Elsevier

Peptides

Volume 78, April 2016, Pages 91-98
Peptides

Glucagon-like peptide-1 regulates calcium homeostasis and electrophysiological activities of HL-1 cardiomyocytes

https://doi.org/10.1016/j.peptides.2016.02.007Get rights and content

Highlights

  • GLP-1 changes calcium homeostasis with an increase of SR calcium contents.

  • Downregulation of phospholamban may increase SERCA2a activity.

  • Decreases of phosphorylated RyR2 S2814 may modulate the arrhythmogenetic activity.

Abstract

Glucagon like-peptide-1 (GLP-1) is an incretin hormone with antidiabetic effects through stimulating insulin secretion, β cell neogenesis, satiety sensation, and inhibiting glucagon secretion. Administration of GLP-1 provides cardioprotective effects through attenuating cardiac inflammation and insulin resistance. GLP-1 also modulates the heart rate and systolic pressure, which suggests that GLP-1 may have cardiac electrical effects. Therefore, the purposes of this study were to evaluate whether GLP-1 has direct cardiac effects and identify the underlying mechanisms. Patch clamp, confocal microscopy with Fluo-3 fluorescence, and Western blot analyses were used to evaluate the electrophysiological characteristics, calcium homeostasis, and calcium regulatory proteins in HL-1 atrial myocytes with and without GLP-1 (1 and 10 nM) incubation for 24 h. GLP-1 (1 and 10 nM) and control cells had similar action potential durations. However, GLP-1 at 10 nM significantly increased calcium transients and sarcoplasmic reticular Ca2+ contents. Compared to the control, GLP-1 (10 nM)—treated cells significantly decreased phosphorylation of the ryanodine receptor at S2814 and total phospholamban, but there were similar protein levels of sarcoplasmic reticular Ca2+-ATPase and the sodium–calcium exchanger. Moreover, exendin (9–39) amide (a GLP-1 receptor antagonist, 10 nM) attenuated GLP-1-mediated effects on total SR content and phosphorylated ryanodine receptor S2814. This study demonstrates GLP-1 may regulate HL-1 cell arrhythmogenesis through modulating calcium handling proteins.

Introduction

The antidiabetic hormone, glucagon like peptide-1 (GLP-1), is an incretin which is rapidly released after a meal intake and stimulates insulin secretion in a glucose-dependent manner [2]. Emerging cardiovascular actions of GLP-1 were reported from human trials and animal models with GLP-1 infusion [13], [30], [31]. GLP-1 improved cardiac function through modulating cardiac metabolism, inflammation, and cell apoptosis in ischemia/reperfusion models [5], [25], [33]. GLP-1 was also reported to improve left ventricular function and remodeling and prolong survival in rats with chronic heart failure. Cardiac beneficial effects of GLP-1 might occur through increasing myocardial insulin activity and glucose uptake and activating the reperfusion injury salvage kinase (RISK) pathway [5], [16], [19], [23], [29], [39]. The cardiovascular actions of GLP-1R agonists and dipeptidyl peptidase 4 (DPP-4, a GLP-1 degradation enzyme) inhibitors include cardiovascular protection and reductions in blood pressure, inflammatory markers, and oxidative stress [3], [10]. Studies showed that GLP-1 possesses ionotropic and chronotropic effects. Hence, it is thought that GLP-1 may modulate the cardiac electrophysiology. However, whether GLP-1 has direct effects on cardiomyocytes remains unknown.

Calcium (Ca2+) regulation plays important roles in cardiac electrical activity. A previous study showed that a DPP-4 inhibitor reversed downregulation of sarcoplasmic reticular (SR) Ca2+-ATPase (SERCA2a) and ryanodine receptor 2 (RyR2) and upregulation of CaV1.2 in hypertensive hearts [18]. Moreover, the GLP-1 receptor agonist can increase the heart rate and lower the blood pressure. This implies that GLP-1 may regulate cardiac activity and calcium handling. Therefore, the purposes of this study were to evaluate whether GLP-1 has direct cardiac effects and identify the underlying mechanisms.

Section snippets

Cell culture

HL-1 cells derived from mouse atrial cardiac muscle cells [37] (kindly provided by Dr. Claycomb, Louisiana State University Medical Center, New Orleans, LA) were cultured in a humidified atmosphere of 5% CO2 at 37 °C in Claycomb medium (JRH Biosciences, Lenexa, KS). For investigating GLP-1 effect on calcium homeostasis and electrophysiology, HL-1 cells from passage 70–90 were seeded at a density of 1 × 106 cells in a 6 cm dish and treated with GLP-1 (7–36) amide (1 and 10 nM, American Peptide

Effect of GLP-1 on calcium handling

Fig. 1 shows calcium transients from control and GLP-1-treated cells, whereas GLP-1 (10 nM)-treated HL-1 cells had larger calcium transients (1.45 ± 0.38 vs. 0.81 ± 0.14 and 0.77 ± 0.09, p < 0.05) than GLP-1-treated (1 nM) and control cells by 79% and 88%, respectively. In addition, GLP-1-treated (10 nM) HL-1 cells had larger SR calcium contents (3.98 ± 0.45 vs. 2.55 ± 0.41 and 2.44 ± 0.40, p < 0.05) than GLP-1-treated (1 nM) and control cells by 56% and 63%, respectively (Fig. 2). Moreover, exendin (9–39) amide

Discussion

This paper demonstrates that GLP-1 has direct effects on cardiac electric activities through modulating calcium handling. The majority of circulating active GLP-1 is a 30-amino acid peptide GLP-1 (7–36) amide, which acts through a G protein-coupled GLP-1R on pancreatic cells to exert glucoregulatory and insulinotropic actions [11]. However, the GLP-1 (7–36) amide is short-lived, being rapidly degraded to the non-insulinotropic metabolite, GLP-1 (9–36) amide, by the ubiquitously expressed

Conflict of interest

The authors declare that no conflicts of interest exist.

Acknowledgements

This study was supported by grants from the Ministry of Science and Technology (NSC102-2314-B-038-018 and NSC102-2314-B-038-003-MY2), Taipei Medical University Hospital (103TMU-TMUH-07), and Wan Fang Hospital, Taipei Medical University (104swf01 and 105swf09).

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