Differential effects of sympathetic activity on AV junctional automaticity and AV conduction

Rapid ventricular response during episodes of supraventricular tachycardia are often followed, on abrupt cessation of the tachycardia, by prolonged pauses terminated by a sluggish and sometimes erratic escape of a supraventricular pacemaker. Such chronotropic-dromotropic paradoxes are readily reproduced in the animal laboratory following elimination of the sinus node and bilateral decentralization of the stellate ganglia and vagi. This study examined whether left stellate stimulation (0.5, 1, 2, 4, 8 and 16 Hz) or lack thereof differentially affected AV junctional automaticity and AV conduction. In the absence of any sympathetic neural activity (maximal sympathetic deficit), the AV junctional rate averaged a mere 22 +/- 2 percent of its peak performance, whereas under the same conditions, anterograde AV conduction averaged 73 +/- 5 percent and retrograde VA conduction 56 +/- 13 percent of their respective peak performances. On comparing the response curve (normalized responses) for AV junctional automaticity with that obtained for anterograde AV conduction the differences were significant at all frequencies between 0 and 4 Hz. Retrograde VA conduction (as assessed by the fastest ventricular pacing rate still conducted 1:1 to the atria) was always significantly less than anterograde AV conduction (as assessed by the fastest atrial pacing still conducted 1:1 to the ventricles). These results indicate that AV junctional automaticity is considerably more affected by sympathetic deficit than are either anterograde or retrograde AV conduction. In other words, AV junctional automaticity is far more dependent upon sympathetic input than AV conduction. While sympathetic influence is critical to the escape and maintenance of AV junctional automaticity both anterograde and retrograde AV conduction are remarkably resilient even under conditions of severe sympathetic deficit.

Atrioventricular junctional tachycardia during heart block

The response of the atrioventricular (AV) junction to brief intense adrenergic stimulation applied during episodes of second degree heart block achieved by acetylcholinesterase paralysis in the AV junction was examined in six dogs. Despite profound depression of AV conduction due to enhanced cholinergic activity, strong local adrenergic stimulation still readily elicited AV junctional tachycardia. Increase in cholinomimetic influences in the AV junction did not prolong transatrial or His bundle-ventricular conduction times. During AV junctional rhythm and retrograde atrial capture (n = 4), neither the sequence of retrograde atrial activation nor the atrial electrogram configurations were altered. In the two remaining dogs the AV junctional tachycardia was associated with AV dissociation. These findings suggest that the acetylcholine-induced depression of AV conduction is located in the AV node region exclusively. More important, however, is the demonstration that retrograde atrial activation originating from a pacemaker located in the AV node or immediate vicinity could actually precede the inscription of the H spike by a considerable amount of time, further suggesting that anterograde conduction from the pacemaker site to the bundle of His is far more depressed by acetylcholine than is the concomitant retrograde conduction from the pacemaker site to the atrium. Thus, inference of the origin of a subsidiary pacemaker from the P wave configuration or the relation of the A wave to the His bundle electrogram, or both, may lead to erroneous conclusions.

Cardiac autonomic efferent activity during baroreflex in puppies and adult dogs

Blood pressure and heart rate were recorded in 15 anesthetized puppies (6-10 wk, 1-6 kg) and 18 adult mongrel dogs (greater than 1 yr, 18-26 kg) before and during acute blood pressure changes achieved with nitroglycerin or phenylephrine (4 and 8 micrograms/kg iv). Overall heart rate responses to blood pressure changes in adults were significantly (P less than 0.05) greater than those in puppies. Following control baroreflex responses, two multifiber efferent preparations from the discrete thoracic cardiac nerves (sympathetic, n = 48; parasympathetic, n = 18) were simultaneously recorded and analyzed by microprocessor. Severing of the nerves significantly attenuated the heart rate responses to blood pressure changes in puppies only, suggesting less redundancy of the neural regulation of the sinus node in the puppy. The pressure-induced reflex changes in the sympathetic or parasympathetic efferent nerve activities were not significantly different between adult dogs and puppies. There were no significant differences in reflex activities in right-sided (n = 29) vs. left-sided (n = 19) sympathetic nerves in either puppies or adult dogs. Preganglionic sympathetic fibers in puppies (but not adult dogs) were more responsive to blood pressure changes than were postganglionic sympathetic fibers. Thus baroreceptor reflex control in the puppy is less developed than in the adult canine heart, and the maturational difference in neural regulation of the heart is at or beyond the efferent nerve terminals.

Serial production of controlled periods of temporary heart block used to unmask and assess latent ventricular automaticity during experimental acute myocardial ischemia

This study examined the onset, time course of development and response to overdrive stimulation of ventricular tachycardia in 10 dogs that underwent a Harris two-stage ligation of the left anterior descending coronary artery. Transient (12 +/- 3 minutes) complete atrioventricular (AV) block was produced 2, 3, 4, 5, 8, 12, 16, 20 and 24 hours after onset of infarction through selective injection of physostigmine salicylate into the AV node artery. Seven of the 10 dogs had early transient arrhythmic episodes that occurred within 20 to 40 minutes after coronary occlusion but none of the dogs had any spontaneous ventricular tachycardia in the ensuing 2 hours. Two hours after left anterior descending coronary artery ligation, complete AV block unmasked in every dog a slow (37 +/- 9 beats/min) AV junctional rhythm readily suppressed by overdrive. Three hours after coronary ligation, AV block revealed a monomorphic ventricular tachycardia (106 +/- 10 beats/min) in 3 of the 10 dogs. Four and five hours after coronary ligation, five and eight dogs, respectively, had ventricular tachycardia during AV block and in three the tachycardia was polymorphic. The two remaining dogs did not develop ventricular tachycardia during the 24 hours of observation. Ventricular tachycardia always began abruptly, first with brief and then longer bursts. Soon after onset the rate of tachycardia began to increase to reach a plateau 2 to 3 hours later at frequencies 21 +/- 9% greater than the initial tachycardia rate. Concomitant with this increase in rate there was a steady decline of overdrive suppressibility and during the plateau phase there was little or no overdrive suppression.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential modulation of autonomic activity by ethmozin and ethacizin (analog of ethmozin) on the canine sinus node and atrioventricular junction

The chronotropic and dromotropic actions of ethmozin and its diethylamine analog ethacizin were studied in the presence and absence of combined muscarinic, beta- and alpha-adrenoreceptor blockade in the intact canine heart in situ (n = 38). Injections of ethacizin, 5, 10 and 25 micrograms/ml, into the sinus node artery caused an immediate and significant (p less than 0.001) sinus bradycardia of 2, 6 and 11%, respectively. Injection of 25 and 50 micrograms/ml of ethacizin into the atrioventricular (AV) node artery significantly (p less than 0.001) prolonged AV conduction time with occasional second degree heart block. Conduction delay was located exclusively during the AH interval of the His bundle electrogram. Autonomic blockade did not alter the negative chronotropic or negative dromotropic effects of ethacizin. Ethacizin, 25 micrograms/ml, injected into the sinus node artery immediately reduced the sinus node response to vagal stimulations by 30% and the effect of acetylcholine, 0.1 micrograms/ml, injected into the sinus node artery by 50%. Ethacizin, 25 micrograms/ml, injected into the AV node artery immediately reduced the duration of complete AV block elicited by vagal stimulation or intranodal acetylcholine, 0.5 micrograms/ml, by 90%. Ethacizin caused a minor reduction in sinus node response to right stellate stimulations without, however, altering the sinus node response to intranodal norepinephrine. Ethmozin injections of up to 50 micrograms/ml into the sinus and AV node arteries had no chronotropic or dromotropic effects. Ethmozin had a minor and variable vagolytic action but significantly (p less than 0.05) reduced the sinus node response to sympathetic nerve stimulation. Hence, ethacizin, in contrast to ethmozin, has a direct depressing action on both the sinus node and the AV junction.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventricular echo beats during ventricular pacing or junctional bradycardia: studies in the normal canine heart

In 15 adult dogs ventricular echoes were elicited during sinus rhythm by incremental ventricular pacing and during atrioventricular (AV) junctional rhythm by depressing simultaneously AV junctional automaticity and retrograde AV nodal conduction. Concomitant slowing of AV junctional automaticity and conduction was achieved by selective intranodal administration of verapamil. In three dogs incremental pacing from either ventricle failed to retrogradely activate the atria, and in each case the site of block was found to be in the AV node. In two dogs with retrograde atrial capture there was little or no rate-dependency of retrograde ventriculoatrial (VA) conduction. During incremental ventricular pacing a single ventricular echo beat was observed in 10 of the 12 dogs that had atrial capture, and the atrium appears to be an essential link in the production of each ventricular echo. Ventricular echo occurred when the time allotted for retrograde VA conduction amounted to 70 +/- 4% of the duration of the ventricular pacing cycle length. During AV junctional rhythm, a single ventricular echo was elicited in half of the dogs and in each of those cases intranodal verapamil produced a profound depression of retrograde VA conduction. These experiments suggest that retrograde AV nodal longitudinal dissociation occurs in the slow current-dependent cells of the AV node.

Differential inotropic actions of ethmozine and ethacizin (diethylamine analog of ethmozine)

The direct inotropic actions of ethmozine and of its diethylamine analog, ethacizin, were studied in the presence of muscarinic and beta-adrenoreceptor blockade in 12 ferret right ventricular papillary muscles. In each muscle ethmozine caused a small but consistent and significant (p less than 0.05) increase in contractile performance, whereas ethacizin significantly (p less than 0.05) diminished contractility. Although both phenothiazines are fast channel blockers, it appears that the net positive inotropic action of ethmozine is due to its stimulatory effect on the slow inward current and that the negative inotropic action of ethacizin is largely due to its recently demonstrated decreases of the slow inward current.

Differential sympathetic-parasympathetic interactions in sinus node and AV junction

This study examined the effects and interactions evoked by various combinations of parasympathetic (P) and sympathetic (S) stimulations on sinus node automaticity and atrioventricular (AV) conduction (both anterograde and retrograde) in 11 two-mo old anesthetized puppies. The respective effects of these autonomic maneuvers were assessed by covariant analysis using a linear regression model of the form Y = alpha 0 + alpha 1 X base line + alpha 2 X sequence + alpha 3 X S + alpha 4 X S2 + alpha 5 X P + alpha 6 X P2 + alpha 7 X (P X S).... The effects of parasympathetic and sympathetic stimulation, of the quadratic term S2, and of the interaction term (P X S) were highly significant (P less than 0.0001) on both sinus node automaticity and AV conduction (anterograde and retrograde). In contrast, the effect of the quadratic term P2 was significant on sinus node automaticity only. Sequence of stimulation whether (SP) or (PS) had no significant effect on either chronotropic or dromotropic properties. Furthermore, whereas augmented antagonism with parasympathetic predominance over sympathetic activity was readily demonstrable in the control of sinus rate, the reverse, i.e., augmented antagonism with sympathetic preponderance over parasympathetic activity, was found to apply to the control of AV conduction (both anterograde and retrograde). Hence, the nature and extent of parasympathetic-sympathetic interactions vary considerably, depending on which cardiac structure and/or function is under scrutiny. It appears that AV conduction is especially sensitive to modulation of sympathoadrenergic activity, whereas sinus node automaticity is particularly responsive to cholinomimetic influences.

Effects of the diethylamino analog of ethmozin (ethacizin) upon sympathetic and parasympathetic efferent activity to the canine heart

This study examined the effects of the diethylamino analog of ethmozin (ethacizin) (1 mg/kg i.v.) on the spontaneous and reflexly elicited efferent activity in thoracic cardiac sympathetic and parasympathetic nerves. Nitroglycerin and phenylephrine (4 and 8 micrograms/kg i.v.) were administered to 15 anesthetized mongrel dogs while monitoring blood pressure and heart rate. In each dog two cardiac nerves were isolated and efferent neurograms were recorded simultaneously and analyzed by microprocessor. Ethacizin significantly attenuated the spontaneous sympathetic efferent activity in both left and right, preganglionic (n = 8) and postganglionic (n = 14) sympathetic nerves to the heart. In contrast, reflex changes in sympathetic activity elicited by baroreceptor challenges were not affected by ethacizin. Also, ethacizin did not significantly affect either spontaneous or baroreceptor reflex-induced parasympathetic efferent activities in eight preganglionic nerves. Thus, this new phenothiazine derivative may exert part of its antiarrhythmic action through a reduction of the spontaneous sympathetic tonic discharges to the heart. The fact that ethacizin reduced neither the reflex-induced changes in sympathetic or parasympathetic activities nor influenced the tonic vagal discharges further suggests that the compound is not likely to interfere with reflexly mediated cardiovascular adaptive changes.

Differences in the determinants of overdrive suppression between sinus rhythm and slow atrioventricular junctional rhythm

Sinus node recovery time was compared to the recovery time of a slow atrioventricular junctional rhythm in each of the same seven pentobarbital anesthetized dogs. Recovery time and the first five cardiac cycles were examined after pacing atria and ventricles for 20, 40, and 60 seconds at four or more pacing cycle lengths. Data relating recovery times and return to control conditions to prepacing cycle length, pacing cycle length, duration of pacing, site of pacing, and origin of rhythms were analyzed by covariance analysis. From the analyses, the relative contribution of the determinants are: the prepacing cycle length 73%, the site of pacing 3.5%, the pacing cycle length 2%, and the interaction of the site of pacing and pacing cycle length 1% for sinus node recovery time; and for slow atrioventricular junctional rhythm recovery time, the duration of pacing 40%, the interactions between the duration of pacing and the pacing cycle length 27%, and the prepacing cycle length 9%. A modified exponential decay model predicted 8 beats for return to prepacing conditions during sinus rhythm and 66-100 beats during atrioventricular junctional rhythm. We conclude that the single most important determinant of sinus node recovery time is the prepacing cycle length. Pacing cycle length and site of pacing have a significant but small influence on sinus node recovery time and duration of pacing, beyond 20 seconds, has no significant influence. In contrast, duration of pacing is the most important determinant of slow atrioventricular junctional recovery time. Another major determinant of slow atrioventricular junctional recovery time is the interactions between pacing cycle length and duration of pacing. Prepacing cycle length has a minor influence, and site of pacing has no influence, on slow atrioventricular junctional recovery time.

Negative chronotropic and parasympatholytic effects of alinidine on canine sinus node and AV junction

The direct effects of alinidine (N-allyl-clonidine) on the sinus node and atrioventricular (AV) junction were studied in 18 anesthetized dogs. Stimulus frequency-response curves to right stellate ganglion and right cervical vagus stimulations as well as responses to norepinephrine or acetylcholine were determined before and after selective perfusion of alinidine into the sinus node artery. Alinidine (1 microgram/ml) had no effect on spontaneous sinus rate [148 +/- 5 (SE) beats/min]. However, alinidine concentrations of 5, 10, and 25 micrograms/ml produced significant (P less than 0.05) sinus slowing to 138, 127, and 121 beats/min, respectively. Recovery to control rate was dose dependent and took from 4 to 33 min. Sinus rate increases with right stellate stimulations were not affected by alinidine. However, sinus rate decreases with right vagal stimulations were significantly (P less than 0.01) attenuated by alinidine. The negative chronotropic effects of acetylcholine were not influenced by alinidine. Alinidine (1-100 micrograms/ml into AV node artery) had no effect on the A-H interval of the His bundle electrogram. However, alinidine (10 and 25 micrograms/ml) diminished the AV block produced by stimulation of the left vagus in electrically paced hearts but not the negative dromotropic actions of directly administered acetylcholine. Thus alinidine has direct negative chronotropic effects, no effect on sinus node responses to sympathetic stimulation, ability to diminish sinus node and AV junctional responses to vagal stimulations without interference at the cholinergic muscarinic receptor, and 4) no effect on AV nodal conduction.

Parasympathetic and sympathetic efferent traffic during a cardiogenic hypertensive chemoreflex

Activation of a cardiac chemoreceptor with serotonin elicits a reflex which includes changes in heart rate, contractile force, regional blood flow and hypertension. In six anaesthetised dogs we simultaneously recorded parasympathetic and sympathetic efferent traffic elicited during this cardiogenic reflex. The parasympathetic fibres were confirmed by reciprocal frequency changes with changes in blood pressure. The sympathetic fibre activity (anterior ansa subclavia) was attenuated or eliminated by ganglionic blockade or by clonidine. Whereas the phasic sympathetic multifibre discharge was only followed by a quiet period, the parasympathetic multifibre discharge was both preceded and followed by a quiet period. The sympathetic discharge preceded the parasympathetic discharge by 683 +/- 170 ms. These autonomic efferent discharges were not elicited by administration of serotonin into the carotid artery, but were abolished by pretreatment with the serotonin antagonist, cyproheptadine. Cyproheptadine blockade could be overcome by increasing the serotonin concentration tenfold. This remarkable neural asynchrony has important implications concerning the electrical stability of the heart.

Indirect stimulatory action of the calcium channel blocker AQA-39

AQA-39 is a new bradycardia-inducing drug chemically related to verapamil that reduces potassium conductance and blocks calcium channels. In canine ventricular trabeculae studied at 25 degrees C and at a pacing rate of 12 stimuli per minute. AQA-39, in concentrations of 1 and 2 X 10(-5) M, had a significant positive (and only positive) inotropic effect. Propranolol significantly diminished this positive inotropic action of AQA-39. After catecholamine depletion with reserpine, AQA-39 still elicited a significant increase in contractility but the magnitude of the increment in contractile performance was considerably less than the one observed when normal muscles were exposed to AQA-39. AQA-39 had no significant inotropic action on reserpinized and atropinized muscles. From these results we can conclude that AQA-39 is not a beta adrenoceptor agonist although adrenergic influences via neuronal norepinephrine release are mediating part of the inotropic effect of the drug. Furthermore, at this low rate of stimulation and at the concentrations used, AQA-39 has no direct inotropic action of its own but part of its indirect stimulatory action is mediated through an antimuscarinic effect.

No electrical instability after intracoronary streptokinase administered into sinus node or AV node of dogs

Intracoronary streptokinase (SK) is being used to treat acute myocardial infarction. Complications including atrioventricular (AV) block and both supraventricular and ventricular arrhythmias have been described. Selective cannulation of the nutrient arteries of the sinus node and AV node in the pentobarbital-anesthetized dog provides a model to study the specific pharmacologic effects of drugs administered directly to these structures. Selective perfusion of 2 ml normal saline into the sinus node artery of five dogs resulted in a transient 38.6% decrease in heart rate (148 +/- 9.6 to 91 +/- 12.7 bpm). This control response is typical of the method. Administration of 100, 500, 1000, 2000, and 4000 IU of SK/ml in these same dogs resulted in similarly brief bradycardias of 41.2%, 40.2%, 39.4%, 41.2%, and 40.3% (p = NS) below the control rate, respectively. The tachycardia induced by 0.0125 and 0.025 micrograms/ml of norepinephrine (24.2% and 31.4% sinus acceleration) was unaffected by the simultaneous administration of 500, 1000, and 2000 IU SK/ml. AV conduction was unchanged by selective perfusion of 1000, 2000, and 4000 IU SK/ml into the AV node artery of seven dogs, respective AV intervals being 111, 114, and 116 msec, with the control interval being 109 msec. Thus after selective perfusion of either the sinus node or AV node with SK in clinically used concentrations, we found no significant chronotropic or dromotropic effect in the dog.

Studies of changes in breathing and blood pressure accompanying a cardiogenic hypertensive chemoreflex compared in conscious and anesthetized dogs

Serotonin activates a cardiogenic hypertensive chemoreflex characterized by sudden hypertension. This study in 16 awake and 12 anesthetized dogs was conducted to examine the influence of changes in breathing or heart rate as they may influence the reflex hypertension. In all 28 dogs, either anesthetized or awake, there is a variable but brief initial reflex bradycardia and in all dogs there is transient ventilatory stimulation, but the response in every dog was characterized by hypertension within at most 10 seconds after administering serotonin. Treatment with atropine regularly eliminated the reflex bradycardia and caused the hypertension to appear soon (average of 6 seconds to maximal level). Atropine did not alter the reflex stimulation of breathing. Since atropine eliminated any transient bradycardia (and associated hypertension) but did not prevent reflex changes in breathing, we do not believe that breathing alteration plays any significant hemodynamic role in this reflex.

Acute effects of amiodarone upon the canine sinus node and atrioventricular junctional region

Amiodarone was selectively perfused into the sinus node artery and atrioventricular node artery of 51 dogs. Amiodarone had an immediate negative chronotropic and dromotropic effect. Threshold concentration was 2.5 micrograms/ml. 25 and 50 micrograms/ml of amiodarone injected into the sinus node artery slowed the heart by 25.6 +/- 3.1 and 33.7 +/- 2.6 beats/min (mean +/- 1 SEM), respectively. Amiodarone 25 and 50 micrograms/ml injected into the AV node artery during AV junctional rhythm slowed the AV junctional pacemaker by 12.2 +/- 1.8 and 17.4 +/- 1.7 beats/min, respectively. Injections of amiodarone into the AV node artery during sinus rhythm regularly increased AV conduction time sometimes causing 2 degrees AV block at the highest concentration used. Impaired conduction was exclusively measured at the level of the A-H interval in the His electrogram. Neither atropine nor propranolol prevented the negative chronotropic effects of amiodarone. Amiodarone had no significant effect on sinus node response to either stellate stimulation or intranodal administration of norepinephrine. The negative chronotropic action of amiodarone was significantly enhanced when amiodarone was administered in a perfusate containing low (0.6 mM) instead of normal calcium. Taken collectively these observations indicate that amiodarone has immediate depressant electrophysiologic effects on both the sinus node and the AV junction and that these early effects might involve the blockade of the slow channel.

Differential effect of verapamil isomers on sinus node and AV junctional region

The l- and d-isomers of verapamil were selectively perfused into the sinus node artery and atrioventricular (AV) node artery of 48 dogs. Injection of l-verapamil into the sinus node artery during sinus rhythm and into the AV node artery during AV junctional rhythm depresses both sinus rhythm and AV junctional rhythm significantly more than does the d-isomer. l-Verapamil is three to four times more powerful than d-verapamil. Injection of the isomers into the AV node artery during sinus rhythm rapidly impairs AV conduction. Increments in conduction time are measured exclusively at the level of the A-H interval of the His bundle electrogram, and l-verapamil is six times more powerful than d-verapamil. Neither d- nor l-verapamil in concentrations that exert a profound negative chronotropic effect or cause AV block, has any significant effect on transatrial or His bundle conduction. Thus these concentrations of d-verapamil have little or no significant effect on the fast sodium channel, but both verapamil isomers affect the slow channel. The main difference in action between l- and d-verapamil appears to be only quantitative in nature. The sinus node is significantly more sensitive to the negative chronotropic action of verapamil than is the AV junctional pacemaker, and this differential responsiveness appears to be related to the different intrinsic rates of the two pacemakers. During sinus rhythm (either in the presence or absence of atropine) sinus node automaticity is less affected than AV conduction when verapamil is given parenterally. We propose that this greater negative dromotropic effect of verapamil is also in part due to a rate-dependent process, since during sinus rhythm AV junctional cells have to be depolarized at frequencies significantly higher than their intrinsic rates.

Post-excitatory depression in thoracic sympathetic efferent neural traffic during a cardiogenic hypertensive chemoreflex

Serotonin injected in the left atrium activates a cardiogenic hypertensive chemoreflex in dogs. To elucidate patterns of the neural traffic, records were obtained from thoracic sympathetic efferent nerves (either the anterior ansa of the left stellate ganglion or the T4 input to the left stellate) in 8 anesthetized dogs with chest open. Serotonin (200 micrograms, left atrium) caused a massive sympathetic discharge during the hypertension and bracardia characteristic of the chemoreflex. Following the initial sympathetic discharge, there was a consistent post-excitatory depression of neural traffic, to a level significantly less than control discharge (two-tailed p less than .05). This post-excitatory depression began 11 +/- 5.4 (S.D.) seconds after injection of serotonin and 6.6 +/- 5.3 seconds after the peak neural discharge. It lasted 140 +/- 94 seconds, being maximal initially with gradual recovery. Complete block of the hypertension by the combined administration of phentolamine, propranolol, and nitroglycerin failed to abolish the efferent neural events, including post-excitatory depression, in all but one dog. We conclude that post-excitatory depression in thoracic sympathetic efferent neural traffic cannot be mediated exclusively through the secondary engagement of a baroreceptor mechanism and that it most likely is an integral part of the cardiogenic hypertensive chemoreflex.

The direct negative chronotropic action of prazosin on the canine sinus node

The direct and indirect chronotropic effects of prazosin were studied in 22 anesthetized dogs using the technique of selective perfusion of the sinus node. Concentrations of 1, 10, 50 and 100 micrograms/ml of prazosin produced cumulative, dose-related reductions in mean sinus rat of 4 +/- 2; 10 +/- 6; 15 +/- 9 and 27 +/- 15 beats/min, respectively (P less than .0001). Intranodal prazosin did not significantly alter the positive chronotropic sinus node response to norepinephrine (0.05 and 0.1 micrograms/ml) administered via the same route. Sinus rate increases during stimulus frequency response curves after right stellate stimulation were not influenced by prior administration of prazosin. Sinus bradycardia elicited by submaximal vagal stimulation also was not affected by intranodal prazosin.l In six reserpinized dogs, methoxamine (10 and 25 micrograms/ml) injected into the sinus node artery caused a sinus rate increase of 10 4/- 2 and 14 +/- 3 beats/min (P less than .001). Intranodal prazosin (50 micrograms/ml) prevented this effect. Taken together, these results indicate that prazosin exerts a direct negative chronotropic action on the canine sinus node and that this effect is at least in part mediated through interference with alpha-1 receptors located in or near the sinus node. Prazosin has little or no effect on the presynaptic alpha-2 receptors nor does it interfere with the cholinergic mechanism in the sinus node.

Effects of selectively produced changes in cardiac rhythm or conduction upon flow in the superior vena cava

The effects of changes in cardiac rhythm or conduction on the rate of volume flow in the superior vena cava (SVC) were studied in 25 dogs anesthetized with sodium pentobarbital. Slowing of sinus rate from 145 +/- 16 to 114 +/- 12 beats/min was achieved by selective perfusion of Ringer's solution into the sinus node artery. During sinus slowing there was a continuous change in the systolic-diastolic flow ratio measured in the SVC. At control sinus rate (net), total forward flow as 6.3 ml/beat with a systolic volume of 6.5 ml and a diastolic volume of -0.2 ml. At the slowest bradycardia, total forward flow had increased by 25%. Since there was no significant change in the systolic volume, the entire 25% increase in forward flow occurred during ventricular diastole. During atrioventricular (AV) block (second-degree or complete), achieved by selective perfusion of eserine into the AV node artery, unimpaired acceleration of flow was readily demonstrable in the SVC even in the absence of ventricular contraction. Thus systolic ventricular suction (vis a fronte) has little or no effect on right atrial filling. During high-degree AV block, the normal atrial contraction readily opened the tricuspid valve, and when both atrium and ventricle were filled to or near maximum capacity, atrial contraction was consistently vigorous enough to propel blood into the pulmonary artery. Given appropriate hemodynamic circumstances, these results confirm the remarkable effectiveness of right atrial contractile performance.