Hannes Jacob

Dopamine can exert effects in the mammalian heart via five different dopamine receptors. There is controversy whether dopamine receptors increase contractility in the human heart. Therefore, we have generated mice that overexpress the human D₁-dopamine receptor in the heart (D₁-TG) and hypothesized that dopamine increases force of contraction and beating rate compared to wild-type mice (WT). In D₁-TG hearts, we ascertained the presence of D₁-dopamine receptors by autoradiography using [³H]SKF 38393. The mRNA for human D₁-dopamine receptors was present in D₁-TG hearts and absent in WT. We detected by in-situ-hybridization mRNA for D₁-dopamine receptors in atrial and ventricular D₁-TG cardiomyocytes compared to WT but also in human atrial preparations. We noted that in the presence of 10 µM propranolol (to antagonize β-adrenoceptors), dopamine alone and the D₁- and D₅-dopamine receptor agonist SKF 38393 (0.1–10 µM cumulatively applied) exerted concentration- and time-dependent positive inotropic effects and positive chronotropic effects in left or right atrial preparations from D₁-TG. The positive inotropic effects of SKF 38393 in left atrial preparations from D₁-TG led to an increased rate of relaxation and accompanied by and probably caused by an augmented phosphorylation state of the inhibitory subunit of troponin. In the presence of 0.4 µM propranolol, 1 µM dopamine could increase left ventricular force of contraction in isolated perfused hearts from D₁-TG. In this model, we have demonstrated a positive inotropic and chronotropic effect of dopamine. Thus, in principle, the human D₁-dopamine receptor can couple to contractility in the mammalian heart.

Lysergic acid diethylamide (LSD) is an artificial hallucinogenic drug. Thus, we hypothesized that LSD might act 5-HT₄ serotonin receptors and/or H₂ histamine receptors. We studied isolated electrically stimulated left atrial preparations, spontaneously beating right atrial preparations, and spontaneously beating Langendorff-perfused hearts from transgenic mice with cardiomyocyte-specific overexpression of the human 5-HT₄ receptor (5-HT₄-TG) or of the H₂-histamine receptor (H₂-TG). For comparison, we used wild type littermate mice (WT). Finally, we measured isometric force of contraction in isolated electrically stimulated muscle strips from the human right atrium obtained from patients during bypass surgery. LSD (up to 10 µM) concentration dependently increased force of contraction and beating rate in left or right atrial preparations from 5-HT₄-TG (n = 6, p < 0.05) in 5-HT₄-TG atrial preparations. The inotropic and chronotropic effects of LSD were antagonized by 10 µM tropisetron in 5-HT₄-TG. In contrast, LSD (10 µM) increased force of contraction and beating rate in left or right atrial preparations, from H₂-TG. After pre-stimulation with cilostamide (1 µM), LSD (10 µM) increased force of contraction in human atrial preparations (n = 6, p < 0.05). The contractile effects of LSD in human atrial preparations could be antagonized by 10 µM cimetidine and 1 µM GR 125487. LSD leads to H₂-histamine receptor and 5-HT₄-receptor mediated cardiac effects in humans.

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.

Ergotamine (2′-methyl-5′α-benzyl-12′-hydroxy-3′,6′,18-trioxoergotaman) is a tryptamine-related alkaloid from the fungus Claviceps purpurea. Ergotamine is used to treat migraine. Ergotamine can bind to and activate several types of 5-HT₁-serotonin receptors. Based on the structural formula of ergotamine, we hypothesized that ergotamine might stimulate 5-HT₄-serotonin receptors or H₂-histamine receptors in the human heart. We observed that ergotamine exerted concentration- and time-dependent positive inotropic effects in isolated left atrial preparations in H₂-TG (mouse which exhibits cardiac-specific overexpression of the human H₂-histamine receptor). Similarly, ergotamine increased force of contraction in left atrial preparations from 5-HT₄-TG (mouse which exhibits cardiac-specific overexpression of the human 5-HT₄-serotonin receptor). An amount of 10 µM ergotamine increased the left ventricular force of contraction in isolated retrogradely perfused spontaneously beating heart preparations of both 5-HT₄-TG and H₂-TG. In the presence of the phosphodiesterase inhibitor cilostamide (1 µM), ergotamine 10 µM exerted positive inotropic effects in isolated electrically stimulated human right atrial preparations, obtained during cardiac surgery, that were attenuated by 10 µM of the H₂-histamine receptor antagonist cimetidine, but not by 10 µM of the 5-HT₄-serotonin receptor antagonist tropisetron. These data suggest that ergotamine is in principle an agonist at human 5-HT₄-serotonin receptors as well at human H₂-histamine receptors. Ergotamine acts as an agonist on H₂-histamine receptors in the human atrium.