Electrophysiological properties of in vitro intrinsic cardiac neurons in the pig (Sus scrofa)

Comparative specialization of intrinsic cardiac neurons in humans, mice, and pigs

Intrinsic cardiac neurons (ICNs) play a crucial role in the proper functioning of the heart; yet a paucity of data pertaining to human ICNs exists. We took a multidisciplinary approach to complete a detailed cellular comparison of the structure and function of ICNs from mice, pigs, and humans. Immunohistochemistry of whole and sectioned ganglia, transmission electron microscopy, intracellular microelectrode recording and dye filling for quantitative morphometry were used to define the neurophysiology, histochemistry, and ultrastructure of these cells across species. The densely packed, smaller ICNs of mouse lacked dendrites, formed axosomatic connections, and had high synaptic efficacy constituting an obligatory synapse. At Pig ICNs, a convergence of subthreshold cholinergic inputs onto extensive dendritic arbors supported greater summation and integration of synaptic input. Human ICNs were tonically firing, with synaptic stimulation evoking large suprathreshold excitatory postsynaptic potentials like mouse, and subthreshold potentials like pig. Ultrastructural examination of synaptic terminals revealed conserved architecture, yet small clear vesicles (SCVs) were larger in pigs and humans. The presence and localization of ganglionic neuropeptides was distinct, with abundant VIP observed in human but not pig or mouse ganglia, and little SP or CGRP in pig ganglia. Action potential waveforms were similar, but human ICNs had larger after-hyperpolarizations. Intrinsic excitability differed; 93% of human cells were tonic, all pig neurons were phasic, and both phasic and tonic phenotypes were observed in mouse. In combination, this publicly accessible, multimodal atlas of ICNs from mice, pigs, and humans identifies similarities and differences in the evolution of ICNs.

Effects of chronic cardiac decentralization on functional properties of canine intracardiac neurons in vitro

Although intrinsic cardiac neurons display ongoing activity after chronic interruption of extrinsic autonomic inputs to the heart, the effects of decentralization on individual neurons remain unknown. The objective of this study was to determine the effects of chronic (3-4 wk) surgical decentralization on intracellular properties of, and neurotransmission among, neurons contained within the canine intrinsic right atrial ganglionated plexus in vitro. Properties of neurons from decentralized hearts were compared with those of neurons from sham-operated hearts (controls). Two populations of neurons were identified by their firing behavior in response to intracellular current injection. Fifty-nine percent of control neurons and 72% of decentralized neurons were phasic (discharged one action potential on excitation). Forty-one percent of control neurons and 27% of decentralized neurons were accommodating (multiple discharge with decrementing frequency). After chronic decentralization, input resistance of phasic neurons increased, whereas the duration of afterhyperpolarization of accommodating neurons decreased. Postsynaptic responses to interganglionic nerve stimulation were evoked in 89% of control neurons and 83% of decentralized neurons; the majority of these responses involved nicotinic receptors. These results show that, after chronic decentralization, intrinsic cardiac neurons 1) undergo changes in membrane properties that may lead to increased excitability while 2) maintaining synaptic neurotransmission within the intrinsic cardiac ganglionated plexus.

Chronic decentralization of the heart differentially remodels canine intrinsic cardiac neuron muscarinic receptors

The objective of the study was to determine if chronic interruption of all extrinsic nerve inputs to the heart alters cholinergic-mediated responses within the intrinsic cardiac nervous system (ICN). Extracardiac nerve inputs to the ICN were surgically interrupted (ICN decentralized). Three weeks later, the intrinsic cardiac right atrial ganglionated plexus (RAGP) was removed and intrinsic cardiac neuronal responses were evaluated electrophysiologically. Cholinergic receptor abundance was evaluated using autoradiography. In sham controls and chronic decentralized ICN ganglia, neuronal postsynaptic responses were mediated by acetylcholine, acting at nicotinic and muscarinic receptors. Muscarine- but not nicotine-mediated synaptic responses that were enhanced after chronic ICN decentralization. After chronic decentralization, muscarine facilitation of orthodromic neuronal activation increased. Receptor autoradiography demonstrated that nicotinic and muscarinic receptor density associated with the RAGP was unaffected by decentralization and that muscarinic receptors were tenfold more abundant than nicotinic receptors in the right atrial ganglia in each group. After chronic decentralization of the ICN, intrinsic cardiac neurons remain viable and responsive to cholinergic synaptic inputs. Enhanced muscarinic responsiveness of intrinsic cardiac neurons occurs without changes in receptor abundance.

The heart reinnervates after transplantation

BACKGROUND

Whether cardiac reinnervation occurs after transplantation remains controversial. If reinnervation does occur, how sympathetic and parasympathetic efferent neurons do this remains unknown.

METHODS

Power spectral analysis of heart rate variability was assessed for 1 year after cardiac autotransplantation in 9 dogs. After induction of anesthesia 13 months after transplantation, cardiac and intrinsic cardiac neuronal responses elicited by both electrical stimulation of parasympathetic or sympathetic efferent neurons and systemic or local coronary artery administration of nicotine (5 microg/kg), angiotensin II (0.75 microg/kg), and tyramine (1.2 microg/kg) were studied. The transmembrane electrical properties of intrinsic cardiac neurons were studied in vitro. Ventricular tissue catecholamine content, alpha-tubulin expression, and beta-adrenergic receptor density and affinity were studied. The presence of axons crossing suture lines was sought histologically.

RESULTS

Nerves were identified crossing suture lines. Electrical or chemical (ie, nicotine or angiotensin II) activation of sympathetic efferent neurons enhanced cardiodynamics, as did tyramine. Stimulating vagal efferent preganglionic axons induced bradycardia in half of the dogs. Functional reinnervation did not correlate with specific power spectra derived from rate variability in the conscious state. Responding to nicotine and angiotensin II in situ, transplanted intrinsic cardiac neurons generated spontaneous activity. These neurons displayed nicotine-dependent synaptic inputs in vitro. Ventricular tissue had normal beta-adrenergic receptor affinity and density but reduced catecholamine and alpha-tubulin contents.

CONCLUSIONS

The intrinsic cardiac nervous system receives reduced input from extracardiac sympathetic efferent neurons after transplantation and inconsistent input from parasympathetic efferent preganglionic neurons. These heterogeneous neuronal inputs are not reflected in heart rate variability or ventricular beta-adrenergic receptor function. Transplanted angiotensin II-sensitive intrinsic cardiac neurons exert greater cardiac control than do nicotine-sensitive ones. The intrinsic cardiac nervous system remodels itself after cardiac transplantation, and this indicates that direct assessment of extracardiac and intrinsic cardiac neuronal behavior is required to fully understand cardiac control after transplantation.

Localisation and quantitation of autonomic innervation in the porcine heart II: endocardium, myocardium and epicardium

The immunological problems of pig hearts supporting life in human recipients have potentially been solved by transgenic technology. Nevertheless, other problems still remain. Autonomic innervation is important for the control of cardiac dynamics and there is evidence suggesting that some neurons remain intact after transplantation. Previous studies in the human heart have established regional differences in both general autonomic innervation and in its component neural subpopulations. Such studies are lacking in the pig heart. Quantitative immunohistochemical and histochemical techniques were used to demonstrate the pattern of innervation in pig hearts (Sus scrofa). Gradients of immunoreactivity for the general neural marker protein gene product 9.5 were observed both within and between the endocardial, myocardial and epicardial plexuses throughout the 4 cardiac chambers. An extensive ganglionated plexus was observed in the epicardial tissues and, to a lesser extent, in the myocardial tissues. The predominant neural subpopulation displayed acetylcholinesterase activity, throughout the endocardium, myocardium and epicardium. These nerves showed a right to left gradient in density in the endocardial plexus, which was not observed in either the myocardial or epicardial plexuses. A large proportion of nerves in the ganglionated plexus of the atrial epicardial tissues displayed AChE activity, together with their cell bodies. Tyrosine hydroxylase (TH)-immunoreactive nerves were the next most prominent subpopulation throughout the heart. TH-immunoreactive cell bodies were observed in the atrial ganglionated plexuses. Endocardial TH- and NPY-immunoreactive nerves also displayed a right to left gradient in density, whereas in the epicardial tissues they showed a ventricular to atrial gradient. Calcitonin gene-related peptide (CGRP)-immunoreactive nerves were the most abundant peptide-containing subpopulation after those possessing NPY immunoreactivity. They were most abundant in the epicardial tissues of the ventricles. Several important differences were observed between the innervation of the pig heart compared with the human heart. These differences may have implications for the function of donor transgenic pig hearts within human recipients.

Localisation and quantitation of autonomic innervation in the porcine heart I: conduction system

This study was prompted by the prospect of transgenic pigs providing donor hearts for transplantation in human recipients. Autonomic innervation is important for the control of cardiac dynamics, especially in the conduction system. Our objective was to assess the relative distribution of autonomic nerves in the pig heart, focusing initially on the conduction system but addressing also the myocardium, endocardium and epicardium (see Crick et al. 1999). Quantitative immunohistochemical and histochemical techniques were adopted. All regions of the conduction system possessed a significantly higher relative density of the total neural population immunoreactive for the general neuronal marker protein gene product 9.5 (PGP 9.5) than did the adjacent myocardium. A similar density of PGP 9.5-immunoreactive innervation was observed between the sinus node, the transitional region of the atrioventricular node, and the penetrating atrioventricular bundle. A differential pattern of PGP 9.5-immunoreactive innervation was present within the atrioventricular node and between the components of the ventricular conduction tissues, the latter being formed by an intricate network of Purkinje fibres. Numerous ganglion cell bodies were present in the peripheral regions of the sinus node, in the tissues of the atrioventricular groove, and even in the interstices of the compact atrioventricular node. Acetylcholinesterase (AChE)-containing nerves were the dominant subpopulation observed, representing 60-70% of the total pattern of innervation in the nodal tissues and penetrating atrioventricular bundle. Tyrosine hydroxylase (TH)-immunoreactive nerves were the next most abundant neural subpopulation, representing 37% of the total pattern of innervation in the compact atrioventricular node compared with 25% in the transitional nodal region. A minor population of ganglion cell bodies within the atrioventricular nodal region displayed TH immunoreactivity. The dominant peptidergic nerve supply possessed immunoreactivity for neuropeptide Y (NPY), which displayed a similar pattern of distribution to that of TH-immunoreactive nerve fibres. Calcitonin gene-related peptide (CGRP)-immunoreactive nerves represented 8-9% of the total innervation of the nodal tissues and penetrating atrioventricular bundle, increasing to 14-19% in the bundle branches. Somatostatin-immunoreactive nerve fibres were relatively sparse (4-13% of total innervation) and were most abundant in the nodes, especially the compact atrioventricular node. The total pattern of innervation of the porcine conduction system was relatively homogeneous. A substantial proportion of nerve fibres innervating the nodal tissues could be traced to intracardiac ganglia indicative of an extensive intrinsic supply. The innervation of the atrioventricular node and ventricular conduction tissues was similar to that observed in the bovine heart, but markedly different to that of the human heart. It is important that we are aware of these findings in view of the future use of transgenic pig hearts in human xenotransplantation.

Extrinsic inputs to intrinsic neurons in the porcine heart in vitro

Convergence of inputs from extrinsic cardiac nerves [vagus and cardiopulmonary (CPN)] on intrinsic cardiac neurons was investigated in the pig (Sus scrofa). A segment of the right atrial wall containing epicardial neurons along with attached stumps of the right vagus nerve and CPN was maintained in vitro; intracellular recordings were made from 57 neurons. Three types of neuron were identified by their responses to long intracellular depolarizing current pulses: phasic [discharged 1 action potential (AP); 40%]; accommodating (discharged multiple APs decrementing in frequency during pulse; 33%); and tonic (discharged multiple APs at a high frequency; 27%). Sixty-six percent of the neurons responded with excitatory postsynaptic potentials (EPSP) to vagal nerve stimulation; two-thirds of these cells fired APs when EPSP amplitude exceeded threshold level. Postsynaptic responses to vagal nerve stimulation were mediated by nicotinic ion channels; responses were eliminated by hexamethonium. CPN stimulation produced EPSPs but no APs in 17% of the neurons. All neurons responding with postsynaptic depolarizations to CPN stimulation also received vagal inputs. Combined stimulation of the vagus nerve and CPN produced APs in all but one of these neurons. Timolol eliminated postsynaptic responses from CPN stimulation, indicating that these responses involved beta-adrenergic receptors and likely resulted from activation of sympathetic postganglionic terminals. These results show that some intrinsic cardiac neurons receive convergent inputs from the CPN and vagus nerve. It is suggested that such neurons represent intraganglionic sites for sympathetic-parasympathetic interactions in neural control of the heart.

3-D characterization of ganglion cells of the terminal nerve plexus of rat atrioventricular junction

Three-dimensional (3-D) morphology of neurons of the terminal nerve plexus of the atrioventricular junction was examined in a scanning electron microscope. Distributions of different cell types encountered as well as their relations to different structures of the atrioventricular specialized tissue were also studied. Most neurons were found disseminated in a thin connective tissue layer separating different segments of the atrioventricular conductive tissue from the interventricular septum. Sometimes, they formed small pluricellular ganglia (up to 5 neurons) but, frequently, they occurred isolated in the terminal ramifications of the intramural nerve plexus of specialized tissue. Some intranodal neurons could also be identified. According to their 3-D morphology, nerve cells of the perinodal ganglionated plexus could be divided into three categories: (1) Large unipolar neurons were scattered throughout the atrioventricular junction. Their long and thin axonal projections were often directed towards the interventricular septum. (2) Large pseudounipolar or bipolar neurons were located at a few specific loci, namely all along the bundle of His and its bifurcation into the right and left bundle branches. Frequently, they occurred solitary and immersed amongst strands of surrounding muscle cells. Only occasional synaptic impacts could be identified on the surface of neuronal bodies of these bipolar neurons. On the other hand, their dendritic varicosities were richly innervated. Due to their irregular shape, intimate association with muscular elements and their topographical superposition with occasional spindle-like structures, these nerve cells recall prospective sensory neurons involved in integration of mechanical and neural stimuli to the heart. (3) Small multipolar interneurons could be identified in the retronodal ganglion and within right and left bundle branches. The present description of morphological heterogeneity of intramural nerve cells agrees with recent morphological and functional classifications of autonomic neurons and supports the idea that, at the level of the atrioventricular junction, a self-governed neuronal network may be operating.

Passive and active membrane properties of isolated rat intracardiac neurons: regulation by H- and M-currents

The electrical characteristics of isolated neonatal rat intracardiac neurons were examined at 22 and 37 degrees C using the perforated-patch whole cell recording technique. The mean resting membrane potential was -52.0 mV at 37 degrees C and exhibited no temperature dependence. Lowering the temperature from 37 to 22 degrees C decreased the mean input resistance from 854 to 345 Momega, respectively, and reduced the membrane time constant approximately threefold yielding a Q10 of 2.1. Hyperpolarizing current pulses induced time-dependent rectification of the voltage response in all neurons at both temperatures. This behavior was previously not observed in dialyzed neurons and was reversibly blocked by external Cs+ (2 mM) but not Ba2+ (1 mM). Voltage-clamp studies of isolated neurons revealed a hyperpolarization-activated inward current. This inwardly rectifying conductance was isolated from other membrane currents using external Cs+. The time and voltage dependence of this current is consistent with Ih and contributes to the passive electrical properties of rat intracardiac neurons. In >90% of the neurons studied, depolarizing currents evoked firing of multiple, adapting, action potentials at 22 degrees C. The number of action potentials increased with current strength producing a mean discharge of 5.1 (+100 pA, 1 s pulse), which was attenuated at 37 degrees C to a mean of 1.4. The amplitude and kinetics of the slow, muscarine-sensitive inward and outward currents (IM) were highly temperature dependent. Lowering the temperature from 37 to 22 degrees C reduced the steady-state current amplitude by approximately one-third and the rate of deactivation of IM by six- to ninefold at all voltages examined. The average Q10 for the time constant of deactivation of IM was 3.7 +/- 0.3 (mean +/- SE). Acetylcholine (ACh) induced tonic discharges in response to depolarizing currents (+100 pA, 1 s pulse) at both temperatures. This effect of ACh was inhibited by the muscarinic receptor antagonists, pirenzepine (100 nM), and mL-toxin (60 nM). At 37 degrees C, a mean discharge of 1.5 was increased to 23.5 in the presence of ACh. A similar switch from phasic to tonic discharge was also produced by the potassium channel inhibitors, Ba2+ (1 mM) and uridine-5'-triphosphate (UTP; 100 microM), whereas cadmium, 4-aminopyridine, apamin, charybdotoxin, and dendrotoxin did not alter discharge activity. The pharmacological sensitivity profile and temperature dependence of the active membrane properties are consistent with the muscarine-sensitive potassium current (IM) regulating the discharge activity in rat intracardiac neurons.

Cardiac neurones of autonomic ganglia

The properties of the postganglionic sympathetic neurones supplying the heart and arising in the stellate and adjacent paravertebral ganglia of various species are discussed with respect to their location, morphology, synaptic input and membrane characteristics. Results from our laboratory on the morphology of rat stellate neurones projecting to the heart were obtained either by intracellular injection of hexammine cobaltic (III) chloride or by retrograde labelling of cells using cobalt-lysine complex. Intracellular recordings were made from cells using electrodes filled either with potassium chloride plus hexammine cobaltic chloride or potassium acetate. Neurones which projected axons into cardiac nerve branches arising from the stellate ganglion were termed putative cardiac neurones, because of the possibility that some supply pulmonary targets. Putative cardiac neurones had unbranched axons and were ovoid or polygonal in shape, but showed considerable variation in soma size and in the complexity of dendritic trees. The mean two-dimensional surface area was 463 microns2 and the mean number of primary dendrites was seven. Other studies have found that the morphology of rat stellate ganglion neurones is similar to that of superior cervical ganglion cells. However, in strains of rat displaying spontaneous hypertension, dendritic length may be increased. Histochemical studies do not, as yet, seem to have demonstrated a distinctive neurochemical profile for stellate cardiac neurones, but various types of peptide-containing intraganglionic nerve fibres have been identified in the guinea pig. In our electrophysiological studies, putative cardiac neurones were found to receive a complex presynaptic input arising from the caudal sympathetic trunk and from T1 and T2 thoracic rami. In addition, 16% of cardiac neurones received a synaptic input from the cardiac nerve. The properties of postganglionic parasympathetic neurones distributed in the cardiac plexus and termed intrinsic cardiac neurones are discussed, including the results of studies on cultures of these neurones.

Morphological and immunohistochemical properties of primary long-term cultures of adult guinea-pig ventricular cardiomyocytes with peripheral cardiac neurons

Long-term (2-12 weeks) cultures of adult guinea-pig ventricular myocytes, cocultured with neurons derived from stellate or intrinsic cardiac ganglia, retain their functional properties (Horackova et al., 1993, 1994, 1995). The present study was designed to investigate the morphological and immunochemical properties of such neurons and their associated cardiomyocytes. Cultured myocytes studied by means of phalloidin-rhodamine (for F-actin) and an antibody raised against myomes revealed parallel myofibrils with striations typical of rod-shaped cardiomyocytes, even while myocytes changed from cylindrical to flattened form as they established intercellular contacts. Microtubular networks, identified by alpha-tubulin DM1A antibody, were arrayed longitudinally in myofibrils, being especially prominent during the formation of intercellular contacts between myocytes. Histochemically identified adult peripheral autonomic neurons cultured alone or with myocytes displayed a variety of shapes. alpha-Tubulin staining was associated with the somata and neurites of various-shaped neurons whether cultured alone or with myocytes. Cultured neurons derived from stellate and intrinsic cardiac ganglia also exhibited staining for the general neuronal marker PGP 9.5 (protein gene product 9.5), and for specific markers of the following neurochemicals: tyrosine hydroxylase, acetylcholinesterase, choline acetyltransferase, neuropeptide Y, vasoactive intestinal peptide, calcitonin gene-related peptide, bradykinin, oxytocin, and NADPH-diaphorase. These data indicate that: (a) adult ventricular myocytes cocultured with intrathoracic neurons retain the structural properties of adult myocytes found in vivo; (b) intrinsic cardiac and extrinsic intrathoracic neurons cultured alone or with cardiomyocytes display morphological characteristics similar to those of neurons studied in situ; (c) intrinsic cardiac and intrathoracic extracardiac neurons cultured alone or with cardiomyocytes display a variety of morphologies (unipolar, bipolar, and multipolar), larger and more multipolar neurons being present in cultures derived from stellate versus intrinsic cardiac ganglia; (d) such cultured neurons are associated with a number of neurochemicals, more than one chemical being associated with each neuron. This model presents an excellent opportunity to study the morphology of individual peripheral extracardiac and intracardiac neurons as well as their potential to produce various neurochemicals that are known to be involved in the neuromodulation of cardiomyocyte function.

Evidence for afferent fiber innervation of parasympathetic neurons of the guinea-pig cardiac ganglion

The present study was done to establish whether peptidergic afferent inputs can modulate parasympathetic neurons of the guinea-pig cardiac ganglion. Whole mount preparations from the guinea-pig heart were utilized to localize afferent terminals by immunohistochemistry and for intracellular recordings from individual neurons in situ. Action potentials could be elicited by both intracellular current injection and stimulation of interganglionic fiber bundles. Two types of neuron, phasic (95%) and tonic (5%) as defined by their firing properties, were observed. High frequency (5-10 Hz) interganglionic fiber stimulation produced a calcium-dependent, slow depolarization in many cells which was not blocked by 100 microM hexamethonium or 1 microM atropine. A prolonged depolarization was also produced by local application of capsaicin (1 mM), which releases substance P and CGRP from afferent nerve terminals. Microinjection of the mammalian tachykinins substance P, neurokinin A and neurokinin B (all at 100 microM), also produced a slow depolarization. Application of specific agonists for the tachykinin receptor subtypes indicated that these neurons express both NK2 and NK3 receptors. Individual cells were filled with neurobiotin to examine their morphology and the preparations were counter-stained for SP-like immunoreactivity. The results demonstrated that SP-positive fibers are found in close apposition to both phasic and tonic neurons. From these results, we suggest that the parasympathetic neurons of the guinea-pig cardiac ganglion receive inputs from peptidergic, afferent fibers and that this input provides a pathway for potential local reflex control of cardiac function.

Gross and microscopic anatomy of the canine intrinsic cardiac nervous system

BACKGROUND

A three-dimensional description of the distribution and organization of the canine intrinsic cardiac nervous system was developed in order to characterize its full extent physiologically.

METHODS

The anatomy of the canine intrinsic cardiac nervous system was investigated in 67 mongrel dogs by means of visualization following methylene blue staining as well as by light and electron microscopic analyses.

RESULTS

Collections of ganglia associated with nerves, i.e., ganglionated plexuses, were identified in specific locations in epicardial fat and cardiac tissue. Distinct epicardial ganglionated plexuses were consistently observed in four atrial and three ventricular regions, with occasional neurons being located throughout atrial and ventricular tissues. One ganglionated plexus extended from the ventral to dorsal surfaces of the right atrium. Another ganglionated plexus, with three components, was identified in fat on the left atrial ventral surface. A ganglionated plexus was located on the mid-dorsal surface of the two atria, extending ventrally in the interatrial septum. A fourth atrial ganglionated plexus was located at the origin of the inferior vena cava extending to the dorsal caudal surface of the two atria. On the cranial surface of the ventricles a ganglionated plexus that surrounded the aortic root was identified. This plexus extended to the right and left main coronary arteries and origins of the ventral descending and circumflex coronary arteries. Two other ventricular ganglionated plexuses were identified adjacent to the origins of the right and left marginal coronary arteries. Intrinsic cardiac ganglia ranged in size from ones comprising one or a few neurons along the course of a nerve to ones as large as 1 x 3 mm estimated to contain a few hundred neurons. Intrinsic cardiac neuronal somata varied in size and shape, up to 36% containing multiple nucleoli. Electron microscopic examination demonstrated typical autonomic neurons and satellite cells in intrinsic cardiac ganglia. Many of their axon profiles contained large numbers of clear, round, and dense-core vesicles. Asymmetrical axodendritic synapses were common.

CONCLUSIONS

The canine intrinsic cardiac nervous system contains a variety of neurons interconnected via plexuses of nerves, the distribution of which is wider than previously assumed.

Electrophysiological properties of canine cardiac ganglion cell types

Intracellular recordings were made from 110 canine cardiac ganglion cells to study their electrophysiological properties. According to their discharge responses to depolarizing currents, these neurons were classified as tonic, phasic and non-discharging cells. Of these cell types, the order of the resting membrane potentials was non-discharging > phasic > tonic cells, which was the reverse of the order of their input resistances. Tonic and phasic cells could not be distinguished by the nature of their after hyperpolarizations which involved Ca(2+)-sensitive K+ currents. Although both cell types demonstrated fast Na+ and slow Ca(2+)-mediated action potentials, the tonic cells' action potentials were more sensitive to tetrodotoxin than those of the phasic cells.

Effects of transient coronary artery occlusion on canine intrinsic cardiac neuronal activity

In order to further elucidate the regulatory function of canine epicardial neurons, the effects of transient coronary artery occlusion on their spontaneous activity was studied. Fifty-eight individual, spontaneously active units were identified by means of their action potential configurations in specific loci of atrial and ventricular epicardial fat of 10 anesthetized dogs. The activity of 49 of the units was modified by one minute of coronary artery occlusion. Twenty-four of the 49 responding units exhibited increased activity and 37 decreased activity during coronary artery occlusions. Activity changes were sometimes, but not always, associated with decreased left ventricular intramyocardial systolic pressure. During reperfusion, the activity of 6 units was increased compared to control levels, even though ventricular pressures remained the same. Following acute decentralization, 48% of previously active units generated spontaneous activity; the activity of 89% of these was altered during coronary artery occlusion despite the fact that overall cardiodynamics were unchanged. Following hexamethonium administration, the activity generated by 9 of 10 spontaneously active units was modified by coronary artery occlusion. It is concluded that transient coronary artery occlusion can modify the activity generated by intrinsic cardiac neurons, such modification involving central and peripheral neuronal interactions.