Serotonin Augments Gut Pacemaker Activity Figure 2. Evidence that endogenous 5-HT generates basal pacemaker [Ca2+]i activity via 5-HT3. A-D) Examples of effects of drugs relating to 5-HT. Each time course of changes in [Ca2+]i activity was acquired 3–5 min after application of drugs. All measurements were carried out in the presence of 1 mM nifedipine. LY-278584, 2-Me-5-HT, GR113808 and O-Me-5-HT were applied at 10 mM, and SK&F96365 was applied at 40 mM. E) Graphs summarizing the effects of drugs relating to 5-HT on the amplitude of pacemaker [Ca2+]i oscillations. Blue bar represents the control. Blue and red bars in the left graph include both experiments (n = 10) for control and subsequent application of LY-278584 shown in A (n = 5) and B (n = 5). Statistical significance of the effects of 2-Me-5-HT and O-Me-5-HT were evaluated by comparing with each control observed in the presence of LY- 278584 (n = 5, each). Middle and right graphs represent experiments shown in C (n = 5) and D (n = 5), respectively. Asterisks (P,0.05). n.d. (No [Ca2+]i activity detected.). doi:10.1371/journal.pone.0024928.g002 Results 5-HT augments [Ca2+]i activity in ICC ICC generate pacemaker activity employing [Ca2+]i oscillations [13,14]. To examine the effect of 5-HT on [Ca2+]i activity in ICC, we used cell cluster preparations (100–200 mm in diameter) obtained from the mouse ileum. This preparation contains ICC, smooth muscle cells and enteric neurons, and is considered to be an integrated model system to investigate gut pacemaker activity. Dihydropyridine (DHP) Ca2+ antagonists can selectively suppress [Ca2+]i and contractile activities of smooth muscle cells by blocking L-type Ca2+ channels, while pacemaking [Ca2+]i PLoS ONE | www.plosone.org 4 September 2011 | Volume 6 | Issue 9 | e24928