SPATIAL-DISTRIBUTION OF NERVE PROCESSES AND BETA-ADRENORECEPTORS IN THE RAT ATRIOVENTRICULAR MODE

Atrioventricular (AV) nodal conduction time is known to be modulated b y the autonomic nervous system. The presence of numerous parasympathet ic 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), vasoactiv e intestinal peptide (VIP), calcitonin gene-related peptide (CGRP) and beta(1) and beta(2)-adrenoreceptors. Serial 12 mu m sections of fresh frozen tissue taken from the frontal plane of the rat atrioventricula r node, His bundle and bundle branches were processed for histology, a cetylcholinesterase (AChE) activity and immunohistochemistry. It was f ound that the AV and ventricular conduction systems were more densely innervated than the atrial and ventricular myocardium as revealed by P GP 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 a nd ventricular conduction systems, the spatial distribution of which w as nonuniform exhibiting a paucity of AChE-reactive nerve processes in the central midnodal cell region and a preponderance in the circumfer ential transitional cell region. TH-immunoreactivity was uniformly dis tributed throughout the AV and ventricular conduction systems includin g the central midnodal and circumferential transitional cell regions. beta(1)-adrenoreceptors were found throughout the AV and ventricular c onduction systems with a preponderance in the circumferential transiti onal cell region. beta(2)-adrenoreceptors were localised predominantly in AV and ventricular conduction systems with a paucity of expression in the circumferential transitional cell region. These results demons trate that the overall uniform distribution of total nerve processes i s comprised of nonuniformly distributed subpopulations of parasympathe tic and sympathetic nerve processes. The observation that the midnodal cell region exhibits a differential spatial pattern of parasympatheti c and sympathetic innervation suggests multiple sites for modulation o f impulse conduction within this legion. Moreover, the localisation of beta(2)-ARs in the AV conduction system, with an absence of expressio n in the circumferential transitional cell layer, suggests that subtyp e-specific pharmacological agents may have distinct effects upon AV no dal conduction.