Sympathetic nerve stimulation activates both beta 1- and beta 2-adrenoceptors of SA and AV nodes in anesthetized dog hearts

Targeted disruption of the beta2 adrenergic receptor gene

beta-Adrenergic receptors (beta-ARs) are members of the superfamily of G-protein-coupled receptors that mediate the effects of catecholamines in the sympathetic nervous system. Three distinct beta-AR subtypes have been identified (beta1-AR, beta2-AR, and beta3-AR). In order to define further the role of the different beta-AR subtypes, we have used gene targeting to inactivate selectively the beta2-AR gene in mice. Based on intercrosses of heterozygous knockout (beta2-AR +/-) mice, there is no prenatal lethality associated with this mutation. Adult knockout mice (beta2-AR -/-) appear grossly normal and are fertile. Their resting heart rate and blood pressure are normal, and they have a normal chronotropic response to the beta-AR agonist isoproterenol. The hypotensive response to isoproterenol, however, is significantly blunted compared with wild type mice. Despite this defect in vasodilation, beta2-AR -/- mice can still exercise normally and actually have a greater total exercise capacity than wild type mice. At comparable workloads, beta2-AR -/- mice had a lower respiratory exchange ratio than wild type mice suggesting a difference in energy metabolism. beta2-AR -/- mice become hypertensive during exercise and exhibit a greater hypertensive response to epinephrine compared with wild type mice. In summary, the primary physiologic consequences of the beta2-AR gene disruption are observed only during the stress of exercise and are the result of alterations in both vascular tone and energy metabolism.

Spatial distribution of nerve processes and beta-adrenoreceptors in the rat atrioventricular node

Atrioventricular (AV) nodal conduction time is known to be modulated by the autonomic nervous system. The presence of numerous parasympathetic and sympathetic nerve fibres in association with conduction tissue in the heart is well authenticated. In this study, confocal microscopy was used to image the distribution of antibodies directed against the general neuronal marker PGP 9.5, tyrosine hydroxylase (TH), vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP) and beta1 and beta2-adrenoreceptors. Serial 12 microm sections of fresh frozen tissue taken from the frontal plane of the rat atrioventricular node, His bundle and bundle branches were processed for histology, acetylcholinesterase (AChE) activity and immunohistochemistry. It was found that the AV and ventricular conduction systems were more densely innervated than the atrial and ventricular myocardium as revealed by PGP 9.5 immunoreactivity. Furthermore, the transitional cell region was more densely innervated than the midnodal cell region, while spatial distribution of total innervation was uniform throughout all AV nodal regions. AChE-reactive nerve processes were found throughout the AV and ventricular conduction systems, the spatial distribution of which was nonuniform exhibiting a paucity of AChE-reactive nerve processes in the central midnodal cell region and a preponderance in the circumferential transitional cell region. TH-immunoreactivity was uniformly distributed throughout the AV and ventricular conduction systems including the central midnodal and circumferential transitional cell regions. Beta1-adrenoreceptors were found throughout the AV and ventricular conduction systems with a preponderance in the circumferential transitional cell region. Beta2-adrenoreceptors were localised predominantly in AV and ventricular conduction systems with a paucity of expression in the circumferential transitional cell region. These results demonstrate that the overall uniform distribution of total nerve processes is comprised of nonuniformly distributed subpopulations of parasympathetic and sympathetic nerve processes. The observation that the midnodal cell region exhibits a differential spatial pattern of parasympathetic and sympathetic innervation suggests multiple sites for modulation of impulse conduction within this region. Moreover, the localisation of beta2-ARs in the AV conduction system, with an absence of expression in the circumferential transitional cell layer, suggests that subtype-specific pharmacological agents may have distinct effects upon AV nodal conduction.

Parasympathetic inhibition of sympathetic effects on atrioventricular conduction in anesthetized dogs

To investigate the selective parasympathetic control of atrioventricular (AV) conduction during sympathetic activation, we studied the effects of cervical vagus nerve stimulation on the positive dromotropic responses to sympathetic interventions before and after surgical dissection of dual fatty tissues at the junction of the inferior vena cava and inferior left atrium and at the right atrial side of the atrial junctions of the right pulmonary veins in open-chest anesthetized dogs. In atrial-paced hearts, vagus stimulation at low frequencies prolonged atrio-His (A-H) interval and at high frequencies induced second- and third-degree AV blocks. Vagus stimulation additively prolonged A-H interval shortened by stimulation of the ansae subclaviae or isoproterenol infusion. After dissection of dual fatty tissues, vagus stimulation prolonged A-H interval by only 7%. However, during sympathetic stimulation but not during isoproterenol infusion, vagus stimulation prolonged the shortened A-H interval. Atropine abolished the responses to vagus stimulation. These results suggest that even during sympathetic activation, regional vagus inputs selectively control A-H interval, and even after denervation of the regional parasympathetic nerves, presynaptic parasympathetic inhibition of the positive cardiac responses to sympathetic activation works in the heart in situ.

Targeted disruption of the mouse beta1-adrenergic receptor gene: developmental and cardiovascular effects

At least three distinct beta-adrenergic receptor (beta-AR) subtypes exist in mammals. These receptors modulate a wide variety of processes, from development and behavior, to cardiac function, metabolism, and smooth muscle tone. To understand the roles that individual beta-AR subtypes play in these processes, we have used the technique of gene targeting to create homozygous beta 1-AR null mutants (beta 1-AR -/-) in mice. The majority of beta 1-AR -/- mice die prenatally, and the penetrance of lethality shows strain dependence. Beta l-AR -/- mice that do survive to adulthood appear normal, but lack the chronotropic and inotropic responses seen in wild-type mice when beta-AR agonists such as isoproterenol are administered. Moreover, this lack of responsiveness is accompanied by markedly reduced stimulation of adenylate cyclase in cardiac membranes from beta 1-AR -/- mice. These findings occur despite persistent cardiac beta 2-AR expression, demonstrating the importance of beta 1-ARs for proper mouse development and cardiac function, while highlighting functional differences between beta-AR subtypes.