Modification of spontaneous contractility of smooth muscle preparations from the bovine abomasal antrum by serotonin receptor agonists

The effects of serotonin (5-hydroxytryptamine, 5-HT), a 5-HT2-receptor agonist, alpha-methyl-5-hydroxytryptamine (alpha-M- 5-HT) and RS- 67506, a 5- HT4 receptor partial agonist, on spontaneous contractility of bovine abomasal smooth muscle preparations were investigated in vitro. Preparations from the abomasal antrum of freshly slaughtered healthy dairy cows were cut parallel to the longitudinal fibres, suspended in isolated organ baths, and concentration-response curves were performed by cumulative application of the 5- HT receptor agonists. Blockade of 5-HT2 -induced response was tested with atropine and hexamethonium. Serotonin evoked a significant increase in the area under curve (AUC), whilst the 5- HT2 receptor agonist alpha- M- 5- HT significantly increased the AUC and resting tone (RT). RS- 67506 induced a significant increase in AUC and RT and a significant decrease in the maximum force. The effect of alpha- M- 5- HT was mediated by a muscarinic cholinergic pathway, as the effect of alpha- M- 5- HT was inhibited in the presence of atropine but not hexamethonium. It is concluded that 5- HT2 and 5- HT4 receptors are present in the bovine abomasal antrum. Muscarinic receptors are involved in the increase in RT seen after 5- HT2 receptor stimulation.