LAUSR

It is unclear whether metoclopramide and domperidone act on human cardiac serotonin 5-HT4-receptors. Therefore, we studied transgenic mice that only express the human 5-HT4 receptor in cardiomyocytes in the atrium and in the ventricle (5-HT4-TG), their wild type-littermates (WT) and isolated human atrial preparations. We found that only metoclopramide but not domperidone enhanced the force of contraction in left atrial preparations (pEC50 = 6.0 ± 0.1; n = 7) from 5-HT4-TG, isolated spontaneously beating right atrial preparations (pEC50 = 6.1 ± 0.1; n = 7) from 5-HT4-TG, Langendorff perfused hearts from 5-HT4-TG, living 5-HT4-TG and human right atrial muscle preparations obtained during bypass surgery of patients suffering from coronary heart disease. The maximum inotropic effect of metoclopramide was smaller (81 ± 2 %) than that of 5-HT on the left atria from 5-HT4-TG. The maximum increase in the beating rate due to metoclopramide was 93 ± 2 % of effect of 5-HT on right atrial preparations from 5-HT4-TG. Metoclopramide and domperidone were inactive in WT. We found that metoclopramide but not domperidone increased the phosphorylation state of phospholamban in the isolated perfused hearts or muscle strips of 5-HT4-TG, but not in WT. Metoclopramide, but not domperidone, shifted the positive inotropic or chronotropic effects of 5-HT in isolated left atrial and right atrial preparations from 5-HT4-TG dextrally, resp., to higher concentrations: the pEC50 of 5-HT for increase in force was in the absence of metoclopramide 8.6 ± 0.1 (n = 5) versus 8.0 ± 0.3 in the presence of 1 μM metoclopramide (n = 5; P < 0.05); and the beating rate was 7.8 ± 0.2 (n = 7) in the absence of metoclopramide versus 7.2 ± 0.1 in the presence of 1 μM metoclopramide (n = 6; P < 0.05). These results suggested that metoclopramide had an antagonistic effect on human cardiac 5-HT4 receptors. In summary, we showed that metoclopramide, but not domperidone, was a partial agonist at human cardiac 5-HT4-receptors.