecholamines on ion transport were studied in both the middle and the posterior intestine of Anguilla anguilla, mounted in an Ussing chamber, with the aim of understanding whether these regulators affect different mechanisms in the different tracts. In the middle intestine, acetylcholine analogues and serotonin decreased the serosa negative transepithelial potential and short-circuit current without altering the transepithelial resistance; catecholamines reversed the inhibitory effects of both regulators. Similar opposite effects were produced by both the acetylcholine analogues and noradrenalin in the posterior intestine. However, the lowering of the short-circuit current elicited by serotonin was paralleled by the decrease of the transepithelial resistance, whilst noradrenalin had the opposite effects on both parameters. These observations, together with the results of experiments performed by measuring the dilution potential in the control condition and in the presence of either serotonin or serotonin plus noradrenalin, led us to hypothesize that serotonin increases the anion conductance of the paracellular pathway while noradrenalin decreases it. In both the middle and posterior intestine, these regulators probably affect transcellular transport mechanisms by acting on the Na-K-Cl transporter; both acetycholine and serotonin decrease its activity while noradrenalin increases it.
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