Background: The effects of insulin on cardiomyocytes, such as positive inotropic action and glucose uptake are well described. However, in vitro studies comparing long-acting insulin analogues with regard to cardiomyocyte signalling and function have not been systematically conducted.
Methods: Insulin receptor (IR) binding was assessed using membrane embedded and solubilised IR preparations. Insulin signalling was analysed in adult rat ventricular myocytes (ARVM) and HL-1 cardiac cells. Inotropic effects were examined in ARVM and the contribution of Akt to this effect was assessed by specific inhibition with triciribine. Furthermore, beating-rate in Cor.4U(®) human cardiomyocytes, glucose uptake in HL-1 cells, and prevention from H2O2 induced caspase 3/7 activation in cardiac cells overexpressing the human insulin receptor (H9c2-E2) were analysed. One-way ANOVA was performed to determine significance between conditions.
Results: Insulin degludec showed significant lower IR affinity in membrane embedded IR preparations. In HL-1 cardiomyocytes, stimulation with insulin degludec resulted in a lower Akt(Ser(473)) and Akt(Thr(308)) phosphorylation compared to insulin, insulin glargine and its active metabolite M1 after 5- and 10-min incubation. After 60-min treatment, phosphorylation of Akt was comparable for all insulin analogues. Stimulation of glucose uptake in HL-1 cells was increased by 40-60 %, with a similar result for all analogues. Incubation of electrically paced ARVM resulted for all insulins in a significantly increased sarcomere shortening, contractility- and relaxation-velocity. This positive inotropic effect of all insulins was Akt dependent. Additionally, in Cor.4U(®) cardiomyocytes a 10-20 % increased beating-rate was detected for all insulins, with slower onset of action in cells treated with insulin degludec. H9c2-E2 cells challenged with H2O2 showed a fivefold increase in caspase 3/7 activation, which could be abrogated by all insulins used.
Conclusions: In conclusion, we compared for the first time the signalling and functional impact of the long-acting insulin analogues insulin glargine and insulin degludec in cardiomyocyte cell models. We demonstrated similar efficacy under steady-state conditions relative to regular insulin in functional endpoint experiments. However, it remains to be shown how these results translate to the in vivo situation.
Keywords: Cardiac action; Insulin analogues; Insulin degludec; Insulin glargine.