Ergometrine stimulates histamine H₂ receptors in the isolated human atrium

Ergometrine (6aR,9R)-N-((S)-1-hydroxypropan-2-yl)-7-methyl-4,6,6a,7,8,9-hexa-hydro-indolo-[4,3-fg]chinolin-9-carboxamide or lysergide acid β-ethanolamide or ergonovine) activates several types of serotonin and histamine receptors in the animal heart. We thus examined whether ergometrine can activate human serotonin 5-HT₄ receptors (h5-HT₄R) and/or human histamine H₂ receptors (hH₂R) in the heart of transgenic mice and/or in the human isolated atrium. Force of contraction or beating rates were studied in electrically stimulated left atrial or spontaneously beating right atrial preparations or spontaneously beating isolated retrogradely perfused hearts (Langendorff setup) of mice with cardiac specific overexpression of the h5-HT₄R (5-HT₄-TG) or of mice with cardiac specific overexpression of the hH₂R (H₂-TG) or in electrically stimulated human right atrial preparations obtained during cardiac surgery. Western blots to assess phospholamban (PLB) phosphorylation on serine 16 were performed. Ergometrine exerted concentration- and time-dependent positive inotropic effects and positive chronotropic effects in atrial preparations starting at 0.3 µM and reaching a plateau at 10 µM in H₂-TGs (n = 7). This was accompanied by an increase in PLB phosphorylation at serine 16. Ergometrine up 10 µM failed to increase force of contraction in left atrial preparations from 5-HT₄-TGs (n = 5). Ten micrometer ergometrine increased the force of contraction in isolated retrogradely perfused spontaneously beating heart preparations (Langendorff setup) from H₂-TG but not 5-HT₄-TG. In the presence of the phosphodiesterase inhibitor cilostamide (1 µM), ergometrine at 10 µM exerted positive inotropic effects in isolated electrically stimulated human right atrial preparations, obtained during cardiac surgery, and these effects were eliminated by 10 µM of the H₂R antagonist cimetidine but not by 10 µM of the 5-HT₄R antagonist tropisetron. Furthermore, ergometrine showed binding to human histamine H₂ receptors (at 100 µM and 1 mM) using HEK cells in a recombinant expression system (pK_i < 4.5, n = 3). In conclusion, we suggest that ergometrine is an agonist at cardiac human H₂Rs.