Effect of Clonidine on Cardiac Norepinephrine Spillover in Isolated Rat Heart

Receptors involved in moxonidine-stimulated atrial natriuretic peptide release from isolated normotensive rat hearts

Imidazoline I1-receptors are present in the heart and may be involved in atrial natriuretic peptide (ANP) release. The following studies investigated whether moxonidine (an antihypertensive imidazoline I1-receptor and alpha2-adrenoceptor agonist) acts directly on the heart to stimulate ANP release, and to characterize the receptor type involved in this action. Perfusion of rat (200-225 g) isolated hearts with moxonidine (10(-6) and 10(-5) M), for 30 min, resulted in ANP release (83+/-29 and 277+/-70 ng/30 min, above basal, respectively), significantly (P<0.01) different from perfusion with buffer (-6+/-31 ng/30 min). ANP release stimulated by moxonidine (10(-6) M) was inhibited by co-perfusion with the antagonists, AGN192403 (imidazoline I1-receptor), phenoxybenzamine (alpha2>alpha1-adrenoceptors), and prazosin (alpha1>alpha2-adrenoceptors), but increased by rauwolscine (alpha2-adrenoceptors). Perfusion with 10(-5) M brimonidine (full alpha2-adrenoceptor agonist) inhibited moxonidine-stimulated ANP release. Similarly, moxonidine (10(-6) M) tended to reduce coronary flow, but significantly increased coronary flow in the presence of brimonidine, which was vasoconstrictive when perfused alone. Coronary flow was reduced by 10(-5) M each, brimonidine>clonidine>moxonidine; while similar bradycardia was observed with clonidine and moxonidine, but not with brimonidine. In conclusion, these results argue in favor of moxonidine acting primarily on imidazoline I1-receptors to release ANP, with both alpha2-adrenoceptor and imidazoline I1-receptors exerting inhibitory inter-relation. In contrast, the coronary vasodilatory effect of moxonidine requires full activation of alpha2-adrenoceptor. The sympatholytic and ANP-releasing effects of moxonidine appear to be mediated by cardiac imidazoline receptors that may be differentially localized. Most importantly, moxonidine can stimulate ANP release from the heart without contribution of the central nervous system.

Ultrasonic vocalization production of preweanling rats: effects of central and peripheral administration of alpha2-adrenoceptor agonists

Stimulation of alpha2-adrenoceptors increases the ultrasonic vocalization production of preweanling rats, however it is not known whether these critical alpha2-adrenoceptors are located peripherally or centrally. In a series of three experiments, ultrasonic vocalizations were measured after 11-day-old rats had been administered clonidine or 2-[2,6-diethylphenylamino]-2-imidazole (ST-91) either systemically (i.p.) or into the third ventricle (i.c.v.). These particular alpha2-adrenoceptor agonists were chosen because clonidine is lipophilic and enters the central nervous system, while ST-91 is hydrophilic and does not readily cross the blood-brain barrier. In the third experiment, clonidine- (1 microg, i.c.v.) and ST-91-induced (15 microg, i.c.v.) ultrasonic vocalizations were measured after systemic injection of the alpha2-adrenoceptor antagonist yohimbine (0.5 or 1 mg/kg, i.p.). Results showed that central administration of both clonidine and ST-91 increased the ultrasonic vocalization production of 11-day-old rats, whereas peripheral administration of only clonidine, and not ST-91, increased ultrasonic vocalizations. These results indicate that the alpha2-adrenoceptors mediating ultrasonic vocalization production are located in the central nervous system. Yohimbine fully attenuated clonidine-induced ultrasonic vocalizations but only partially attenuated ST-91-induced vocalizations. This pattern of results may have been due to the differential selectivity of clonidine and ST-91 for alpha2-adrenoceptor subtypes (alpha2A, alpha2B, and alpha2C) or imidazoline receptors. When combined with past research, the present results are consistent with the hypothesis that centrally located alpha2-adrenoceptors are a component of a neural system that mediates ultrasonic vocalization production.

Regulation of postburn ischemia by α- and β-adrenoceptor subtypes

Deep skin burns are characterised by progressive ischemia secondary to vasoconstriction and thrombosis formation. Burn trauma elicits increased sympathetic activity and elevation of circulating catecholamines acting on adrenoceptors in vascular tissue playing an important role in the regulation of organ blood flow. The present study in rats investigated the role of alpha- and beta-adrenoceptors in the circulatory changes taking place in normal skin and in partial- and full-thickness skin burns using laser Doppler flowmetry. Evaluation was based on intravenous administration of the following adrenergic agonists and antagonists: l-phenylephrine (alpha(1)-agonist), prazosin (alpha(1)-antagonist), clonidine (alpha(2)-agonist), yohimbine (alpha(2)-antagonist), prenalterol (beta(1)-agonist), terbutaline (beta(2)-agonist), and propranolol (beta(1)- and beta(2)-antagonist). Blood flow in normal skin was reduced by phenylephrine (p<0.001), clonidine (p<0.001) and propranolol (p<0.01), and increased by prazosin (p<0.05), yohimbine (p<0.05), prenalterol (p<0.05) and terbutaline (p<0.01). In partial-thickness burns, blood flow was reduced by phenylephrine (p<0.01), clonidine (p<0.01) and propranolol (p<0.05). In full-thickness burns, only clonidine reduced perfusion (p<0.05). In conclusion, beta(1)- and beta(2)-adrenoceptors play important role in the physiological regulation of skin perfusion but are of lesser importance for postburn skin perfusion. Vasoconstrictive alpha(1)- and alpha(2)-adrenoceptors were shown to be tonically active in normal skin and in partial-thickness burns, exerting a negative effect on skin perfusion which was further potentiated by exogenous administration of alpha(1)- and alpha(2)-agonists and reversed by selective alpha-blockers. In full-thickness burns, activation of alpha(2)-receptors was shown to significantly impair skin circulation, raising a flag of warning for the use of clonidine to treat pain in burn patients.