Relative Distribution Densities of Cholinergic and Adrenoceptor Structures in the Central Part of the Sinoatrial Node in Rat Heart

Abstract

Characteristics of distribution of cholinergic and adrenoceptor structures along the sinoatrial node artery in rat heart were evaluated by autoradiography on semithin sections by determining the density of (3)H-dihydroalprenolol and (3)H-quinuclidinyl benzilate binding sites. The relative density of binding sites for (3)H-dihydroalprenolol and (3)H-quinuclidinyl benzilate was minimum in the functional nucleus of the sinoatrial node and asymmetrically increased to maximum values to cranial (sharply) and caudal (smoothly) directions. The relative level of binding for (3)H-dihydroalprenolol in the perinodal atrial myocardium tissue was markedly lower than in the periarterial zone of the central part of the sinoatrial node and comparable to that for (3)H-quinuclidinyl benzilate.

Distribution of 3H-Dopamine and 3H-DAGO Binding Sites in the Central Part of Rat Sinoatrial Node

“…Topographic identification of the position of the glass microelectrode inserted into the DPR region after binding, washing, fixation, and embedding in epon showed that, similarly to the previous series of experiments [5], the electrode was located in one of atypical myocytes (Figs. 1, 2) forming a fine (2-3 layers of cells) muff around the SAN artery.…”

“…The morphology of these myocytes was studied previously using intracellular La 3+ treatment and corresponded to the morphology of typical nodal cells [6]. The microelectrode tip in all cases was situated only on one side of the SAN artery in a region, denoted as the lateral SAN area [5]. For evaluation of possible relationships between migration processes the DPR along the SAN artery in response to autonomic neurotransmitters and peculiarities in the distribution densities of receptor structures to these modulators, we carried out densitometry in the narrow area in the immediate vicinity of the SAN artery t. media (only in the lateral area).…”

“…After detection of DPR a glass microelectrode was left in the tissue, and culture medium in the cuvette was replaced with washing medium (50 mM Tris-HCl (pH 7.4) with 0.1% ascorbic acid, 0.1 nM pyrocatechine, 120 mM NaCl, 5 mM KCl, 2 mM CaCl 2 , 1 mM MgCl 2 for 3 H-dopamine; 50 mM Tris-HCl (pH 7.4) with 0.2% BSA and 0.04% bacitracin for 3 H-DAGO) for 15 min. The ligand binding was carried out as described previously [5] by replacing washing medium with medium with 3 H-dopamine (Amersham, 49.7 Ci/mmol) and 3 H-DAGO (Amersham, 48 Ci/mmol) to a final concentration of labeled ligand 2 nM. The binding was carried out for 1 h at ambient temperature (20-25 o C).…”

“…For comparison the density of autoradiographic label in the adjacent perinodal atrial working myocardium was evaluated. The images shown by the videocam were computer-processed [5]. The results were expressed as the percentage of the visual field area occupied by autoradiographic label grains.…”

“…with norepinephrine) leads to the effect of transposition of the dominant pacemaker region (DPR) [1,3]. A possible cause of this effect is uneven distribution of cholinoceptor and adrenoceptor structures in the central part of SAN [5]. Apart from basic transmitters of the autonomic nervous system, myocardial tissue, specifically SAN, contains a spectrum of neurotransmitters; of these, enkephalins [12,13] and dopamine [2,11] are particularly interesting, as the former are responsible for acetylcholine and norepinephrine release from nerve terminals and the latter is a catecholamine precursor in sympathetic terminals, on the one hand, and an independent neurotransmitter with peripheral receptor formations and specific physiological effects, including the chronotropic effect [2,11], on the other.…”