Background. In patients with obesity or diabetes, agonists of the anorexic glucagon like peptide 1 receptor (GLP-1R), such as semaglutide (sema), efficiently reduce body weight and improve cardiovascular outcomes, with a modest effect also on hospitalization for heart failure (HF, by 11% in a meta-analysis). Obesity is an important risk factor also for HF, where defects in excitation-contraction coupling and mitochondrial energetics occur. Since it is currently evaluated in clinical trials whether GLP-1R agonists improve outcome of patients with HF and obesity, we evaluated the impact of sema on mechano-energetic coupling in cardiac myocytes from obese rats.

Methods and results. Male Wistar rats were fed a high fat, high fructose diet (HFD) for 8 weeks to induce obesity. Rats were then randomly assigned to receive sema (120μg/kg s.c.) or saline s.c. (CO) for 8 weeks, leading to a body weight change of -10% (sema) vs. +8% (CO). Rats could choose between HFD and low-fat diet ad libitum while receiving therapy. After 16 weeks, cardiac ventricular myocytes (from n=3 CO and n=5 sema rats) and mitochondria (from n=6 rats per group) were isolated. Sarcomere length, cytosolic Ca²⁺ (Indo1, AM), mitochondrial redox state (autofluorescence of NAD(P)H and FAD), membrane potential (MP,TMRM) and ROS (DCF) were measured in myocytes with an automatic Ionoptix fluorescence setup. Pacing at 0.3 Hz, followed by β-adrenergic stimulation and increasing stimulation rate at 3 Hz for 3 minutes, was used to subject cardiac myocytes to a physiological stress regimen. Sema treatment in vivo lowered systolic and diastolic [Ca²⁺]_i and Ca²⁺-transient amplitude (n=100 cells) compared to CO (n=70), which led to longer sarcomere lengths at both systole and diastole in sema (n=120) vs. CO (n=100), but without altering fractional sarcomere shortening. To further elucidate the underlying mechanism of these Ca²⁺ changes, we measured SR-Ca²⁺-Load and L-Type Ca²⁺ channel current. The SR-Ca²⁺-load was significantly reduced in sema treated rats compared to CO, whereas the L-Type Ca²⁺ channel current was normal under baseline condition. Isoprenalin stimulation (30nM) increased the current, but to a significantly lower extent in sema treated rat’s cells. Mitochondrial redox state (n=46/82 CO/sema) remained unaltered, but MP was diminished in sema by approx. 3% during the protocol and about 50% overall (n=120) vs. CO (n=69), and ROS generation was enhanced in sema by approx. 10% (n=33) vs. CO (n=56). Mitochondrial respiration was measured in isolated mitochondria. Ca²⁺-retention capacity using Calcium-Green, mitochondrial MP using TMRM, and NAD(P)H levels were determined. While pyruvate/malate (P/M, Complex I)- and succinate (Complex II)-linked state 3 respiration (in the presence of ADP) was unchanged, fatty acid-linked state 3 respiration was increased. In contrast, Ca²⁺-retention capacity, mitochondrial membrane potential, and NAD(P)H levels remained unaltered.

Conclusion. In rats with diet-induced obesity, sema substantially reduced body weight and boosted fatty acid-linked respiration of cardiac mitochondria. In cardiac myocytes, cytosolic Ca²⁺ concentrations were lowered, due to lower SR-Ca²⁺ load and less activation of L-Type Ca²⁺ channel, without compromising fractional sarcomere shortening. However, since sema in vivo treatment also lowered mitochondrial MP but increased ROS, further studies are required to resolve the underlying mechanisms.