Glucagon-like peptide-1 directly increases heart rate and shortens atrial refractoriness: an in vivo and ex vivo study in pigs

Funding Acknowledgements Type of funding sources: Foundation. Main funding source(s): Novo Nordisk Foundation Synergy program Novo Nordisk Foundation Center for Basic Metabolic Research Background Treatment with glucagon-like peptide-1 receptor agonists (GLP-1 RAs) in patients with type 2 diabetes not only reduces hyperglycaemia, but also improves cardiovascular outcomes. However, GLP-1 RA treatment also increases heart rate: an apparent paradox. Purpose Whether the heart rate increase is a direct effect, and whether GLP-1 affects other aspects of cardiac electrophysiology, remain unclear. To answer these questions we investigated the effect of GLP-1 infusion on cardiac electrophysiology in vivo and ex vivo in pigs and pig hearts, respectively, during sinus rhythm and pacing. Methods Anaesthetised pigs (n = 8) received infusions of GLP-1 (10 pmol/kg/min). Electrocardiogram, atrial monophasic action potentials and atrial conduction velocity data were collected and atrial and ventricular effective refractory periods (ERP) were measured. For the ex vivo studies, pig hearts (n = 7) were excised, retrogradely perfused and exposed to consecutive bolus perfusions of 2 and 4 nmol GLP-1, 100 nmol of the GLP-1 receptor antagonist exendin-9-39 and a final 4 nmol bolus of GLP-1. The same electrophysiological parameters were measured. Results In anaesthetised pigs, GLP-1 increased heart rate, cardiac output and diastolic pressure, while systemic vascular resistance was decreased. Infusion of GLP-1 decreased PQ interval in sinus rhythm (P = 0.019, n = 8) and during atrial pacing (P = 0.027, n = 6) with 8 ± 3 % and 12 ± 3 %, respectively. Additionally, GLP-1 decreased atrial ERP at all pacing cycle lengths (P = 0.04, n = 7), while ventricular ERP was unaffected (P = 0.29, n = 7). In the isolated perfused heart, GLP-1 increased heart rate with 13 ± 2 bpm (P = 0.001, n = 7). This increase in heart rate was completely abolished by pre-administration of exendin-9-39. Atrial ERP shortened after GLP-1 perfusion (P = 0.01, n = 7) comparable to the in vivo studies, with an average decrease of 11 ± 2 %. This effect was also abolished by exendin-9-39. Conclusion GLP-1 increases heart rate through activation of the GLP-1 receptor in the isolated perfused heart, suggesting a direct effect of GLP-1 rather than activation through the central nervous system. Additionally, GLP-1 affects atrial electrophysiology, but not ventricular electrophysiology, in vivo and ex vivo independent of the increase in heart rate.

Abstract 276: The Role of Diacylglycerol Acetyltransferase 1 and 2 in Cardiac Metabolism and Function

Cardiac triglyceride (TG) plays an important role in myocardial metabolism. TG synthesis is catalyzed by diacylglycerol:acetyltransferase (DGAT). Enhancing cardiac TG synthesis and turnover, by means of overexpression DGAT1, has protected hearts against stresses while blocking the TG turnover causes cardiomyopathy. In the meantime DGAT inhibitors are being developed for lipid lowering therapy, raising concerns whether DGAT inhibition affects cardiac function. Here we determined the role of the two cardiac DGAT isoforms in TG synthesis and turnover in the heart and their contribution to cardiac fatty acid metabolism. Using an inducible cardiac specific DGAT1 deletion mouse (iKO) together with DGAT2-specific inhibitor, we were able to achieve graded inhibition of TG synthesis and turnover as determined by 13 C-NMR spectroscopy of isolated perfused mouse heart. The iKO heart has normal TG level (CON 5.7±1.2 vs. 6.9±0.7 μg/mg wwt) and perfusing hearts with glucose (5.5mM), fatty acids (0.4mM) and lactate (1.2mM) for 1hr did not change the TG content in control (CON, pre-perfusion 3.9 ± 0.6 vs. post-perfusion 4.5 ± 0.7 μg/mg wwt), iKO (3.8±0.6 vs. 4.2±0.5 μg/mg wwt) or iKO+DGAT2 inhibitor (3.8±0.6 vs. 5.0±0.8 μg/mg wwt). Relative to CON, the rate of 13 C labeled fatty acids incorporation into the TG pool decreased by 32% in iKO (AUC 5.170 of 7.547, p<0.05) which was accompanied by an increase the oxidation of exogenous fatty acids (relative FAO: 48.5±5.3 for CON vs. 67.0±4.1% for iKO, p<0.05). Cardiac function, assessed by echocardiography (FS: CON 47.5±1.3 vs. iKO 45.8±3.0%, p>0.05) or by rate pressure product of the isolated perfused heart (CON 39149±1047 vs. iKO 40836±3424 bpm*mmHg, p>0.05) is normal in iKO hearts and remained unchanged after treatment with the DGAT2 inhibitor (37083±8507 bpm*mmHg). Coinhibition of DGAT1 and 2 abrogated 13 C labeled fatty acids incorporation into the TG pool by 58% (AUC 5.042 of 11.82, p<0.05) and suppressed expression of PPARα target genes relative to non-treated control hearts (p<0.05). Taken together, our data show that both DGAT1 and 2 contribute TG synthesis in the heart. Inhibition of both isoforms abrogated TG synthesis and reduced PPARα activity in the heart but did not affect cardiac function in isolated perfused heart.

Abstract 292: The Role of the ATF6 Branch of the ER Stress Response in the Endocrine Heart

Rationale: Atrial natriuretic peptide (ANP) is stored in the heart in large dense core granules of atrial myocytes as a biologically inactive precursor, pro-ANP. Hemodynamic stress and atrial stretch stimulate coordinate secretion and proteolytic cleavage of pro-ANP to its bioactive form, ANP, which promotes renal salt excretion and vasodilation, which, together contribute to decreasing blood pressure. While the ATF6 branch of the ER stress response has been studied in ventricular tissue mouse models of myocardial ischemia and pathological hypertrophy, roles for ATF6 and ER stress on the endocrine function of atrial myocytes have not been studied. Objective/Methods: To address this gap in our knowledge, we knocked down ATF6 in primary cultured neonatal rat atrial myocytes (NRAMs) using a chemical inhibitor of the proteolytic cleavage site enabling ATF6 activation and siRNA and measured ANP expression and secretion basally and in response to alpha- adrenergic agonist stimulation using phenylephrine. We also compared the ANP secretion from wild- type mice and ATF6 knockout mice in an ex vivo Langendorff model of the isolated perfused heart. Results: ATF6 knockdown in NRAMs significantly impaired basal and phenylephrine-stimulated ANP secretion. ATF6 knockout mice displayed lower levels of ANP in atrial tissue at baseline as well as after phenylephrine treatment. Similarly, in the ex vivo isolated perfused heart model, less ANP was detected in effluent of ATF6 knockout hearts compared to wild-type hearts. Conclusions: The ATF6 branch of the ER stress response is necessary for efficient co-secretional processing of pro-ANP to ANP and for agonist-stimulated ANP secretion from atrial myocytes. As ANP is secreted in a regulated manner in response to a stimulus and pro-ANP is synthesized and packaged through the classical secretory pathway, we posit that ATF6 is required for adequate expression, folding, trafficking, processing and secretion of biologically active ANP from the endocrine heart.