Mechanisms mediating serotonin-induced contraction of colonic myocytes

1. 5-Hydroxytryptamine (5-HT) has an important role in the pathogenesis of irritable bowel syndrome. To investigate the effects of 5-HT on the contractile activity of myocytes of the guinea-pig proximal colon, cell imaging before and after contraction was undertaken and images were analysed using image-analysis software. Ion currents and membrane potentials were measured. Cytoplasmic free Ca2+ was recorded using a confocal microscope followoing loading of the cells with the fluorescent probe Fura-2AM. 2. 5-Hydroxytryptamine reduced cell length in a dose-dependent manner (EC50 = 0.189 mu mol/L). Under current clamp, 10 mu mol/L 5-HT reduced action potential amplitude (measured as peak height) and decreased action potential duration, as well as depolarizing the resting potential from -68.4 +/- 3.6 to -22.96 +/- 4.65 mV. Iberiotoxin (1 mu mol/L) blocked the effects of 5-HT in reducing the time to repolarization (T-90) and nicardipine (5 mu mol/L) blocked the effects of 5-HT in reducing action potential amplitude. 3. In the whole-cell mode, 5-HT enhanced L-type Ca2+ currents, large conductance K+ channel (BKCa) currents and spontaneous transient outward currents (STOC). In addition, 5-HT increased intracellular Ca2+ levels. Ondansetron (10 mu mol/L) blocked the effects of 5-HT in enhancing L-type Ca2+ currents, BKCa currents and STOC. 4. In conclusion, 5-HT induces contraction of colonic myocytes, mostly as a result of Ca2+ release from the sarcoplasmic reticulum (SR) following activation of 5-HT3 receptors and the inositol 1,4,5-trisphosphate pathway. In addition, the effect of 5-HT in decreasing action potential amplitude is mediated by the release of Ca2+ from the SR, as well as by enhanced L-type Ca2+ current. 5-Hydroxytryptamine decreased action potential duration by enhancing BKCa current.