Influence of Serotonin 5-HT4 Receptors on Responses to Cardiac Stressors in Transgenic Mouse Models.

The current study aimed to deepen our knowledge on the role of cardiac 5-HT4 receptors under pathophysiological conditions. To this end, we used transgenic (TG) mice that overexpressed human 5-HT4a receptors solely in cardiac myocytes (5-HT4-TG mice) and their wild-type (WT) littermates that do not have functional cardiac 5-HT4 receptors as controls. We found that an inflammation induced by lipopolysaccharide (LPS) was detrimental to cardiac function in both 5-HT4-TG and WT mice. In a hypoxia model, isolated left atrial preparations from the 5-HT4-TG mice went into contracture faster during hypoxia and recovered slower following hypoxia than the WT mice. Similarly, using isolated perfused hearts, 5-HT4-TG mice hearts were more susceptible to ischemia compared to WT hearts. To study the influence of 5-HT4 receptors on cardiac hypertrophy, 5-HT4-TG mice were crossbred with TG mice overexpressing the catalytic subunit of PP2A in cardiac myocytes (PP2A-TG mice, a model for genetically induced hypertrophy). The cardiac contractility, determined by echocardiography, of the resulting double transgenic mice was attenuated like in the mono-transgenic PP2A-TG and, therefore, largely determined by the overexpression of PP2A. In summary, depending on the kind of stress put upon the animal or isolated tissue, 5-HT4 receptor overexpression could be either neutral (genetically induced hypertrophy, sepsis) or possibly detrimental (hypoxia, ischemia) for mechanical function. We suggest that depending on the underlying pathology, the activation or blockade of 5-HT4 receptors might offer novel drug therapy options in patients.

Histamine can be Formed and Degraded in the Human and Mouse Heart.

Histamine is metabolized by several enzymes in vitro and in vivo. The relevance of this metabolism in the mammalian heart in vivo is unclear. However, histamine can exert positive inotropic effects (PIE) and positive chronotropic effects (PCE) in humans via H2-histamine receptors. In transgenic mice (H2-TG) that overexpress the human H2 receptor in cardiomyocytes but not in wild-type littermate mice (WT), histamine induced PIE and PCE in isolated left or right atrial preparations. These H2-TG were used to investigate the putative relevance of histamine degrading enzymes in the mammalian heart. Histidine, the precursor of histamine, increased force of contraction (FOC) in human atrial preparations. Moreover, histamine increased the phosphorylation state of phospholamban in human atrium. Here, we could detect histidine decarboxylase (HDC) and histamine itself in cardiomyocytes of mouse hearts. Moreover, our data indicate that histamine is subject to degradation in the mammalian heart. Inhibition of the histamine metabolizing enzymes diamine oxidase (DAO) and monoamine oxidase (MAO) shifted the concentration response curves for the PIE in H2-TG atria to the left. Moreover, activity of histamine metabolizing enzymes was present in mouse cardiac samples as well as in human atrial samples. Thus, drugs used for other indication (e.g. antidepressants) can alter histamine levels in the heart. Our results deepen our understanding of the physiological role of histamine in the mouse and human heart. Our findings might be clinically relevant because we show enzyme targets for drugs to modify the beating rate and force of the human heart.

Characterization of Stressed Transgenic Mice Overexpressing H2-Histamine Receptors in the Heart.

We studied transgenic mice with cardiac-specific overexpression of H2-histamine receptors (H2-TG) by using the α-myosin heavy-chain promoter. We wanted to address whether this overexpression would protect the heart against paradigmatic stressors. To this end, we studied isolated atrial preparations in an organ bath under normoxic and hypoxic conditions and after prolonged exposure to high histamine concentrations. Moreover, we assessed cardiac function using echocardiography in mice with cardiac hypertrophy due to overexpression of the catalytic subunit of PP2A (PP2A-TG) in the heart [H2-TG × PP2A-TG = double transgenic (DT)] or H2-TG with cardiac systolic failure due to treatment of mice with lipopolysaccharides (LPSs). Furthermore, the effect of ischemia and reperfusion was studied in isolated perfused hearts (Langendorff mode) of H2-TG. We detected evidence for the protective role of the overexpressed H2-histamine receptors in the contractile dysfunction of DT and isolated atrial preparations subjected to hypoxia. In contrast, we noted the detrimental role of H2-histamine receptor overexpression against ischemia (Langendorff perfusion) and LPS-induced systolic heart failure. Hence, the role of H2-histamine receptors in the heart is context-sensitive: the results differ between hypoxia (in atrium) and ischemia (perfused whole heart), as well as between genetically induced hypertrophy (DT) and toxin-induced heart failure (LPS). The underlying molecular mechanisms for the protective or detrimental roles of H2-histamine receptor overexpression in the mammalian heart remain to be elucidated. SIGNIFICANCE STATEMENT: The beneficial and detrimental effects of the cardiac effects of H2-histamine receptors in the heart under stressful conditions, here intended to mimic clinical situations, were studied. The data suggest that depending on the clinically underlying cardiac pathophysiological mechanisms, H2-histamine agonists or H2-histamine antagonists might merit further research efforts to improve clinical drug therapy.

Initial Characterization of Stressed Transgenic Mice With Cardiomyocyte-Specific Overexpression of Protein Phosphatase 2C.

As part of our ongoing studies on the potential pathophysiological role of serine/threonine phosphatases (PP) in the mammalian heart, we have generated mice with cardiac-specific overexpression of PP2Cß (PP2C-TG) and compared them with littermate wild type mice (WT) serving as a control. Cardiac fibrosis was noted histologically in PP2C-TG. Collagen 1a, interleukin-6 and the natriuretic peptides ANP and BNP were augmented in PP2C-TG vs. WT (p < 0.05). Left atrial preparations from PP2C-TG were less resistant to hypoxia than atria from WT. PP2C-TG maintained cardiac function after the injection of lipopolysaccharide (LPS, a model of sepsis) and chronic isoproterenol treatment (a model of heart failure) better than WT. Crossbreeding of PP2C-TG mice with PP2A-TG mice (a genetic model of heart failure) resulted in double transgenic (DT) mice that exhibited a pronounced increase of heart weight in contrast to the mild hypertrophy noted in the mono-transgenic mice. The ejection fraction was reduced in PP2C-TG and in PP2A-TG mice compared with WT, but the reduction was the highest in DT compared with WT. PP2A enzyme activity was enhanced in PP2A-TG and DT mice compared with WT and PP2C-TG mice. In summary, cardiac overexpression of PP2Cß and co-overexpression of both the catalytic subunit of PP2A and PP2Cß were detrimental to cardiac function. PP2Cß overexpression made cardiac preparations less resistant to hypoxia than WT, leading to fibrosis, but PP2Cß overexpression led to better adaptation to some stressors, such as LPS or chronic ß-adrenergic stimulation. Hence, the effect of PP2Cß is context sensitive.