Immunoreactive atrial and brain natriuretic peptides are co-localized in Purkinje fibres but not in the innervation of the bovine heart conduction system

Recently, we observed that atrial natriuretic peptide (ANP) immunoreactivity was present in Purkinje fibres and nerve fibre varicosities in the conduction system of the bovine heart. In order to elucidate further the morphological correlation between natriuretic peptides and the conduction system, the distribution of brain natriuretic peptide (BNP) was examined. The different parts of the conduction system in the bovine heart were dissected out and processed for immunohistochemistry with antisera against BNP and ANP. BNP immunoreactivity was frequently observed in Purkinje fibres of the atrioventricular bundle, whereas only a few Purkinje fibres in the ventricular part of the conduction system showed immunoreaction. BNP immunoreactivity was detected in regions of the Purkinje fibres which also showed ANP immunoreactivity. BNP immunoreactivity was not observed in nerve fibre varicosities. Methodologically, a larger number of small BNP immunofluorescent granular structures was observed by using an elution-restaining technique instead of conventional immunohistochemistry. The present study shows that BNP and ANP immunoreactivities frequently occur in the atrioventricular bundle and that they are co-localized in Purkinje fibres, but not in nerve fibre varicosities, in the conduction system. As previously has been proposed for ANP, the present observations suggest that also BNP may act in an autocrine and/or paracrine way in the conduction cells.

Selective inhibition by zatebradine and discrete parasympathetic stimulation of the positive chronotropic response to sympathetic stimulation in anesthetized dogs

To investigate how to reduce the positive chronotropic response to sympathetic nerve activation selectively without affecting other cardiac actions, we studied the effects of zatebradine, an inhibitor of the hyperpolarization-activated current (I(f)), verapamil and parasympathetic nerve stimulation on the positive chronotropic, dromotropic and inotropic responses to sympathetic nerve stimulation in the autonomically decentralized heart of the open-chest anesthetized dog. Parasympathetic input was activated by stimulation of the cervical vagus (CV) or parasympathetic nerves to the sinoatrial (SA) nodal region (SAP). Zatebradine (0.1-3 mumol/kg i.v.) decreased the heart rate but not other cardiac responses to sympathetic nerve stimulation, i.e., a wave component of the right atrial pressure (RAP), the first derivative of the RAP (dRAP/dt), atrioventricular (AV) conduction time (AVCT), right ventricular pressure (RVP) and its first derivative (dRVP/dt). Zatebradine (1 mumol/kg) inhibited basal heart rate by 28% but inhibited the chronotropic response to sympathetic stimulation by 85%. Verapamil (0.06-0.6 mumol/kg i.v.) attenuated the increases in heart rate, RVP and dRVP/dt elicited by sympathetic stimulation but potentiated shortening of the AVCT from the prolonged basal AVCT. The SAP stimulation attenuated the heart rate and dRAP/dt responses to sympathetic stimulation without affecting other cardiac responses, whereas CV stimulation decreased the positive chronotropic and atrial and ventricular inotropic responses. Cervical vagus stimulation did not change the positive dromotropic response. These results demonstrate that in contrast to CV nerve activation or verapamil, zatebradine and SAP stimulation cause bradycardia but preserve the myocardial contractile force and AVCT in response to sympathetic nerve activation or sympathomimetic drugs in the heart in situ.

Regional distribution of atrial electrical changes induced by stimulation of extracardiac and intracardiac neural elements

Autonomic nerves and intrinsic cardiac neural elements are known to influence the electrophysiologic and dynamic properties of the heart. This study describes the regional distribution in the canine atria of electrophysiologic effects induced by stimulation of the right and left cervical vagosympathetic complexes, the right atrial ganglionated plexus, and the right and left stellate ganglia. Local atrial effects were determined from changes in QRST area of unipolar electrograms recorded from multiple sites with plaque electrodes sewn onto the atria in 16 anesthetized dogs.

RESULTS

(1) Although being very consistent in any given preparation, atrial changes varied between animals when similar neural structures were stimulated. (2) Among the common features identified between preparations, consistent effects were induced by neural stimulation in the region of the sinus node, indicating that this atrial region is the most richly innervated. (3) All other regions of the atria could be affected by stimulation of either right-sided or left-sided efferent nerves. (4) Responses to right atrial ganglionated plexus stimulation after atropine administration indicated that the corresponding fat pad contains both sympathetic and parasympathetic neural elements.

CONCLUSION

This study demonstrates that there is considerable overlapping of atrial innervation affecting all regions of the atria, as well as the sinus node region.

Innervation of the pacemaker in guinea-pig sinoatrial node

Heart rate is regulated by the autonomic nervous system but little is known about the pattern of innervation of the pacemaker in the sinoatrial node, or the subpopulations of nerves involved. Therefore in this study the pacemaker was located using electrophysiological methods and the pattern of innervation established by cholinesterase staining. In subsequent experiments, subpopulations of sympathetic, sensory and parasympathetic nerves were identified. Sympathetic nerves were labelled by glyoxylic acid-induced catecholamine fluorescence or an antiserum raised against tyrosine hydroxylase (TH). These experiments showed that the entire sinoatrial node was densely innervated by sympathetic axons, the majority of which were immunoreactive for neuropeptide Y (NPY). There were a few axons which were only immunoreactive for TH. Sensory nerves which were immunoreactive for both substance P (SP) and calcitonin gene-related peptide (CGRP) were also found throughout the sinoatrial node. In the absence of a selective marker for parasympathetic neurons, hearts were extrinsically denervated by placing them in organotypic culture to allow degeneration of extrinsic axons. In this way intrinsic parasympathetic neurons could be characterised. These experiments revealed several distinct populations of parasympathetic nerves which innervated only a small, discrete part of the sinoatrial node. These populations were immunoreactive for NPY, somatostatin (SOM) or vasoactive intestinal peptide (VIP) alone, or SOM combined with NPY, SOM with dynorphin B, and SOM with SP. These results highlight a remarkable difference in the pattern of innervation of the sinoatrial node by the sympathetic and parasympathetic nervous systems. Furthermore the presence of several distinct populations of autonomic cardiac neurons indicates a further complexity in neuronal regulation of heart rate.

Use of sinoaortic denervation to study the role of baroreceptors in cardiovascular regulation

To assess the role of arterial baroreceptors in cardiovascular regulation, many studies have used rats in which baroreceptor afferents have been surgically destroyed. However, interpretation of studies using sinoaortic-denervated (SAD) rats is complicated by variability in the extent of baroreceptor denervation. We have compared cardiovascular regulation in rats with total sinoaortic cardiovascular regulation in rats with total sinoaortic denervation, as assessed by the abolition of reflex changes in heart rate (HR) to increases and decreases in arterial pressure (AP), with rats that underwent the same denervation procedure but still had residual (although markedly blunted) reflex changes in HR to changes in AP. In totally SAD rats, the lability of AP was greatly exaggerated compared with sham-denervated rats, although the average AP was equivalent. In contrast, partially SAD rats had elevated AP, and although AP was more labile than in sham-denervated rats, it was less labile than in totally SAD rats. In addition, cardiovascular responses elicited by elimination of neural activity in the nucleus tractus solitarius (NTS) were qualitatively different between the two groups of rats; destruction of the NTS increased AP similarly in partially SAD rats and sham-denervated rats, whereas this treatment did not alter AP in totally SAD rats. Thus there are marked differences in SAD rats with no residual arterial baroreceptor reflex function compared with SAD rats with even a small degree of residual baroreceptor reflex function. These studies highlight the importance of carefully characterizing SAD rats used in studying the role of the baroreceptor reflex in cardiovascular regulation.

[The innervation of the conducting and working myocytes in the sinoatrial area of the canine heart (a quantitative study)-]

Quantitative electron microscopic study of the innervation of different types of specialized sinus node conducting myocytes and of working myocytes in the perinodal myocardium of the right atrium was carried out in intact dogs. Differences were found in diameters of unmyelinated nervous fibers and their efferent terminals and distances between them and sarcolemmas of myocytes of the types studied. It was demonstrated that qualitatively different synaptic vesicles in the efferent terminals near the nodal and working myocytes are encountered with different frequency. Similar parameters of innervation of the nodal and working myocytes in sinauricular region of the heart were compared in intact dog and rat.

Metabolic response to ambient temperature and hypoxia in sinoaortic-denervated rats

We tested the hypothesis that the sinoaortic afferents may contribute to normoxic thermogenesis and to the magnitude of the hypometabolic response to hypoxia. Adult rats were either sinoaortic denervated (SAX; n = 20) or sham operated (Sham; n = 20). A few days after the operation, gaseous metabolism [O2 uptake (VO2) and CO2 production (VCO2)] was measured with an open-flow system at ambient temperatures (Tamb) of 20, 25, 30, and 35 degrees C as the animal was resting awake. At thermoneutrality (Tamb 30 degrees C) or higher Tamb there was no difference in VO2 or VCO2. Below thermoneutrality, metabolic rate was significantly lower in SAX than in Sham animals (-14 and -16% at 20 and 25 degrees C, respectively). Colonic temperature and arterial PO2 were also slightly less, whereas arterial PCO2 and pH, mean arterial pressure, and heart rate did not differ significantly between the two groups. Exposure to acute hypoxia (10% inspired O2, 20-30 min) at Tamb 20 and 25 degrees C significantly reduced VO2 in both groups to a similar value; hence, at either Tamb, the metabolic drop during hypoxia in Sham animals was larger than that in SAX animals. Hypercapnia (5% CO2 breathing) did not change VO2 in either group. We conclude that in the rat at Tamb slightly below thermoneutrality, the sinoaortic afferents 1) provide a small but significant contribution to normoxic thermogenesis and 2) are not required for the manifestation of the drop in metabolism during hypoxia.

Neural and humoral mechanisms involved in the generation of arterial pressure lability in rats

1. The role of the renin-angiotensin system (RAS) and sympathetic nervous system (SNS) in the generation of the arterial pressure lability (APL) observed after sino-aortic deafferentation (SAD) in rats was evaluated. 2. SAD was performed in normotensive (N = 8), renal hypertensive (2K-1C, N = 8) and spontaneously hypertensive rats (SHR, N = 8) and mean arterial pressure (MAP) recordings were performed 24 h after SAD. 3. MAP was recorded by a computerized technique using a sampling frequency of 30 Hz for 30 min and the data obtained were used to calculate APL. After MAP measurements the activity of the RAS and SNS was pharmacologically evaluated in all groups by the changes in MAP in response to iv injection of captopril and hexamethonium chloride, respectively. 4. SAD produced an increase in MAP (118 +/- 4 vs 99 +/- 2 mmHg) and a large increase in APL (13.4 +/- 1.3 vs 3.8 +/- 0.3 mmHg) in normotensive rats. SAD produced no changes in MAP (161 +/- 7 vs 167 +/- 7 mmHg) in 2K-1C hypertensive rats but induced a large increase in APL (6.7 +/- 0.5 vs 12 +/- 1 mmHg). SAD also produced no changes in MAP (152 +/- 3 vs 152 +/- 4 mmHg) in SHR but induced a marked increase in APL (6.7 +/- 0.3 vs 21 +/- 2.3 mmHg). 5. All SAD rats presented a larger fall in MAP in response to captopril and hexamethonium than the respective control group with intact baroreceptors suggesting an overactivity of both systems after SAD in normotensive, renal hypertensive and spontaneously hypertensive rats. 6. The data also show that SAD produced no additional increase in MAP but promoted a significant increase in APL in renal and spontaneously hypertensive rats. 7. We suggest that APL observed after SAD in different experimental models is dependent on an interaction of RAS and SNS, both of which seem to be overactive after removal of arterial baroreceptors.

An abnormal neural reflex plays a role in causing syncope in sinus bradycardia

OBJECTIVES

This study investigates the role of an abnormal neural reflex in causing syncope in patients with sinus bradycardia.

BACKGROUND

Syncope is commonly considered an indication of severity in sinus bradycardia. However, the occurrence of syncope is unpredictable, and the prognosis appears to be similar in patients with and without syncope.

METHODS

Head-up tilt testing (60 degrees for 60 min), carotid sinus massage in the supine and standing positions, 24-h Holter ambulatory electrocardiographic (ECG) recording and electrophysiologic study before and after pharmacologic autonomic blockade were performed in 25 patients with sinus bradycardia and syncope (group I, sinus rate < 50 beats/min, age 71 +/- 12 years) and 25 patients with sinus bradycardia and no neurologic symptoms (group II, sinus rate < 50 beats/min, age 67 +/- 16 years).

RESULTS

Clinical characteristics and ambulatory ECG monitoring data were similar in the two study groups. A positive response (induction of syncope or presyncope with hypotension and/or bradycardia) was obtained by head-up tilt testing in 15 group I (60%) and in 3 group II (12%) patients (p < 0.001) and by carotid sinus massage in 11 group I (44%) and 6 group II (24%) patients (p = NS). Results of at least one test (head-up tilt testing or carotid sinus massage, or both) were positive in 19 group I (76%) and 9 group II (36%) patients (p < 0.01). Basal and intrinsic corrected sinus node recovery time did not differ significantly between the two groups. An abnormal intrinsic heart rate was present in 66% of group I and 26% of group II patients (p < 0.01). The different percentage of positive findings on head-up tilt testing and carotid sinus massage in the two groups was independent of the presence of intrinsic sinus node dysfunction.

CONCLUSIONS

These results indicate that an abnormal neural reflex plays a role in causing syncope in patients with sinus bradycardia. This reflex seems to be unrelated to the severity of sinus node dysfunction, even if the latter could enhance the cardioinhibitory response.

Age-related changes of sinus node function and autonomic regulation in subjects without sinus node disease--assessment by pharmacologic autonomic blockade

To assess the relationship between aging and autonomic regulation of sinus node function, 56 subjects of various ages (range 14-75 years, mean 43 +/- 19 years) without sinus node disease were studied. Heart rate, corrected sinus node recovery time and sinoatrial conduction time were measured before (basic) and after (intrinsic) pharmacologic autonomic blockade (propranolol + atropine i.v.). Percent chronotropies of the above parameters were calculated by a modified Jordan's method. Basic heart rate and basic corrected sinus node recovery time did not vary with age (r = -0.15, r = 0.08, respectively), while basic sinoatrial conduction time tended to increase with age (r = 0.32, p < 0.05). Intrinsic heart rate decreased (r = -0.76, p < 0.001), and intrinsic corrected sinus node recovery time and intrinsic sinoatrial conduction time both increased with age (r = 0.55, p < 0.001; r = 0.56, p < 0.001, respectively). The younger the subject, the more negative the percent chronotropies of the above parameters were, and the percent chronotropies correlated positively with age (r = 0.68, p < 0.001; r = 0.52, p < 0.001 and r = 0.34, p < 0.05, respectively). In conclusion, intrinsic sinus node functions deteriorated with age. Furthermore, parasympathetic tone on the sinus node functions decreased with age, which may compensate for age-related deterioration of intrinsic sinus node function.

Regional differences in sympathetic reinnervation after human orthotopic cardiac transplantation

BACKGROUND

In the majority of humans > or = 1 year after cardiac transplantation, cardiac norepinephrine (NE) stores reappear, suggesting late sympathetic reinnervation.

METHODS AND RESULTS

To determine whether there are regional differences in reinnervation, we measured markers of sympathetic reinnervation of the sinus node (SN) and left ventricle (LV) in five early transplant recipients (< or = 4 months after cardiac transplantation), 45 late transplant recipients (> or = 1 year after cardiac transplantation), and seven normally innervated control patients. SN reinnervation was defined as an increase in heart rate by more than five beats per minute after injection of tyramine into the artery supplying the SN. LV reinnervation was defined as a measurable LV NE release after left main coronary injection of 8 micrograms/kg tyramine. In 13 patients with previously known LV reinnervation, regional LV reinnervation was assessed by NE release after subselective injection of tyramine (4 micrograms/kg) into the proximal left anterior descending and circumflex arteries. Five of five patients < or = 4 months after cardiac transplantation had no change in heart rate and no LV NE release, confirming early, total denervation. In contrast, > or = 1 year after cardiac transplantation, tyramine caused a heart rate increase (eight to 49 beats per minute) in 32 of 45 patients and LV NE release in 33 of 45. Although LV NE release was correlated with the change in heart rate in late cardiac transplantation recipients (r = .61), eight of 45 had only heart rate response, nine had only LV NE release, and four had neither. In late cardiac transplantation recipients with LV reinnervation, tyramine caused NE release from both the anterior descending and circumflex perfusion fields in 10 of 14, but one of 14 patients released NE only after circumflex tyramine and three of 14 only after left anterior descending tyramine stimulation. Tyramine caused a marked heart rate increase and LV NE release in all control patients.

CONCLUSIONS

Sympathetic reinnervation after cardiac transplantation is regionally heterogeneous. SN reinnervation is not associated necessarily with LV reinnervation, and LV reinnervation can involve the anterior and posterior walls together or separately.

Modification of DiFrancesco-Noble equations to simulate the effects of vagal stimulation on in vivo mammalian sinoatrial node electrical activity

We present a new mathematical model for vagal control of rabbit sinoatrial (SA) node electrical activity based on the DiFrancesco-Noble equations. The original equations were found to be unstable, resulting in progressive cycle by cycle depletion or accumulation of ions in intra- and extracellular compartments. This problem was overcome by modifying the maximum Na-K pump current and the time constant for uptake of intracellular calcium. We also included a formulation for the acetylcholine (ACh)-activated potassium current which was consistent with experimental data. This formulation was based on kinetics first proposed by Osterrieder and later modified by Yanagihara. The resulting model exhibits cycle-cycle ionic stability, and includes an ACh-activated potassium current which accurately reproduces experimentally observed effects of vagal stimulation on both the membrane potential and its time-derivative. Simulations were performed for both brief-burst and prolonged vagal stimulation using simplified square wave profiles for the concentration of ACh in the synaptic cleft space. This protocol permits the isolation of cardiac period dynamics caused by changes in membrane potential and intra- and extracellular ionic concentrations from those caused by other mechanisms including the dynamics of ACh release, diffusion, hydrolysis and washout. Simulation results for the effects of brief-burst single cycle stimulation on the cardiac period agree closely with experimental data reported in the literature, accurately reproducing changes in membrane potential and the phasic dependency of the response to the position of vagal stimulus bursts within the cycle. Simulation of the effects of prolonged vagal stimulation accurately reproduced the steady-state characteristics of heart period response, but did not yield the complex multimodal dynamics of the recovery phase, or the pronounced post vagal tachycardia observed experimentally at the termination of the stimulus. Our results show that the major chronotropic effects of vagal stimulation on the SA cell membrane can be explained in terms of the ACh-activated potassium current. The effects of this membrane current however are generally fast acting and cannot contribute to any long lasting dynamics of the cardiac period response. The modified DiFrancesco-Noble model presented in this article provides a valuable theoretical tool for further analysis of the dynamics of vagal control of the cardiac pacemaker.

Selective stimulation of parasympathetic nerve fibers to the human sinoatrial node

BACKGROUND

In animals, parasympathetic nerve fibers that innervate the sinoatrial node can be selectively stimulated to increase atrial cycle length. These nerve fibers course through an epicardial fat pad at the margin of the right superior pulmonary vein, the superior vena cava, and the right atrium. We hypothesized that similar nerves exist and can be selectively stimulated in humans.

METHODS AND RESULTS

Microscopic examination of fat pads excised from the margin of the right superior pulmonary vein, the superior vena cava, and the right atrium during two human autopsies revealed the presence of nerve fibers and ganglia. We electrically stimulated this epicardial fat pad in 16 patients during cardiac surgery. The fat pads were stimulated with continuous-pulse trains for 15 seconds via a hand-held bipolar electrode using constant current (10-15 mA), constant pulse width (0.02-0.05 msec), and at 6.6, 10, 20, 25, and 30 Hz. The mean atrial cycle length +/- 1 SEM increased from 734 +/- 34 msec at baseline to a maximum of 823 +/- 61 msec at 6.6 Hz, 1,167 +/- 125 msec at 10 Hz, 1,734 +/- 281 msec at 20 Hz, 2,993 +/- 661 msec at 25 Hz, and 2,461 +/- 668 msec at 30 Hz during nerve stimulation. Linear regression analysis showed that the response of atrial cycle length to sinoatrial parasympathetic nerve stimulation was frequency dependent. The maximum response and complete decay of the response occurred within 4-8 seconds of initiation or termination of sinoatrial parasympathetic nerve stimulation. Atrioventricular conduction time and the PR interval did not change during sinoatrial parasympathetic nerve stimulation, even when the atria were paced at the baseline heart rate.

CONCLUSIONS

Electrical stimulation of parasympathetic nerve fibers in a fat pad near the sinoatrial node increased atrial cycle length without affecting atrioventricular nodal conduction. This is the first study in which such nerve fibers that innervate the sinoatrial node have been selectively stimulated in humans.

Chemical renal medullectomy and arterial pressure response to sinoaortic denervation

We investigated in conscious Wistar-Kyoto rats the effect of chemical renal medullectomy on the responses of mean arterial pressure, arterial pressure lability, and heart rate to sinoaortic deafferentation (SAD). Chemical medullectomy was obtained by the intravenous administration of 2-bromoethylamine hydrobromide (2-BEA) 2-3 weeks before SAD or sham SAD. Chemically medullectomized rats were compared with control rats treated with saline. In control rats, the increase in mean arterial pressure elicited by SAD was not significantly greater than that produced by sham SAD. In medullectomized rats, SAD significantly increased mean arterial pressure compared with sham SAD. No direct relation was observed between the response of mean arterial pressure to SAD and the grade (1, 2, or 3) of lesion of the renal papilla. In control rats, SAD increased significantly arterial pressure lability. Chemical medullectomy did not affect basal lability or the increased lability after SAD. No direct relation was observed between increased arterial pressure lability due to SAD and the grade of lesion of the renal papilla. SAD produced a conspicuous tachycardia in control rats. Chemical medullectomy did not affect basal heart rate or the tachycardia produced by SAD. No direct relation was observed between the extent of this tachycardia and the grade of lesion of the renal papilla. These data indicate that lesions of the renal papilla lead to a significant increase in mean arterial pressure after SAD, without affecting basal pressure or heart rate. In addition, SAD per se did not increase significantly the mean arterial pressure in control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of selective denervation of baroreceptors on pulmonary ventilation and arterial pressure lability in rat

Earlier studies report that sinoaortic baroreceptor denervation (SAD) in rats causes moderate elevation of mean arterial pressure along with a marked increase of arterial pressure lability (APL). In this context, we studied the effects of selective aortic denervation (AD) or selective carotid denervation (CD) on the regulation of blood pressure. In addition, we evaluated the effects of selective or total baroreceptor denervation on pulmonary ventilation and ventilation-related changes of arterial pressure. Mean arterial pressure was evaluated by computer-assisted techniques, and ventilation was measured by whole body plethysmography on conscious freely moving rats. With this approach, equal increases of mean arterial pressure were obtained for rats that had undergone AD, CD, and SAD. The APL was higher in SAD rats than in selectively denervated rats. CD and AD rats had an elevated APL relative to sham-operated animals, and its increase was approximately equal for the two selectively denervated groups. Total as well as selective denervation had relatively small effects on ventilation and on the general pattern of breathing. In all groups, this pattern consisted of regular ventilation, periodically interrupted by single deeper breaths. In SAD, AD, and CD animals, these larger tidal volumes were associated with marked transient reductions of mean arterial pressure, whereas small decreases of pressure occurred in sham-operated rats. The results indicate that both groups of baroreceptors must be present to keep mean arterial pressure at its normal level. Moreover, both receptor groups are equally important in reducing APL. Ventilation contributes to generation of APL after total or selective baroreceptor removal. Such ventilation-induced pressure changes are kept at a minimum in baroreceptor-intact rats.

Chronotropic responsiveness of developing sinoatrial and ventricular rat myocytes to autonomic agonists following adrenergic and cholinergic innervation in vitro

The chronotropic responses of isolated sinoatrial node and ventricular muscle cells to neurotransmitters were compared in vitro with and without selective adrenergic and cholinergic innervation. Explants of either thoracolumbar sympathetic ganglion or sacrococcygeal spinal cord were added to cultures of newborn rat sinus node regions or ventricular apexes harvested before the onset of autonomic innervation in vivo. Catecholamine synthesis was detected by glyoxylic acid histofluorescence. Acetylcholine synthesis was indicated by monoclonal antibody labeling of choline acetyltransferase. After electrical or pharmacological stimulation of neurons, the chronotropic response of individual myocardial cells confirmed the presence of neuroeffector transmission; the nature of the myocyte response identified the stimulated neuron as either adrenergic or cholinergic. Chronotropic responses of all myocardial cells to norepinephrine or acetylcholine were transcribed on a recorder coupled to a video photoconductive cell monitor. Isolated sinoatrial node cells were supersensitive to norepinephrine and acetylcholine; thresholds were 3 x 10(-16) M and 6 x 10(-15) M, respectively. These sinoatrial node cells remained sensitive to both norepinephrine and acetylcholine after the development of innervation in vitro. Ventricular cells also were sensitive with thresholds of 3 x 10(-11) M and 6 x 10(-14) M to norepinephrine and acetylcholine, respectively. However, following in vitro innervation, ventricular cells were significantly less sensitive to norepinephrine and acetylcholine (thresholds 3 x 10(-9) M and 6 x 10(-11) M). These data are the first to demonstrate that neurotrophic modulation is not homogeneous throughout the myocardium and that it may be dependent on the specific myocardial cell innervated.

Selective vagal postganglionic innervation of the sinoatrial and atrioventricular nodes in the non-human primate

The distribution of parasympathetic postganglionic nerves to the atrioventricular (AVN) and sinoatrial nodal (SAN) regions was investigated in the non-human primate heart. Eight male monkeys (Macaca fascicularis) weighing 5.5-7.0 kg. were anesthetized (alpha-chloralose, 50 mg/kg and urethane, 500 mg/kg) and instrumented to measure arterial pressure, electrocardiogram, atrial and ventricular electrograms. The cervical vagi were electrically stimulated (20 Hz, 4 V, 2 ms) before and after selective denervation (D) of the AVN and/or SAN. Vagal stimulation was repeated during atrial pacing to assess parasympathetic modulation of AVN conduction. Ablation of parasympathetic pathways to the AVN, accomplished by the disruption of the epicardial fat and surface muscle layer at the junction of the inferior vena cava and inferior left atrium eliminated (P less than 0.01) the dromotropic effects of vagal stimulation without affecting the heart rate response (right vagus, before D, paced: atrial rate 218.0 +/- 6.3, ventricular rate 67.1 +/- 23.7; after D: atrial rate 210.3 +/- 6.4, ventricular rate 210.3 +/- 6.4 beats/min, means +/- S.D.). In sharp contrast, surgical dissection of the fat pad overlying the right pulmonary vein-superior vena cava junction significantly (P greater than 0.01) attenuated negative chronotropic effects of vagal stimulation (left vagus, before D the R-R interval increased by 832.7 +/- 146.4 ms, 209.5% increase; after D 37.4 +/- 18.0 ms, 8.8% increase). These data demonstrate discrete vagal efferent pathways innervate both the SAN and AVN regions of the non-human primate heart.

Selective ganglionic blockade of vagal inputs to sinoatrial and/or atrioventricular regions

Vagal postganglionic neurons to sinoatrial (SAN) and atrioventricular (AVN) nodal regions of the canine heart have been localized surgically around the right pulmonary vein-atrial fat pad and in the fat pad overlying the epicardium at the inferior vena cava-inferior left atrial junction, respectively. Local ganglionic blocking doses (total of 5 mg per injection) of hexamethonium were injected into the pulmonary vein-atrial fat pad to block selectively right and left vagal inputs to the SAN region without interrupting vagal inputs to AVN. Conversely, hexamethonium injected into the inferior vena cava-inferior left atrial pad selectively blocked vagal control of arteriovenous conduction without interfering seriously with vagal control of SAN function. Vagal ganglia situated in pulmonary vein-atrial fat pad also exercise moderate but incomplete control of right atrial contractile force. Lesser vagal control of atrial inotropism is localized in the inferior vena cava-inferior left atrial fat pad. Ganglia situated in a large fat pad on the dorsal epicardial surface of the left atrium (left atrial fat pad) appear to play little or no role in SAN or AVN regulation, although some left preganglionic axons may pass through it en route to the AVN region. Vagal stimulation is associated with prompt and profoundly negative chronotropic and dromotropic responses, but in response to the same stimulation, a negative change in atrial contractile force is much slower in development and is much longer lasting. Such precise anatomical localization and differentiation of the intrinsic vagal regulation of SAN, AVN and contractile force opens new avenues of research on the neural regulation of cardiac performance.

Changes in neural and humoral mechanisms of the heart in sudden death due to myocardial abnormalities

Quantitative neurohistochemical study of adrenergic elements of the myocardium and the adrenal medulla in victims of sudden death revealed an unequal and focal depletion of catecholamines attributable to prior pathologic processes in the myocardium. The greatest changes in cardiac innervation were found in cases of acute myocardial infarction and alcoholic cardiomyopathy, and the adrenergic plexuses were better preserved in cases of coronary heart disease without focal myocardial changes. Ultrastructural study of cardiac innervation in patients who died suddenly showed more pronounced changes in the nerve plexuses of the sinus node than in the perinodal nerves of the working myocardium. The changes in coronary artery innervation were usually related to the severity of stenosis due to fibrous plaque; desympathization of the vessels and the adjoining myocardial zone was also found in cases with 50% or greater occlusion of the lumen. The bulk of chromaffinocytes were depleted and weakly luminescent in the adrenal glands of patients with acute myocardial infarction who died suddenly. In contrast, chromaffinocytes with moderate and bright luminescence were prevalent in cases of sudden death with scarring from previous myocardial infarction and with alcoholic cardiomyopathy. Ultrastructural and histochemical examinations demonstrated that changes in the neurons of sympathetic ganglia increased with more severe atherosclerotic lesions of the aorta and with greater changes in the vessels supplying the ganglia. In experiments on rabbits and dogs, both coronary artery ligation and electrostimulation produced cardiac fibrillation followed by a local increase in luminescence intensity of the myocardial nerve plexuses when their density remained high. Chemoreceptors located along the coronary vessels and pulmonary artery in dogs included small cells with bright fluorescence and adrenergic nerve fibers.

Effects of beta-adrenoreceptor antagonists on sino-aortic baroreflex sensitivity and blood pressure in hypertensive man

1. Sensitivity of the sino-aortic baroreflex was investigated before and after acute (23 patients) and chronic (23 patients) beta-adrenoreceptor antagonism in patients with essential hypertension. 2. Sensitivity was inversely related to age (r = -0.60) and systolic blood pressure (r = -0.46); a positive relationship was noted between sensitivity and initial pulse intervals (r = 0.40). 3. Sensitivity increased significantly in patients less than 40 years of age after chronic treatment. No change occurred after acute treatment or in older patients treated chronically. 4. The fall in ambulatory intra-arterial blood pressure after chronic treatment was unrelated to alteration of baroreflex sensitivity.

[Change in cardiac pacemaker activity during interaction of parasympathetic and sympathetic regulatory effects]

Chronotropic effect on the isolated sinus node of the frog heart and the unit activity of the pacemaker cells were studied during separate and joint stimulation of sympathetic and parasympathetic pathways. The joint stimulation entailed parasympathetic bradycardia or augmented the initial negative chronotropic response occurring in case of the parasympathetic stimulation alone. In most cases the joint stimulation diminished the speed of the slow diastolic depolarization and shortened the pacemaker AP. Thus the sympathetic and parasympathetic effects were not antagonistic in relation to the rhythmicity of the sino-atrial pacemaker. The possibility that catecholamines may increase the efficiency of the cholinergic action is discussed.

Dysrhythmias and sudden death in acromegalic heart disease. A clinicopathologic study

A clinicopathologic investigation was carried out in a patient with acromegaly from acidophylic adenoma of the hypophysis. Pronounced cardiomegaly and disturbances in impulse formation and conduction led to sudden death. The problem of acromegalic cardiomyopathy has been reviewed. Particular attention has been focused on the histopathologic findings in the working and specific myocardium, and in the intrinsic nervous supply to the sinoatrial node, which were correlated with the electrocardiographic features of atrioventricular conduction delay and sick sinus syndrome.