| Background 5-HT (5-hytroxytryptamine, serotonin) is widely distributed throughout the body. It is synthesized from L-tryptophan in enterochromaffin (EC) cells of the gastrointestinal tract and in serotonergic neurons. In mammals, it has been estimated that about 60%-90% of the total amount of 5-HT in the body is in gastrointestinal tract and most of it exists in EC cells. In gastrointestinal tract, 5-HT functions as an important neurotransmitter and intercellular messenger to modulate the gastrointestinal motility and secretion. It is related to various gastrointestinal dysfunctions such as emesis, motility disorders and diarrhea, more recently, irritable bowel syndrome (IBS). 5-HT produces its effects through different membrane-bound receptors in different organs. Currently seven subtypes of 5-HT receptor, including-5-HT1, 5-HT2, 5-HT3, 5-HT4, 5-HT5, 5-HT6 and 5-HT7 have been identified. It is reported that 5-HT implicates its functions mainly via 5-HT1 2,3,4 receptors in gastrointestinal tract. 5-HT-evoked colonic fluid secretion was mediated by both 5-HT3 and 5-HT4 receptors in vivo. However, 5-HT evoked intestinal secretion via either the neural pathway mediated by 5-HT3 receptor or the non-neural pathway mediated by 5-HT4 receptor in vitro.Aim To investigate the effects of 5-HT and receptor subtypes that may be involved in the rat colonic ion transport.Methods Short-circuit Current Technique, Calcium Measurement, cGMP Measurement and RT-PCR.Results In colonic mucosa, TTX (1 uM), a neuronal Na+ channel blocker, and, indomethacin (10 uM), a cyclooxygenase (COX) inhibitor, were routinely added to basolateral side to abolish the effects of enteric neural system and endogenousprostaglandin. Removal of extracellular Cl, HCO, or both of them reduced 49.28% (P.01), 76.41% (P.001) and 90.00% (P<0.05) of 5-HT-elicited responses, respectively. A Ca-dependent Cl" channel blocker (DIDS) didn't affect 5-HT-induced A/so But glibenclamide (1 mM), a non-specific Cl" channel blocker, inhibited 5-HT-induced A by 92.94% (PO.05). Removal of apical Na reduced it by 27.31%. Basolateral pretreatment with inhibitor of Na-K-2Cl" cotransporter, bumetanide (100 yM), inhibitor of Na-HCO transporter or C-HCO" exchanger, DIDS (200 M) or both of them decreased the Ac induced by 5-HT about 75.46% (PO.01), 59.02% (PO.01) and 86.27% (PO.05), respectively. Removal of basolateral Na also reduced the current evoked by 5-HT (PO.05) by 59.40%. In intact colon, indomethacin (10 M) was also routinely added. Even at 100 M, the 5-HT-induced change in /sc was 61.10 + 6.35 mC/cm2 (n=ll) which was still lower than that in colonic mucosa. Basolateral application of H9 (20 uM), a selective inhibitor of protein kinase A (PKA) significantly decreased the A/sc induced by 5-HT (10 uM; PO.05). But, MDL-12330A (lOuM), an adenylate cyclase inhibitor, had no significant effect on colonic mucosa (data not shown), but reduced the A/sc induced by 5-HT in colonic mucosa pretreated with an epithelial sodium channel blocker, amiloride (10 uM; PO.01). In colonic mucosa, IB MX (100 M), a phosphodiesterase (PDE) inhibitor also increased the sensitive of colonic mucosa to 5-HT. BAPTA-AM (100 uM, basolateral), membrane permeable Ca chelator, decreased the A/sc induced by 5-HT (10 uM; PO.05). In calcium measurement, 5-HT, at 10 uM, failed to effect intracellular calcium concentration. In cGMP measurement, 5-HT (10 uM) did not cause significant change of the levels of cGMP compared to control cells. To investigate the receptor mechanism underlying 5-HT-induced responses, specific 5-HT agonist 2-methyl-5-HT, 5-HT antagonists MDL72222, Tropanyl-3, 5-dimethylbenzoate and ICS 205-930, 5-HT antagonist GR113808 were used. In colonic mucosa, 1 uM ICS 205-930 did not have significant effects on 5-HT-induced A/sc; 10 uM 2-methyl-5-HT did not induce any Ac; GR113808 (0.1 M~l uM) totally blocked 5-HT-induced response. In intact colon, 10 uM MDL72222 and 10 iM Tropanyl-3, 5-dimethylbenzoate did not significantly inhibit 5-HT (100 uM) produce... |
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