Role of imidazoline-preferring receptors in the genesis of epinephrine-induced arrhythmias in halothane-anesthetized dogs

Reduced Hemodynamic Responses to Physical and Mental Stress Under Low-Dose Rilmenidine in Healthy Subjects

Activation of the sympathetic nervous system plays a major role in the pathogenesis and prognosis of cardiovascular diseases. Rilmenidine is an I(1)-imidazoline receptor agonist that reduces blood pressure by modulation of central sympathetic activity, but the effects of low-dose rilmenidine on the hemodynamic responses to physiological maneuvers that increase adrenergic drive is not known. To assess the effects of low-dose rilmenidine on the hemodynamic responses to stress, 32 healthy subjects (20-56 years old) underwent acute physical exercise (n = 15, individualized ramp protocol on treadmill) and mental stress (n = 17, word color Stroop and mental arithmetics tests) two hours after the oral administration of 0.5 mg of rilmenidine (RIL) or placebo (PLA) following a randomized, double-blind, placebo controlled crossover study. No subject complained of any side effect. Rilmenidine reduced peak exercise heart rate (PLA: 187 +/- 7; RIL: 181 +/- 9 bpm; P = 0.003), but did not modify peak aerobic power (VO(2max) - PLA: 41.7 +/- 6.2; RIL: 42.3 +/- 6.7 ml/kg/min; P = 0.26). During mental stress, rilmenidine inhibited the peak systolic (PLA: 123 +/- 10; RIL: 114 +/- 8 mmHg; P = 0.02) and diastolic (PLA: 86 +/- 7; RIL: 81 +/- 7 mmHg; P <0.05) blood pressure responses. In conclusion, rilmenidine reduced the hemodynamic response to physical and mental stress stimuli without limiting exercise capacity. These results support the concept that rilmenidine, at a dose lower than the ones recommended to treat hypertension, reduced the myocardial oxygen demand to stress and may carry potential clinical impact.

Protective effects of centrally acting sympathomodulatory drugs on myocardial ischemia induced by sympathetic overactivity in rabbits

It is recognized that an imbalance of the autonomic nervous system is involved in the genesis of ventricular arrhythmia and sudden death during myocardial ischemia. In the present study we investigated the effects of clonidine and rilmenidine, two centrally acting sympathomodulatory drugs, on an experimental model of centrally induced sympathetic hyperactivity in pentobarbital-anesthetized New Zealand albino rabbits of either sex (2-3 kg, N = 89). We also compared the effects of clonidine and rilmenidine with those of propranolol, a beta-blocker, known to induce protective cardiovascular effects in patients with ischemic heart disease. Central sympathetic stimulation was achieved by intracerebroventricular injection of the excitatory amino acid L-glutamate (10 micro mol), associated with inhibition of nitric oxide synthesis with L-NAME (40 mg/kg, iv). Glutamate triggered ventricular arrhythmia and persistent ST-segment shifts in the ECG, indicating myocardial ischemia. The intracisternal administration of clonidine (1 microg/kg) and rilmenidine (30 microg/kg) or of a nonhypotensive dose of rilmenidine (3 microg/kg) decreased the incidence of myocardial ischemia (25, 14 and 25%, respectively, versus 60% in controls) and reduced the mortality rate from 40% to 0.0, 0.0 and 12%, respectively. The total number of ventricular premature beats per minute fell from 30 +/- 9 in the control group to 7 +/- 3, 6 +/- 3 and 2 +/- 2, respectively. Intravenous administration of clonidine (10 micro g/kg), rilmenidine (300 microg/kg) or propranolol (500 microg/kg) elicited similar protective effects. We conclude that clonidine and rilmenidine present cardioprotective effects of central origin, which can be reproduced by propranolol, a lipophilic beta-blocking agent.

Facilitation of spontaneous defibrillation by moxonidine during regional ischaemia in an isolated working rabbit heart model

Moxonidine has been shown to be antiarrhythmic during ischaemia in vivo. This study aimed to investigate its electrophysiological effects in isolated working rabbit hearts in vitro. Monophasic action potential duration, effective refractory period and conduction delay were measured at three ventricular sites. The hearts were treated before and during ischaemia and reperfusion with vehicle, moxonidine (0.01, 0.1 and 1 microM) or labetalol (1 microM). In all groups, ventricular fibrillation was always induced during ischaemia. Only 0.1 microM moxonidine decreased the incidence of sustained ventricular fibrillation from 86 to 17%, although it did not affect any electrophysiological parameters measured. Similarly, labetolol, an adrenoceptor blocker, facilitated spontaneous defibrillation without any electrophysiological effects. In conclusion, moxonidine directly facilitates spontaneous defibrillation of ventricular fibrillation during ischaemia. Since the same effect is observed with labetalol, it is possible that the defibrillatory action of moxonidine is related to its peripheral antiadrenergic activity, although other mechanisms cannot be excluded.

The involvement of pertussis toxin-sensitive G proteins in the post receptor mechanism of central I1-imidazoline receptors

1. To elucidate the possible involvement of pertussis toxin (PTX)-sensitive G proteins in the post receptor mechanism of alpha 2-adrenoceptors and imidazoline receptors, we examined the effect of pretreatment of the central nervous system with PTX on the antidysrhythmic effect of dexmedetomidine, a selective alpha 2-adrenoceptor agonist, and rilmenidine, a selective I1-imidazoline receptor agonist on halothane-adrenaline dysrhythmias in rats. 2. Dexmedetomidine (0, 1.0, 2.0, 5.0 micrograms kg-1 min-1.i.v.) and rilmenidine (0, 1.0, 3.0, 10, 20 micrograms kg-1, i.v.) prevented the genesis of halothane-adrenaline dysrhythmias in a dose-dependent fashion. Both idazoxan (10, 20 micrograms kg-1, intracerebroventricularly (i.c.v.)), an alpha 2-adrenoceptor antagonist with high affinity for imidazoline receptors, and rauwolscine, (40 micrograms kg-1, i.c.v.), an alpha 2-adrenoceptor antagonist with low affinity for imidazoline receptors inhibited the action of dexmedetomidine (5.0 micrograms kg-1, min-1, i.v.), but the inhibitory potency of idazoxan was much greater than that of rauwolscine. While the pretreatment with PTX (0.1, 0.5, 1.0 micrograms kg-1, i.c.v.) did not change the dysrhythmogenecity of adrenaline, this treatment completely blocked the antidysrhythmic property of rilmenidine (20 micrograms kg-1, i.v.) as well as dexmedetomidine (5.0 micrograms kg-1 min-1, i.v.). 3. It is suggested that central I1-imidazoline receptors as well as alpha 2-adrenoceptors may be functionally coupled to PTX-sensitive G proteins.

Antiarrhythmic action of rilmenidine on adrenaline-induced arrhythmia via central imidazoline receptors in halothane-anaesthetized dogs

1. To elucidate the role of central imidazoline receptors in the genesis of adrenaline-induced arrhythmias under halothane anaesthesia, we investigated the effects of rilmenidine, a selective agonist at imidazoline receptors, on this type of arrhythmia in dogs. Rilmenidine (1, 3, 10 micrograms kg-1, i.v.) did not affect basal haemodynamic parameters (heart rate and blood pressure), but dose-dependently inhibited adrenaline-induced arrhythmias under halothane anaesthesia. 2. Although, rilmenidine has a weak affinity for alpha(2)-adrenoceptors, pretreatment with idazoxan (10 micrograms kg-1, intracisternally i.c.), an imidazoline receptor antagonist which has also alpha(2)-adrenoceptor blocking potency, blocked the antiarrhythmic effect of rilmenidine (10 micrograms kg-1, i.v.). In contrast, pretreatment with rauwolscine (20 micrograms kg-1, i.c.), a classical alpha(2)-adrenoceptor antagonist with little affinity for imidazoline receptors, did not affect the effect of rilmenidine (10 micrograms kg-1, i.v.). Furthermore, bilateral vagotomy completely blocked the antiarrhythmic action of rilmenidine (10 micrograms kg-1, i.v.). 3. It is suggested that the antiarrhythmic action of rilmenidine is due to the activation of central imidazoline receptors and that vagal tone is critical for this action of rilmenidine.

Antiarrhythmic effect of the selective I1-imidazoline receptor modulator moxonidine on ouabain-induced cardiac arrhythmia in guinea pigs

Moxonidine is a centrally acting antihypertensive agent with potent action on I1-imidazoline receptors. Moxonidine as an SIR modulator elicits a persistent reduction in circulating levels of epinephrine, demonstrating a reduction in sympathetic tone. In the first experiment the threshold dose of ouabain needed to induce ventricular arrhythmia and asystole was determined in guinea pigs, and the influence of moxonidine was tested. In a dose range of 0.1-0.4 mg/kg body weight i.v., moxonidine increased the threshold dose needed to induce ventricular tachycardia, premature ventricular beats, ventricular flutter, ventricular fibrillation, and asystole. The effect was dose-dependent and statistically significant. Clonidine, in a dose range of 0.2-0.8 mg/kg body weight i.v., also increased the threshold dose of ouabain necessary to induce different cardiac rhythm disturbances. Moxonidine was more effective than clonidine. Pretreatment with the alpha 2-receptor and I1-receptor-influencing substances efaroxan, idazoxan, and SKF 86466 attenuated the effect of moxonidine and clonidine. Efaroxan, idazoxan, or SKF 86466 alone reduced the threshold dose of ouabain necessary to induce cardiac arrhythmia as a sign for arrhythmogenic effects. The alpha 1-receptor antagonist prazosin had no influence on ouabain-induced arrhythmia. Pretreatment with prazosin reduced the moxonidine but not the clonidine effect. In the second experiment the influence of moxonidine on aconitine-induced extrasystoles (ES) in the spontaneously beating guinea pig auricle was investigated. Moxonidine in a dose of 10(-7)-10(-8) M reduced the number of ES. A 10-fold higher dose had no influence on ES number. The beta-blocking agent propranolol showed antiarrhythmic effects in both methods. The ouabain-induced cardiac arrhythmia is associated with increased sympathetic tone on central stimulation. The reduced sympathetic tone by centrally acting moxonidine via imidazoline receptors seems responsible for the antiarrhythmic effect of this drug. Clonidine also reduced the sympathetic tone via imidazoline receptor. The selectivity of clonidine to imidazoline receptors is less pronounced than is that of moxonidine. The interaction of moxonidine with imidazoline receptors is not clear. The possible interaction between imidazoline and alpha-adrenoceptors in relation to the antiarrhythmic effect of moxonidine or clonidine is also unknown. Modulation of imidazoline receptors by moxonidine could be an agonistic effect or an antagonism to an endogenous agonistic or antagonistic substance and vice versa.