Effects of glucagon on atrioventricular conduction and ventricular automaticity in dogs

Glucagon and Its Receptors in the Mammalian Heart

Glucagon exerts effects on the mammalian heart. These effects include alterations in the force of contraction, beating rate, and changes in the cardiac conduction system axis. The cardiac effects of glucagon vary according to species, region, age, and concomitant disease. Depending on the species and region studied, the contractile effects of glucagon can be robust, modest, or even absent. Glucagon is detected in the mammalian heart and might act with an autocrine or paracrine effect on the cardiac glucagon receptors. The glucagon levels in the blood and glucagon receptor levels in the heart can change with disease or simultaneous drug application. Glucagon might signal via the glucagon receptors but, albeit less potently, glucagon might also signal via glucagon-like-peptide-1-receptors (GLP1-receptors). Glucagon receptors signal in a species- and region-dependent fashion. Small molecules or antibodies act as antagonists to glucagon receptors, which may become an additional treatment option for diabetes mellitus. Hence, a novel review of the role of glucagon and the glucagon receptors in the mammalian heart, with an eye on the mouse and human heart, appears relevant. Mouse hearts are addressed here because they can be easily genetically modified to generate mice that may serve as models for better studying the human glucagon receptor.

Oxyntomodulin increases intrinsic heart rate through the glucagon receptor

Two hormones from the gastrointestinal tract, glucagon and oxyntomodulin (OXM), vigorously elevate the intrinsic heart rate (I_HR) of mice. We have previously shown that OXM influences murine heart rate (HR) independent of the glucagon-like peptide 1 (GLP-1) receptor. Here, we demonstrate using radiotelemetry in mice deficient in the glucagon receptor (Gcgr −/−) that both OXM and glucagon require the glucagon receptor for their chronotropic effects on the heart. Furthermore, we found that other hormones associated with hunger and satiety (ghrelin, leptin, and PYY_3-36) had no effect on I_HR, while cholecystokinin moderately elevated the I_HR. Finally, the resting HR of Gcgr −/− mice was higher than in control mice (Gcgr +/+ and Gcgr +/−) at thermal neutral temperature (30°C). Using atropine, we demonstrated that Gcgr −/− mice have diminished parasympathetic (PNS) influence of the heart at this temperature. Gcgr −/− mice displayed a normal bradycardia as compared to controls in response to administration of either methacholine (to activate the muscarinic acetylcholine receptor) or methoxamine (to activate the baroreflex through agonism of the α1 adrenergic receptor agonist) suggesting that vagal pathways are intact in the Gcgr −/− mice. As OXM is an agonist of the GLP-1 receptor and Gcgr with antidiabetic activity, we suggest OXM may be an alternative to glucagon in the treatment of overdose of beta-blockers to elevate HR in clinical conditions.

Glucagon therapy in the treatment of symptomatic bradycardia

Symptomatic bradycardia is commonly seen in the emergency department. Effective drug therapy for this clinical scenario is limited. Although glucagon has been used in no clinical trial in this setting, its cardiac activity may prove useful, particularly in the setting of beta-adrenergic blockade. We report a case series comprising three patients taking maintenance beta-blocker therapy who presented to the ED with symptomatic bradycardia and hypotension and in whom glucagon therapy obviated the need for further treatment. Further study is warranted to evaluate and define the role of glucagon in the treatment of symptomatic bradycardia.

Effects of amrinone on atrioventricular conduction in the intact canine heart

The effects of amrinone on conduction in the intact canine heart were studied. Intracardiac His-electrode catheter recordings were used to measure the functional refractory period (FRP) of the AV node and conduction time through the AV node (A2H2 interval) and in the His-Purkinje system (H2V2 interval). Amrinone (2.5 to 10 mg/kg) shortened the FRP and A2H2 in a dose-dependent manner but had no significant effect on H2V2. In hearts where AV conduction was depressed by treatment with verapamil, propranolol, or ouabain, amrinone partially reversed this depression. Amrinone also shortened the recovery time of spontaneous sinoatrial (SA) node activity following a train of rapid atrial stimulation. This effect was also observed after depression of SA nodal recovery with verapamil, propranolol, or ouabain. These results indicate that amrinone enhances AV conduction and SA nodal activity in the normal heart and may favorably influence depressed AV conduction and SA nodal activity induced by a variety of cardioactive agents.

Influence of glucagon on the cardiovascular effects of procainamide

Effects of glucagon on procainamide-induced cardiac toxicity were studied in anesthetized dogs. Procainamide in doses of 50 and 100 mg/kg produced dose-dependent decreases in the blood pressure, cardiac output, left ventricular work index, and left ventricular systolic pressure; and increases in the left ventricular end diastolic and right atrial pressure, and total systemic vascular resistance. Glucagon antagonized most of the effects of procainamide on the cardiovascular system. Glucagon may be effective in antagonizing procainamide-induced cardiac toxicity.

Calcium ions and the cardiotonic action of glucagon (calcium fluxes-myocardial function-asystole-membrane transport-cat)

The positive inotropic action of glucagon on the cat heart is dependent upon the extracellular Ca concentration. The inotropic effect is proportionately greater, the lower the concentration of Ca in the perfusate (above zero, at which glucagon is without significant effect). Nevertheless, Ca influx (as measured with (45)Ca) into the myocardium is not demonstrably influenced by glucagon, except at 0.09 mM Ca, where the interpretation of the effect is complicated by the fact that there is mechanical asystole at that Ca concentration in the absence of glucagon, while excitation-contraction coupling still occurs when the hormone is added. The possibility is suggested that glucagon may influence intracellular movement of Ca, even when its transmembrane movement is affected only slightly.

Cardiac actions in the dog of a new antagonist of adrenergic excitation which does not produce competitive blockade of adrenoceptors

1. The cardiac actions of amiodarone, a benzofuran derivative used in the treatment of angina pectoris, have been compared with those of (+/-)-propranolol in anaesthetized dogs.2. After three successive intravenous injections of propranolol, 0.5 mg/kg, had reduced the heart rate by 25%, a fourth dose had no further negative chronotropic action, but amiodarone, 10 mg/kg intravenously, at this point reduced the heart rate by 23%.3. Amiodarone, 10 mg/kg intravenously, reduced, but did not abolish, cardiac responses to isoprenaline, 2 mug/kg intravenously. Subsequent successive injections of 10 mg/kg of amiodarone did not further block the responses to isoprenaline, but propranolol, 1 mg/kg intravenously, abolished them.4. Amiodarone reduced cardiac chronotropic and inotropic responses to glucagon, which were not affected by propranolol.5. Cardiac output was increased 5 min after amiodarone, 10 mg/kg intravenously, but at 10 min and thereafter it did not differ from control values. Propranolol, 1 mg/kg intravenously, reduced cardiac output by 17% at 5 min, and by 30% after 30 min.6. From this and other evidence which is discussed, it is concluded that the cardiac actions of amiodarone are not produced by competitive blockade of beta-adrenoceptors.