Effect of increasing concentrations of serotonin and adrenaline on contractility myocardium of the right ventricle of the heart of adult rats

In recent years progress has been evident in studies of the importance of serotonin in the physiological and pathological processes of the body and its mechanisms. The role of the serotonin system in the development of diseases such as atherosclerosis, arterial hypertension, and ischemic heart disease is largely discussed. In the myocardium of mammals and humans two types of serotonin receptors (5-HT2 and 5-HT4) have been identified. The activity of the heart is also controlled by the action of catecholamines on the adrenergic receptors of cardiomyocytes. In the implementation of the contraction of cardiomyocytes in the hearts of humans and animals there is also activation of adrenergic receptors, such as β1, β4 and α1A. Serotonin and adrenaline are regulators and modulators of physiological processes in organism, which, under pathological conditions, turn into factors contributing to the development of the disease. In studies on myocardial contractility in vitro in adult rats we found that with an increase in each concentration of serotonin, depending on the dose, a positive inotropic response to the right ventricular myocardium was observed. The effect of serotonin at the last dose on the force of contraction of the right ventricle compared with the first dose increased by 48.3 %. However, with an increase in the dose of epinephrine, the positive inotropic response weakened. At the maximum concentration of 10.0 mM epinephrine, a negative inotropic effect of 10.4 % was observed compared to the previous concentration. Thus, despite the fact that the distribution and functional role of serotonergic receptors in the heart repeats the role of adrenergic receptors, the inotropic response of cardiomyocytes to serotonin and adrenaline is different.

Abstract 16878: Cardiac Overexpression of RXFP1 via AAV9 Gene Therapy Sensitizes Myocardial Inotropic Response to Serelaxin

Background: The pleiotropic hormone relaxin has been shown to mediate physiologic responses during pregnancy by facilitating increases in vascular compliance and cardiac output. While relaxin signaling has been attributed to increases in atrial inotropy, the hormonal effects on ventricular contractility have not been explored in vivo . In this study, we investigated the dose-response effects of recombinant human relaxin (serelaxin) on ventricular mechanics. Methods and Results: Adult male CD1 mice were anesthetized and mechanically ventilated prior to Millar cardiac catheterization. A pressure-volume catheter was advanced into the left ventricle (LV), and baseline systolic and diastolic pressure/volume (PV) loops were recorded (Pre). Mice were injected with either saline, 10 μg/kg (low dose) or 1 mg/kg (high dose) of serelaxin, and PV loops were obtained after stabilization (Post) . Mice injected with 1 mg/kg demonstrated significantly higher LV contractility ( Fig A ) independent of preload ( Fig B ). These responses were not observed with low dose. In order to further examine the role of RXFP1 in modulating these responses, healthy CD1 were injected with AAV9-RXFP1 or empty vector as control and receptor overexpression was confirmed four weeks later via qPCR ( Fig C ). PV loop analysis showed an increased inotropic response [ΔP Max , Δ(dP/dt) Max ] and improved relaxation kinetics [Δ(dP/dt) Min , Δτ] in RXFP1-overexpressing mice at a lower dose of serelaxin, when compared to empty vector mice ( Fig D ). Conclusions: Our data show a novel, dose-dependent correlation between serelaxin and ventricular inotropic and lusitropic responses in mice. These responses were observed at a lower drug dose in cardiac RXFP1-overexpressing mice. We propose that further investigation of this phenomenon could advance the therapeutic, cardioprotective effects associated with relaxin therapy in different etiologies of heart failure that may exhibit changes in RXFP1 expression.

Abstract 17108: Natural History of Cardiomyopathy and Cardiac Stress Response in Young Dogs With Golden Retriever Muscular Dystrophy

Introduction: Duchenne muscular dystrophy (DMD) and the genetically homologous model, golden retriever muscular dystrophy (GRMD), are x-linked conditions that cause progressive muscle wasting and cardiomyopathy. We previously defined a late onset DMD-like dilated cardiomyopathy in adult GRMD dogs and aimed to extend that work to young dogs used for preclinical studies. The goal of this study was to characterize the early natural history and cardiac stress response of GRMD cardiomyopathy. Methods: A prospective imaging study was completed in 10 GRMD dogs and 8 age-matched normal littermates at 3, 6, and 12 months of age. Electrocardiography (ECG), echocardiography with speckle tracking, and cardiac magnetic resonance (CMR) with late gadolinium enhancement (LGE) and myocardial T1 mapping were assessed. In an effort to identify early subclinical evidence of cardiomyopathy, we conducted a dobutamine stress test in an additional subset of dogs (17 GRMD vs. 6 normal). Fractional shortening (FS) was assessed using echocardiography during dobutamine infusion (5-25 μg/kg/min) at 2 months of age, with follow up studies (4 GRMD vs. 6 normal) at 4.5 and 6 months. Results: Heart rate and ECG Q/R ratios (lead II, III, and aVF) were greater in GRMD dogs at 12 months (p<0.04 for all), and PR interval was shortened at 6 months (p=0.02). No systolic functional differences were seen between GRMD and normal dogs using echocardiography. No differences in circumferential strain were seen. CMR left ventricular end-diastolic volume, end-systolic volume, and myocardial mass were smaller in GRMD dogs from 6 months onward (p=0.014, 0.008, and 0.0013). LGE and extracellular volume calculated from T1 mapping did not differ on CMR. A diminished inotropic response, shown by a lower increase of FS (ΔFS), was seen in GRMD dogs during the dobutamine stress test (beginning at a dose of 5 μg/kg/min, 13.9±4.8% vs. 21.8±3.0%, p=0.003) at 2 months of age but not at 4.5 and 6 months. Conclusions: We demonstrated ECG changes and heart size differences in GRMD dogs as early as 6-12 months of age. While cardiac function was preserved, the inotropic response to dobutamine stress was blunted in 2-month-old GRMD dogs, potentially providing an early subclinical marker for DMD therapeutic research.

Functional β2-adrenoceptors in rat left atria: effect of foot-shock stress

Altered sensitivity to the chronotropic effect of catecholamines and a reduction in the β1/β2-adrenoceptor ratio have previously been reported in right atria of stressed rats, human failing heart, and aging. In this report, we investigated whether left atrial inotropism was affected by foot-shock stress. Male rats were submitted to 3 foot-shock sessions and the left atrial inotropic response, adenylyl cyclase activity, and β-adrenoceptor expression were investigated. Left atria of stressed rats were supersensitive to isoprenaline when compared with control rats and this effect was abolished by ICI118,551, a selective β2-receptor antagonist. Schild plot slopes for the antagonism between CGP20712A (a selective β1-receptor antagonist) and isoprenaline differed from unity in atria of stressed but not control rats. Atrial sensitivity to norepinephrine, as well as basal and forskolin- or isoprenaline-stimulated adenylyl cyclase activities were not altered by stress. The effect of isoprenaline on adenylyl cyclase stimulation was partially blocked by ICI118,551 in atrial membranes of stressed rats. These findings indicate that foot-shock stress equally affects inotropism and chronotropism and that β2-adrenoceptor upregulation contributes to the enhanced inotropic response to isoprenaline.