Evidence for a lack of inotropic and chronotropic effects of glucagon and glucagon receptors in the human heart

Fig. 1. Representative traces showing that glucagon did not exert inotropic effects on the failing human myocardium. Increasing concentrations of glucagon (1–1000 nM) were added to trabecular strips obtained from the right atrium (RA), left atrium (LA), right ventricle (RV) and left ventricle (LV) obtained from a failing heart. After being washed (W), the samples were allowed to stabilize for 30 min, and then the nonselective phosphodiesterase inhibitor IBMX (30 μM) was applied and left in contact with the tissue for 15 min. Afterwards, increasing concentrations of glucagon (1–1000 nM) were again applied to all samples, and again, no inotropic effect was observed. Experiments were finalized with the addition of 9 mM CaCl2 to determine the responsiveness of the samples

Fig. 2. Representative traces showing that glucagon did not exert inotropic effects on the nonfailing human myocardium in the absence or presence of IBMX. Further details are provided in the legend to Fig. 1

Fig. 3. Representative traces of a strip of the right ventricle of a rat heart showing A the contractile effect of glucagon (1–1000 nM) alone and, after washing, in the presence of 30 µM IBMX B, which increased contractility on its own and enhanced the inotropic effect of glucagon. C Cumulative concentration‒response curves showing the inotropic effect of glucagon alone (●) and in the presence of 30 µM IBMX (■). Contractility in the presence of IBMX was used as the baseline value for the second concentration‒response curve of glucagon. Inotropic responses are expressed as percent contractility increase compared to baseline. Each point represents the mean value ± SEM (vertical bars) of 5 experiments. *P < 0.05 compared to baseline (paired Student’s t test)

Fig. 4. Concentration‒response curves for the positive chronotropic effects of glucagon in the absence (●) and presence of the nonselective phosphodiesterase inhibitor IBMX (30 μM, ■) in spontaneously beating rat right atria. The results are expressed as the increase in the basal rate (beats min−1). IBMX increased the beating rate by 33 ± 12 beats min−1 (n = 5). The beating rate in the presence of IBMX was used as the basal rate for the second concentration‒response curve of glucagon. Each point represents the mean value ± SEM (vertical bars) of 5 experiments. No significant differences were found when comparing any concentrations of glucagon in the absence or presence of IBMX (paired Student’s t test)

Fig. 5. A Western blot analysis showing the absence of glucagon receptor expression in samples of sinoatrial node (SN), right atrium (RA), left atrium (LA), right ventricle (RV) and left ventricle (LV) from a failing (F) and a nonfailing (NF) human heart. Samples correspond to heart donors number 5 F and number 6 (NF) in Table 1. α-Tubulin was used as the loading control. Human liver (HL) was used as the positive control. Band intensity was determined with densitometry using the program Image Quant TL Plus (General Electrics, USA). B The absence of glucagon receptor mRNA in failing (n = 5) and nonfailing (n = 5) human myocardium samples. Samples were obtained from the sinoatrial node (SN), right atrium (RA), left atrium (LA), right ventricle (RV), and left ventricle (LV). Human liver (HL) tissue was used as the positive control. Each bar represents the combined result of samples obtained from failing and nonfailing hearts (n = 10). Only one sample of the human liver was used as the control. *P < 0.015 compared to HL (parametric binomial analysis)