Localization of multiple neurotransmitters in surgically derived specimens of human atrial ganglia.

Dysfunction of the intrinsic cardiac nervous system is implicated in the genesis of atrial and ventricular arrhythmias. While this system has been studied extensively in animal models, far less is known about the intrinsic cardiac nervous system of humans. This study was initiated to anatomically identify neurotransmitters associated with the right atrial ganglionated plexus (RAGP) of the human heart. Biopsies of epicardial fat containing a portion of the RAGP were collected from eight patients during cardiothoracic surgery and processed for immunofluorescent detection of specific neuronal markers. Colocalization of markers was evaluated by confocal microscopy. Most intrinsic cardiac neuronal somata displayed immunoreactivity for the cholinergic marker choline acetyltransferase and the nitrergic marker neuronal nitric oxide synthase. A subpopulation of intrinsic cardiac neurons also stained for noradrenergic markers. While most intrinsic cardiac neurons received cholinergic innervation evident as punctate immunostaining for the high affinity choline transporter, some lacked cholinergic inputs. Moreover, peptidergic, nitrergic, and noradrenergic nerves provided substantial innervation of intrinsic cardiac ganglia. These findings demonstrate that the human RAGP has a complex neurochemical anatomy, which includes the presence of a dual cholinergic/nitrergic phenotype for most of its neurons, the presence of noradrenergic markers in a subpopulation of neurons, and innervation by a host of neurochemically distinct nerves. The putative role of multiple neurotransmitters in controlling intrinsic cardiac neurons and mediating efferent signaling to the heart indicates the possibility of novel therapeutic targets for arrhythmia prevention.

Cholinergic neurons of mouse intrinsic cardiac ganglia contain noradrenergic enzymes, norepinephrine transporters, and the neurotrophin receptors tropomyosin-related kinase A and p75.

Half of the cholinergic neurons of human and primate intrinsic cardiac ganglia (ICG) have a dual cholinergic/noradrenergic phenotype. Likewise, a large subpopulation of cholinergic neurons of the mouse heart expresses enzymes needed for synthesis of norepinephrine (NE), but they lack the vesicular monoamine transporter type 2 (VMAT2) required for catecholamine storage. In the present study, we determined the full scope of noradrenergic properties (i.e. synthetic enzymes and transporters) expressed by cholinergic neurons of mouse ICG, estimated the relative abundance of neurons expressing different elements of the noradrenergic phenotype, and evaluated the colocalization of cholinergic and noradrenergic markers in atrial nerve fibers. Stellate ganglia were used as a positive control for noradrenergic markers. Using fluorescence immunohistochemistry and confocal microscopy, we found that about 30% of cholinergic cell bodies contained tyrosine hydroxylase (TH), including the activated form that is phosphorylated at Ser-40 (pSer40 TH). Dopamine beta-hydroxylase (DBH) and norepinephrine transporter (NET) were present in all cholinergic somata, indicating a wider capability for dopamine metabolism and catecholamine uptake. Yet, cholinergic somata lacked VMAT2, precluding the potential for NE storage and vesicular release. In contrast to cholinergic somata, cardiac nerve fibers rarely showed colocalization of cholinergic and noradrenergic markers. Instead, these labels were closely apposed but clearly distinct from each other. Since cholinergic somata expressed several noradrenergic proteins, we questioned whether these neurons might also contain trophic factor receptors typical of noradrenergic neurons. Indeed, we found that all cholinergic cell bodies of mouse ICG, like noradrenergic cell bodies of the stellate ganglia, contained both tropomyosin-related kinase A (TrkA) and p75 neurotrophin receptors. Collectively, these findings demonstrate that mouse intrinsic cardiac neurons (ICNs), like those of humans, have a complex neurochemical phenotype that goes beyond the classical view of cardiac parasympathetic neurons. They also suggest that neurotrophins and local NE synthesis might have important effects on neurons of the mouse ICG.

Stereochemical trigger for initiating cooperative interaction of the subunits during the oxygenation of cobaltohemoglobin.

Relative to the standard provided by an unconstrained five-coordinate imidazoleiron(II) porphyrin, the axial separation (N(epsilon)...P(mu)) in deoxyhemoglobin of the complexed histidine-nitrogen (N(epsilon)) atom from the mean plane (P(mu)) of the protoporphyrin is stretched by [unk] 0.30 A. Retention of the same globin framework (quaternary structure) in deoxycobaltohemoglobin implies an axial connection with the cobalt protoporphyrin that carries enhanced tension, but a somewhat smaller value of N(epsilon)...P(mu) than the 2.90 A observed in deoxyhemoglobin. Structural constraints imposed on and by the globin in support of tension in the axial linkage are conducive to doming of the entire 24-atom porphine skeleton of the protoporphyrin toward the metal (M) atom. Furthermore, the presence of an odd electron in the 3d(z) (2) orbital of the metal atom is responsible for an easily stretched M-N(epsilon) bond. It then appears that moderate doming of the porphinato core in combination with modest stretch in the Co-N(epsilon) bond can readily lead to an N(epsilon)...P(mu) distance approaching 2.90 A in deoxycobaltohemoglobin and, consequently, to compatibility with Perutz's postulated trigger for the initiation of cooperative interaction of the subunits during the reversible oxygenation of hemoglobin.

Stereochemistry of hemes and other metalloporphyrins.

Metalloporphyrins, most notably the iron porphyrins, observe clearly defined, internally consistent, structural principles that promise to be fully applicable to the hemes in the several families of the hemoproteins.