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

BACKGROUND: Glucagon is thought to increase heart rate and contractility by stimulating glucagon receptors and increasing 3',5'-cyclic adenosine monophosphate (cAMP) production in the myocardium. This has been confirmed in animal studies but not in the human heart. The cardiostimulatory effects of glucagon have been correlated with the degree of cardiac dysfunction, as well as with the enzymatic activity of phosphodiesterase (PDE), which hydrolyses cAMP. In this study, the presence of glucagon receptors in the human heart and the inotropic and chronotropic effects of glucagon in samples of failing and nonfailing (NF) human hearts were investigated. METHODS: Concentration‒response curves for glucagon in the absence and presence of the PDE inhibitor IBMX were performed on samples obtained from the right (RA) and left atria (LA), the right (RV) and left ventricles (LV), and the sinoatrial nodes (SNs) of failing and NF human hearts. The expression of glucagon receptors was also investigated. Furthermore, the inotropic and chronotropic effects of glucagon were examined in rat hearts. RESULTS: In tissues obtained from failing and NF human hearts, glucagon did not exert inotropic or chronotropic effects in the absence or presence of IBMX. IBMX (30 µM) induced a marked increase in contractility in NF hearts (RA: 83 ± 28% (n = 5), LA: 80 ± 20% (n = 5), RV: 75 ± 12% (n = 5), and LV: 40 ± 8% (n = 5), weaker inotropic responses in the ventricular myocardium of failing hearts (RV: 25 ± 10% (n = 5) and LV: 10 ± 5% (n = 5) and no inotropic responses in the atrial myocardium of failing hearts. IBMX (30 µM) increased the SN rate in failing and NF human hearts (27.4 ± 3.0 beats min-1, n = 10). In rat hearts, glucagon induced contractile and chronotropic responses, but only contractility was enhanced by 30 µM IBMX (maximal inotropic effect of glucagon 40 ± 8% vs. 75 ± 10%, in the absence or presence of IBMX, n = 5, P < 0.05; maximal chronotropic response 77.7 ± 6.4 beats min-1 vs. 73 ± 11 beats min-1, in the absence or presence of IBMX, n = 5, P > 0.05). Glucagon receptors were not detected in the human heart samples. CONCLUSIONS: Our results conflict with the view that glucagon induces inotropic and chronotropic effects and that glucagon receptors are expressed in the human heart.

Does glucagon have a positive inotropic effect in the human heart?

Glucagon is considered to exert cardiostimulant effects, most notably the enhancement of heart rate and contractility, due to the stimulation of glucagon receptors associated with Gs protein stimulation which causes adenylyl cyclase activation and the consequent increase in 3',5'-cyclic adenosine monophosphate production in the myocardium. These effects have been extensively demonstrated in experimental studies in different animal species. However, efforts to extrapolate the experimental data to patients with low cardiac output states, such as acute heart failure or cardiogenic shock, have been disappointing. The experimental and clinical data on the cardiac effects of glucagon are described here.

Resveratrol enhances the inotropic effect but inhibits the proarrhythmic effect of sympathomimetic agents in rat myocardium.

BACKGROUND: Resveratrol is a cardioprotective agent with known antiarrhythmic effects that has recently been shown to inhibit phosphodiesterase (PDE) enzyme activity. Thus, it is possible that resveratrol increases the inotropic effect of sympathomimetic agents, as PDE inhibitors do but, unlike other PDE inhibitors, its effect may not be accompanied by proarrhythmia due to its antiarrhythmic action. This work is aimed to test this hypothesis. METHODS: This is an "in vitro" concentration-response relationship study. The effects of noradrenaline, tyramine and isoproterenol, alone or in combination with either resveratrol or with the typical PDE inhibitor 3-isobutylmethylxantine (IBMX), were studied in electrically driven strips of right ventricle or in the spontaneously beating free wall of the right ventricle of rat heart in order to investigate inotropic or proarrhythmic effects respectively. Also, the effects of resveratrol or IBMX on the sinoatrial node rate were examined in the isolated right atria of rat heart. RESULTS: Resveratrol (10 µM and 100 µM) produces a leftward shift in the concentration-response curves for the contractile effects of noradrenaline, tyramine or isoproterenol and reduces the -log EC50 values of these three agents. IBMX produces similar effects. The spontaneous ventricular beating rate was increased by all three compounds, an effect that was further enhanced by the addition of IBMX. In contrast, resveratrol (100 µM) abolished the effects of these sympathomimetic agents on the ventricular rate. Resveratrol (1-100 µM) had no effect on the sinoatrial node rate, while IBMX produce a concentration dependent sinoatrial tachycardia. DISCUSSION: Taken together, the finding, indicate that resveratrol, like the PDE inhibitor IBMX enhances the contractile effects of sympathomimetic agents but, in contrast to IBMX, it does not enhance their proarrhythmic effect or produce sinoatrial tachycardia. This is most probably consequence of the antiarrhythmic effect of resveratrol which protect against the proarrhythmic effects resulting from PDE inhibition.

Single inhibition of either PDE3 or PDE4 unmasks ß2-adrenoceptor-mediated inotropic and lusitropic effects in the left but not right ventricular myocardium of rat.

Cyclic nucleotide phosphodiesterase (PDE)3 and PDE4 provide the major PDE activity in cardiac myocytes and shape ß1-adrenoceptor-dependent cardiac cAMP signaling but their role in regulating ß2-adrenoceptor-mediated responses is less well known. We investigated potential differences in PDE3 and PDE4 activities between right (RV) and left (LV) ventricular myocardium, and their role in regulating ß2-adrenoceptor effects. PDE3 activity in the microsomal fraction was lower in RV than in LV but was the same in the cytosolic fraction. However, no significant difference between RV and LV was found when the PDE4 activity was studied. ß2-adrenoceptor activation increased inotropism and lusitropism in LV when measured in the presence of either the PDE3 inhibitor cilostamide, the PDE4 inhibitor rolipram or a non-selective PDE inhibitor IBMX. However, the joint inhibition of both PDE3 and PDE4 was necessary in RV to uncover ß2-adrenoceptor-induced inotropic and lusitropic effects. Our results indicate different regulation of ß2-adrenoceptor-mediated contractility by PDE3 and PDE4 in RV and LV of the rat heart. In the case of PDE3 due to a different contribution of the enzyme in the microsomal fraction whereas in the case of PDE4 it can be attributed to differences in the intracellular distribution and coupling to ß2-adrenoceptors.

Glucagon Increases Beating Rate but Not Contractility in Rat Right Atrium. Comparison with Isoproterenol.

This study evaluated the chronotropic and inotropic responses to glucagon in spontaneously beating isolated right atria of rat heart. For comparison, we also investigated the effects resulting from stimulating ß-adrenoceptors with isoproterenol in this tissue. Isoproterenol increased both atrial frequency and contractility but glucagon only enhanced atrial rate. The transcript levels of glucagon receptors were about three times higher in sinoatrial node than in the atrial myocardium. Chronotropic responses to glucagon and isoproterenol were blunted by the funny current (If) inhibitor ZD 7288. Inhibitors of protein kinase A, H-89 and KT-5720 reduced the chronotropic response to glucagon but not to isoproterenol. Inhibition of ryanodine receptors and calcium/calmodulin dependent protein kinase II (important regulators of sarcoplasmic reticulum Ca2+ release), with ruthenium red and KN-62 respectively, failed to alter chronotropic responses of either glucagon or isoproterenol. Non selective inhibition of phosphodiesterase (PDE) with 3-isobutylmethylxantine or selective inhibition of PDE3 or PDE4 with cilostamide or rolipram respectively did not affect chronotropic effects of glucagon or isoproterenol. Our results indicate that glucagon increases beating rate but not contractility in rat right atria which could be a consequence of lower levels of glucagon receptors in atrial myocardium than in sinoatrial node. Chronotropic responses to glucagon or isoproterenol are mediated by If current but not by sarcoplasmic reticulum Ca2+ release, neither are regulated by PDE activity.

PDE2 activity differs in right and left rat ventricular myocardium and differentially regulates ß2 adrenoceptor-mediated effects.

The important regulator of cardiac function, cAMP, is hydrolyzed by different cyclic nucleotide phosphodiesterases (PDEs), whose expression and activity are not uniform throughout the heart. Of these enzymes, PDE2 shapes ß1 adrenoceptor-dependent cardiac cAMP signaling, both in the right and left ventricular myocardium, but its role in regulating ß2 adrenoceptor-mediated responses is less well known. Our aim was to investigate possible differences in PDE2 transcription and activity between right (RV) and left (LV) rat ventricular myocardium, as well as its role in regulating ß2 adrenoceptor effects. The free walls of the RV and the LV were obtained from Sprague-Dawley rat hearts. Relative mRNA for PDE2 (quantified by qPCR) and PDE2 activity (evaluated by a colorimetric procedure and using the PDE2 inhibitor EHNA) were determined in RV and LV. Also, ß2 adrenoceptor-mediated effects (ß2-adrenoceptor agonist salbutamol + ß1 adrenoceptor antagonist CGP-20712A) on contractility and cAMP concentrations, in the absence or presence of EHNA, were studied in the RV and LV. PDE2 transcript levels were less abundant in RV than in LV and the contribution of PDE2 to the total PDE activity was around 25% lower in the microsomal fraction of the RV compared with the LV. ß2 adrenoceptor activation increased inotropy and cAMP levels in the LV when measured in the presence of EHNA, but no such effects were observed in the RV, either in the presence or absence of EHNA. These results indicate interventricular differences in PDE2 transcript and activity levels, which may distinctly regulate ß2 adrenoceptor-mediated contractility and cAMP concentrations in the RV and in the LV of the rat heart.

Pathophysiological relevance of the cardiac ß2-adrenergic receptor and its potential as a therapeutic target to improve cardiac function.

ß-adrenoceptors are members of the G protein-coupled receptor superfamily which play a key role in the regulation of myocardial function. Their activation increases cardiac performance but can also induce deleterious effects such as cardiac arrhythmias or myocardial apoptosis. In fact, inhibition of ß-adrenoceptors exerts a protective effect in patients with sympathetic over-stimulation during heart failure. Although ß(2)-adrenoceptor is not the predominant subtype in the heart, it seems to importantly contribute to the cardiac effects of adrenergic stimulation; however, the mechanism by which this occurs is not fully understood. This review summarizes the current knowledge on the role of ß(2)-adrenoceptors in the regulation of cardiac contractility, metabolism, cardiomyocyte survival and cardiac arrhythmias. In addition, therapeutic considerations relating to stimulation of the ß(2)-adrenoceptor such as an increase in cardiac contractility with low arrythmogenic effect, protection of the myocardium again apoptosis or positive regulation of heart metabolism are discussed.

The increase in rat ventricular automaticity induced by salbutamol is mediated through ß(1)- but not ß(2)-adrenoceptors: role of phosphodiesterases.

AIMS: While ß(2)-adrenoceptor (AR) agonists are useful bronchodilators, they also produce cardiac arrhythmias. These agents are not fully selective and also activate ß(1)-AR, but the involvement of ß(1)-AR and ß(2)-AR in the observed pro-arrhythmic effect has not been established. We studied the effect of ß(1)-AR and ß(2)-AR activation on ventricular automaticity and the role of phosphodiesterases (PDE) in regulating this effect. MAIN METHODS: Experiments were performed in the spontaneously beating isolated right ventricle of the rat heart. We also measured cAMP production in this tissue. KEY FINDINGS: The ß(2)-AR agonist salbutamol (1-100 µM) produced a concentration-dependent increase in ventricular automaticity that was not affected by 50nM of the ß(2)-AR antagonist ICI 118551. This effect was enhanced by the non-selective PDE inhibitor theophylline (100 µM) and by the selective PDE4 inhibitors rolipram (1 µM) and Ro 201724 (2 µM), but not modified by the selective PDE3 inhibitors cilostamide (0.3 µM) or milrinone (0.2 µM). The effects of salbutamol alone and in the presence of either theophylline or rolipram were virtually abolished by 0.1 µM ß(1)-AR antagonist CGP 20712A. Salbutamol (10 µM) increased the cAMP concentration, and this effect was abolished by CGP 20712A (0.1 µM) but enhanced by theophylline (100 µM) or rolipram (1 µM). Cilostamide (0.3 µM) failed to modify the effect of salbutamol on cAMP concentration. SIGNIFICANCE: These results indicate that the increase of ventricular automaticity elicited by salbutamol was exclusively mediated through ß(1)-AR and enhanced by non-selective PDE inhibition with theophylline or selective PDE4 inhibition. However, PDE3 did not appear to regulate this effect.

Regulation of contractility and metabolic signaling by the ß2-adrenergic receptor in rat ventricular muscle.

AIMS: Cardiac function is modulated by the sympathetic nervous system through ß-adrenergic receptor (ß-AR) activity and this represents the main regulatory mechanism for cardiac performance. To date, however, the metabolic and molecular responses to ß(2)-agonists are not well characterized. Therefore, we studied the inotropic effect and signaling response to selective ß(2)-AR activation by tulobuterol. MAIN METHODS: Strips of rat right ventricle were electrically stimulated (1Hz) in standard Tyrode solution (95% O(2), 5% CO(2)) in the presence of the ß(1)-antagonist CGP-20712A (1µM). A cumulative dose-response curve for tulobuterol (0.1-10µM), in the presence or absence of the phosphodiesterase (PDE) inhibitor IBMX (30µM), or 10min incubation (1µM) with the ß(2)-agonist tulobuterol was performed. KEY FINDINGS: ß(2)-AR stimulation induced a positive inotropic effect (maximal effect=33±3.3%) and a decrease in the time required for half relaxation (from 45±0.6 to 31±1.8ms, -30%, p<0.001) after the inhibition of PDEs. After 10min of ß(2)-AR stimulation, p-AMPKα(T172) (54%), p-PKB(T308) (38%), p-AS160(T642) (46%) and p-CREB(S133) (63%) increased, without any change in p-PKA(T197). SIGNIFICANCE: These results suggest that the regulation of ventricular contractility is not the primary function of the ß(2)-AR. Rather, ß(2)-AR could function to activate PKB and AMPK signaling, thereby modulating muscle mass and energetic metabolism of rat ventricular muscle.

Glucagon increases contractility in ventricle but not in atrium of the rat heart.

This study evaluates the inotropic responses to glucagon in electrically driven isolated left and right atria as well as in right ventricular strips of rat heart. For comparison, the contractile effects resulting from stimulating beta-adrenoceptors with isoprenaline in atrial and ventricular tissues were also obtained. Glucagon (0.01-1 microM) produces a concentration-dependent positive inotropic effect in ventricular but not in atrial myocardium. Isoprenaline, however, increases contractility both in atrial and ventricular tissues. The nonselective phosphodiesterase (PDE) inhibitor 3-isobutylmethylxantine (IBMX, 10 microM) enhances the contractile effect of glucagon on ventricular myocardium. However, glucagon still failed to increase contractility in atrial myocardium in the presence of 10 microM, IBMX. Also, in left atria of rats pretreated with pertussis toxin, glucagon did not produce any positive inotropic effect, either alone or in the presence of 10 microM, IBMX. Western blotting analysis indicates that glucagon receptors expression is 5 times higher in ventricular than in atrial myocardium. Taken together, these results indicate that the lack of inotropic effect of glucagon in atrium is not due to Gi protein or PDEs activity but seems to be a consequence of a lower glucagon receptor density in this tissue.

Rolipram reduces the inotropic tachyphylaxis of glucagon in rat ventricular myocardium.

Glucagon increases cardiac contractility through G(s) protein-coupled glucagon receptors, but the inotropic responses fade. The fade could be due to receptor desensitisation or to the action of phosphodiesterases (PDE), or to both mechanisms. We investigated the effects of the PDE4 inhibitor rolipram (1 microM) on the inotropic and cAMP-responses to glucagon in paced right ventricular strips of the rat heart. Responses to the partial agonist dobutamine, mediated through beta(1)-adrenoceptors, were studied for comparison. Glucagon increased contractility (-logEC(50)M=7.3 for maximum responses with E(max)=32% of the response to 9 mM Ca(2+)), but the responses tended to fade (-logEC(50)M=7.1 for faded responses with E(max)=11.5%). Dobutamine (-logEC(50)M=5.8, E(max)=56%) produced positive inotropic effects that did not fade. Rolipram did not affect basal contractility and cAMP levels. Rolipram enhanced the contractile responses to glucagon and reduced fade (-logEC(50)M=7.5 and 7.3 with E(max)=74% and 45% for maximum and faded responses respectively). The response to glucagon (0.1 microM) completely faded in the absence of rolipram, but only partially faded and then remained stable in the presence of rolipram (1 microM). Rolipram enhanced contractile responses to dobutamine (-logEC(50)M=6.0, E(max)=75%). Dobutamine (3 microM), but not glucagon (0.1 microM), increased tissue levels of cAMP. Consistent with the inotropic data, rolipram caused glucagon to augment cAMP and enhanced the effects of dobutamine. Thus, PDE4 activity limits the responses mediated through both glucagon receptors and beta(1)-adrenoceptors. PDE4-catalysed hydrolysis of cAMP contributes to the inotropic tachyphylaxis of glucagon.