Pinacidil suppression on 5-HT3 receptor-mediated contraction of guinea pig ileum in vitro.

AIM To study the effects of the K+ channel opener pinacidil on 5-HT3 receptor-mediated contractions of the isolated guinea pig ileum (GPI) longitudinal muscle-myenteric plexus strip preparations. METHODS GPI contractions were recorded with a chart recorder through isometric transducers. The effect of pinacidil on binding properties of 5-HT3 receptors was assessed using [3H]GR65630 binding assay in membrane preparations of rat entorhinal cortex. RESULTS (1) A selective 5-HT3 receptor agonist 2-methyl-5-HT 0.1-300 mumol.L-1 and 5-HT 0.001-50 mumol.L-1 elicited GPI contractile responses in concentration-dependent manners, the EC50 values (and 95% confidence limits) for 2-methyl-5-HT and 5-HT were 10.0 (8.9-11.2) mumol.L-1 and 1.6 (1.3-1.9) mumol.L-1, respectively. Selective 5-HT3 receptor antagonist tropisetron 0.1 mumol.L-1 competitively inhibited the responses to 2-methyl-5-HT and 5-HT. (2) Pinacidil 0.5-5 mumol.L-1 inhibited 5-HT3 receptor-mediated contractions. (3) Pinacidil 1 mumol.L-1 enhanced the inhibitory effects of tropisetron 0.1 mumol.L-1 or another selective 5-HT3 receptor antagonist benesetron 1 mumol.L-1 on 5-HT-induced GPI contractile responses. (4) Pinacidil 1-5 mumol.L-1 did not affect GPI contractile responses evoked by a selective M-ACh receptor agonist carbachol 1 mumol.L-1. (5) Pinacidil 1-5 mumol.L-1 had no effect on binding properties of 5-HT3 receptors with selective 5-HT3 receptor radioligand [3H]GR65630 in the entorhinal cortex of rat brain. CONCLUSION The inhibition by pinacidil of 5-HT3 receptor-mediated GPI contractile responses may be mediated through activation of ATP-sensitive K+ channels located in prejunctional myenteric neurons.