Alpha 1-adrenoceptors in the conduction system of rat hearts

Neurohumoral Control of Sinoatrial Node Activity and Heart Rate: Insight From Experimental Models and Findings From Humans

The sinoatrial node is perhaps one of the most important tissues in the entire body: it is the natural pacemaker of the heart, making it responsible for initiating each-and-every normal heartbeat. As such, its activity is heavily controlled, allowing heart rate to rapidly adapt to changes in physiological demand. Control of sinoatrial node activity, however, is complex, occurring through the autonomic nervous system and various circulating and locally released factors. In this review we discuss the coupled-clock pacemaker system and how its manipulation by neurohumoral signaling alters heart rate, considering the multitude of canonical and non-canonical agents that are known to modulate sinoatrial node activity. For each, we discuss the principal receptors involved and known intracellular signaling and protein targets, highlighting gaps in our knowledge and understanding from experimental models and human studies that represent areas for future research.

Effects of Pre- and Postconditioning on Arrhythmogenesis in the In Vivo Rat Model

The antiarrhythmic potential of postconditioning in in vivo models remains poorly defined. We compared the effects of pre- and postconditioning on ventricular arrhythmogenesis against controls with and without reperfusion. Wistar rats (n = 40, 269 ± 3 g) subjected to ischemia (30 minutes)–reperfusion (24 hours) were assigned to the following groups: (1) preconditioning (2 cycles), (2) postconditioning (6 cycles), or (3) no intervention and were compared with (4) nonreperfused infarcts and (5) sham-operated animals. Infarct size was measured, and arrhythmogenesis was evaluated with continuous telemetric electrocardiographic recording, heart rate variability indices, and monophasic action potentials (MAPs). During a 24-hour observation period, no differences in mortality were observed. Reperfusion decreased infarct size and ameliorated sympathetic activation during the late reperfusion phase. Preconditioning decreased infarct size by a further 35% ( P = .0017), but only a marginal decrease (by 18%, P = .075) was noted after postconditioning. Preconditioning decreased arrhythmias during ischemia and early reperfusion, whereas postconditioning almost abolished them during the entire reperfusion period. No differences were noted in MAPs or in the magnitude of sympathetic activation between the 2 interventions. Compared to postconditioning, preconditioning affords more powerful cytoprotection, but both interventions exert antiarrhythmic actions. In the latter, these are mainly evident during the ischemic phase and continue during early reperfusion. Postconditioning markedly decreases reperfusion arrhythmias during a prolonged observation period. The mechanisms underlying the antiarrhythmic effects of pre- and postconditioning are likely different but remain elusive.

Topography of 3H-DHA, 3H-QNB, 3H-dopamine, and 3H-DAGO binding sites distribution in the central part of the sinoatrial node in rat heart

The topography of distribution of 3H-dihydroalprenolol, 3H-quinucledinyl benzilate, 3H-dopamine, and 3H-DAGO binding sites in the central part of the sinoatrial node in rat heart was studied by autoradiography after electrophysiological identification of the dominant pacemaker region location. Receptor asymmetry between the lateral and median regions of the central part of the sinoatrial node was shown. The dominant pacemaker region lay in the lateral area of the sinoatrial node; the number of binding sites for all four ligands was minimum in it. The number of binding sites gradually increased in the cranial and caudal directions from the dominant pacemaker region along the sinoatrial node artery (more smoothly in the caudal direction). The relative densities of bindings sites for 3H-dihydroalprenolol and 3H-dopamine were higher in the lateral region compared to the perinodal working myocardium, while the densities for 3H-quinucledinyl benzilate and 3H-DAGO were virtually the same. The distribution of binding sites along the artery in the median region of the sinoatrial node was even for 3H-quinucledinyl benzilate and 3H-DAGO. For 3H-DAGO these parameters were close to those in the perinodal atrial myocardium, for 3H-quinucledinyl benzilate somewhat lower. Curves presenting the distribution of binding site densities for 3H-dihydroalprenolol and 3H-dopamine in the median region of the sinoatrial node were similar, with a pronounced peak in the region contralateral to the dominant pacemaker region, and significantly higher binding parameters compared to those for the perinodal atrial myocardium. The difference consisted in higher density of 3H-dopamine binding sites in the median region of the sinoatrial node in comparison with the lateral region. Binding activity was maximum in the wall of the sinoatrial node artery. The distribution of binding sites for ligands to the main autonomic nervous system neurotransmitters in the rat heart sinoatrial node is heterogeneous.

Localization of alpha 1-adrenoceptors in rat and human hearts by immunocytochemistry

The localization of the alpha 1 adrenoceptors (alpha 1-AR) in the heart tissues from rat and human and in the cultured heart cells from neonatal rats was studied by indirect immunofluorescence and postembedding electronmicroscopical immuno-gold technique. With antipeptide antibodies directed against the second extracellular loop of the human alpha 1-AR (AS sequence 192-218), this receptor was found to be localized along the sarcolemma in both human and rat hearts. Similar localization sites were detected in cultivated rat neonatal cardiomyocytes. Beside the localization in cardiomyocytes, alpha 1-AR were identified in endothelial cells of capillaries and smooth muscle cells of coronary vessels, in neuronal endings, in mast cells of cultivated heart cells but not, or in less amount in fibroblasts. Interestingly, in the right atrium of rat heart the localization of alpha 1-AR was found to be near or on atrial natriuretic factor (ANF) granules, providing the basis for the alpha-adrenergic influence on ANF release. The immunocytochemical studies further confirm and complete the findings known by using autoradiographic binding studies with specific ligands.