Biomedical subjects

M R Warner

Publications and source records attributed to M R Warner.

At least 19 recordsLinked to original sources

PubMed1996

Functional evaluation of gap vs. abutment repair of peripheral nerves in the rat.

Various expressions of nerve regeneration specificity (tissue, target, and end-organ) have been demonstrated histologically, but the influence of nerve regeneration specificity on functional outcome remains to be investigated. Specificity expression appears to be dependent, to some extent, on the distance between the proximal and distal nerve stumps. Regeneration specificity is inhibited by end-to-end alignment. This study examined the effect of gap distance repair on the return of rat hind-limb function, as measured by the sciatic function index (SFI). Sixty-four Sprague-Dawley rats were randomized into three groups: 0-mm gap (abutment), 2-mm gap, and 5-mm gap repair. The sciatic nerve was divided and secured in a Silastic conduit to maintain the gap length. The SFI was measured preoperatively and then at postoperative weeks 2, 4, 8, 12, 16, and 20. Results showed no significant difference in functional return between the abutment and the two gap groups at any time period (SFI at 20 weeks = -73.9, 0 mm; -73.5 2 mm; -75.2, 5mm; p = 0.90). This lack of difference in functional outcomes suggested an insignificant functional influence of regeneration specificity within single chamber conduits. The study also demonstrated that small gaps between nerve ends within a conduit yielded functional results equal to end-to-end repair of rat sciatic nerves.

Anastomosis, Surgical↗

PubMed1995

Time-course and frequency dependence of sympathetic stimulation-evoked inhibition of vagal effects at the sinus node.

Vagally-induced chronotropic responses have been shown to be inhibited after the termination of sympathetic stimulation. We sought to characterize the onset and time-course of the sympathetically evoked inhibition of vagal effects by measuring vagally induced chronotropic responses during concomitant sympathetic stimulation. In anesthetized dogs we recorded lead II of the electrocardiogram, arterial pressure and cardiac cycle length. In 7 dogs, the vagi were stimulated for 15 s every minute before, during and after 10-min trains of sympathetic stimulation. The sympathetic stimulation was applied at frequencies of 0.5, 1, 2, 5 and 10 Hz. During the 1, 2 and 5-Hz trains of sympathetic stimulation, the vagally induced changes in cycle length diminished progressively and thus, were less (P < 0.001) at 3, 5 and 10 min compared with 1 min into the sympathetic stimulation. The magnitude of the attenuation of vagal effects on cycle length depended (P < 0.001) on the frequency of sympathetic stimulation. To determine the role of alpha- and beta-adrenergic receptors, we measured vagally-induced changes in cycle length during 5-min trains of sympathetic stimulation (1, 2, 5, 10 Hz) in the presence and absence of phentolamine and propranolol (n = 6). Both before and after combined alpha- and beta-adrenergic receptor blockade, the vagally-induced changes in cycle length decreased (P < 0.03) progressively during the 1, 2, 5 and 10-Hz trains of sympathetic stimulation and the magnitude of the inhibition depended (P < 0.002) on the frequency of sympathetic stimulation. These data show that the effects of short trains of vagal stimulation on cardiac cycle length are inhibited progressively during continuous trains of sympathetic stimulation before and after combined alpha- and beta-adrenergic receptor blockade. Thus, substances other than norepinephrine may contribute to the inhibition of cardiac vagal effects that occurs during a continuous train of sympathetic stimulation.

Adrenergic Fibers↗

PubMed1995

In vivo and in vitro electrophysiologic effects of terodiline on dog myocardium.

INTRODUCTION: Terodiline hydrochloride, widely prescribed for urinary incontinence, has been reported to cause bradycardia and torsades de pointes. METHODS AND RESULTS: In this study, we characterized the electrophysiologic effects of terodiline in dog cardiac tissues in vivo and in isolated canine cardiac Purkinje fibers. Terodiline (1 to 10 microM) resulted in dose-dependent reduction of action potential amplitude and maximal upstroke velocity (Vmax). The threshold for these effects was approximately 2 microM (0.6 mg/L), and the changes were cycle-length dependent. Terodiline (> or = 2 microM) also depressed the action potential plateau but did not significantly alter action potential duration at concentrations < or = 10 microM. In vivo studies demonstrated that high doses of terodiline (3 mg/kg) lengthened AH and HV intervals, slowed spontaneous sinus rate, prolonged ventricular refractoriness, and inhibited vagally induced slowing of the sinus node. Sympathetic effects on spontaneous sinus rate were unchanged. In both isolated canine Purkinje fibers and anesthetized dogs, terodiline did not evoke afterdepolarizations, repetitive firing, or ventricular tachyarrhythmias under normal or hypokalemic conditions. CONCLUSION: Our findings suggest that terodiline (> or = 1 to 2 microM) leads to blockade of sodium and calcium channels as well as muscarinic receptors in canine cardiac tissues.

Action Potentials↗

PubMed1993

Autotomy and the sciatic functional index.

The rat sciatic nerve serves as a good model of nerve regeneration and, as such, is often used in investigations of nerve repair. After transection of the sciatic nerve, rodents frequently scratch and bite their anesthetic foot, resulting in amputation of one or more toes. This is termed autotomy or autophagy. When these rats are part of a study using the sciatic functional index (SFI), autotomy results in unusable data, since necessary foot landmarks have been removed. It would be helpful, therefore, to be familiar with the phenomenon of autotomy and to know which rats are least likely to mutilate themselves. In our experiment involving 64 rats in which the sciatic nerve was transected and repaired, we found that female Sprague-Dawley rats were significantly less likely to perform autotomy than males (33% vs. 65%, P = .04). In addition, we noted that two-thirds of the autotomies that occurred did so by postoperative week 4 and that tabasco sauce did not decrease this activity. We present our experience and a survey of the literature on autotomy and the SFI.

Animals↗

PubMed1993

Simulated ischemia does not protect against efferent sympathetic denervation following acute myocardial infarction in canine hearts.

INTRODUCTION: Preconditioning the myocardium with brief episodes of ischemia preserves efferent autonomic responsiveness of noninfarcted myocardium apical to a site of acute transmural ischemia by mechanism(s) still unknown. We hypothesized that repeated brief exposure of the myocardium to a simulated ischemic milieu including hypoxia, high K+, low pH, and adenosine would be as effective as brief coronary occlusions in creating this protection. METHODS AND RESULTS: Open chest anesthetized dogs received an extracorporeal bypass between the left carotid artery and a diagonal branch of the left anterior descending coronary artery. We analyzed the effects of simulated ischemia on the time course and extent of efferent sympathetic denervation during a subsequent 3-hour sustained ischemia in three groups of dogs: two groups of dogs underwent four cycles of 5-minute intracoronary perfusion with either hypoxic altered Tyrode's solution (12 mM K+, 6.8 pH, and 10 microM adenosine; n = 11) or normal Tyrode's solution (n = 11). Each Tyrode's perfusion was separated by 5 minutes of blood perfusion prior to permanent coronary occlusion by latex embolization of the cannulated coronary artery. A third group received a continuous 3-hour blood perfusion before the final ischemic episode (n = 5). Shortening of effective refractory periods (ERPs) induced by bilateral ansae subclaviae stimulation (2 to 4 Hz) basal and apical to the intervention site was determined before and after perfusions and 20, 60, 120, and 180 minutes after sustained occlusion. In all groups, sympathetically-induced ERP shortening was unchanged at basal sites throughout the experiment. ERP shortening at apical sites was unchanged after perfusions with either the altered or normal Tyrode's solution or after a continuous 3-hour blood perfusion. However, ERP shortening became significantly attenuated at apical sites after coronary occlusion in all groups. Neither the size in reduction of sympathetically-induced ERP shortening at apical test sites nor the cumulative percentage of denervated apical test sites (< or = 2-msec shortening) during a 3-hour period of permanent ischemia differed significantly among groups (P = 0.052 and P = 0.752, respectively). The degree of subepicardial involvement in the myocardial infarction was comparable among groups. CONCLUSION: Thus, brief exposure of the left ventricular myocardium to ischemic metabolites prior to a subsequent permanent coronary occlusion does not trigger mechanism(s) that are responsible for protection against efferent sympathetic denervation apical to an area of transmural myocardial infarction/ischemia.

Adenosine↗

PubMed1993

Mechanisms of denervation supersensitivity in regionally denervated canine hearts.

Mechanisms responsible for "denervation supersensitivity" in regionally denervated canine hearts were examined by measuring beta-adrenergic receptor density and affinity and the density of the alpha-subunit of the stimulatory G protein (Gs alpha). Sympathetic denervation was produced by applying an epicardial strip of phenol midway between the left ventricular (LV) base and apex. Six to eight days after denervation, dogs were anesthetized and then underwent functional studies (n = 4) or hearts were excised for biochemical analyses (n = 6). Biochemical studies were also done on 3 nondenervated hearts. Effective refractory periods (ERPs) were measured in innervated (base) and denervated (apex) LV myocardium. During sympathetic stimulation (2 and 4 Hz), the ERP shortened more (P < 0.05) at basal than at apical sites, whereas during norepinephrine infusion (0.05 to 0.5 mg.kg-1 x min-1), the ERP shortened more (P < 0.001) at apical than at basal sites. In regionally denervated hearts, however, the density and affinity of beta-adrenergic receptors did not differ significantly (P > 0.2) in nondenervated basal compared with denervated apical myocardium. Quantitative immunoblotting of the Gs alpha demonstrated that the density of the 47- and 52-kDa subunits was also similar (P > 0.6) in basal compared with apical myocardium from regionally denervated hearts. In addition, beta-adrenergic receptor density and affinity and Gs alpha density did not differ significantly (P > 0.5) in basal compared with apical myocardium from nondenervated control hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Sympathetic stimulation and norepinephrine infusion modulate extracellular potassium concentration during acute myocardial ischemia.

The purpose of this study was to investigate whether sympathetic stimulation modulated the rise in extracellular K+ concentration ([K+]o) evoked by acute myocardial ischemia. In 35 alpha-chloralose-anesthetized dogs, we measured changes in [K+]o during acute myocardial ischemia in the presence and absence of sympathetic stimulation or norepinephrine infusion. A series of four 5-minute occlusions of the distal left anterior descending coronary artery (LAD) was completed in 18 dogs. Thirty minutes of reperfusion separated each LAD occlusion. Four to five K(+)-sensitive electrodes were inserted into the left ventricular midmyocardium that was perfused by the distal LAD. Lead II of the electrocardiogram, arterial pressure, and [K+]o were recorded, and the right atrium was paced at a constant cycle length. The first, second, and fourth LAD occlusions were done in the absence of sympathetic stimulation or norepinephrine infusion. The changes in [K+]o evoked by the first LAD occlusion differed (p < 0.05) from those elicited by the second and fourth occlusions. However, the changes in [K+]o during the second and fourth LAD occlusions were similar (p > 0.2) and served as controls for the responses obtained during the third occlusion. Two minutes before the third LAD occlusion, sympathetic stimulation (4 Hz) or norepinephrine infusion (0.25-0.5 micrograms/kg per minute i.v.) was begun and was continued until 2 minutes after reperfusion. We found that sympathetic stimulation and norepinephrine infusion increased (p < 0.05) myocardial blood flow in both normal and ischemic tissue. The mean response recorded by 23 K(+)-sensitive electrodes in 11 dogs showed that sympathetic stimulation increased (p < 0.001) the [K+]o at 1, 2, 3, 4, and 5 minutes after the onset of LAD occlusion compared with the second and fourth occlusions. In contrast, the mean response recorded by 20 K(+)-sensitive electrodes in seven dogs showed that norepinephrine infusion reduced (p < 0.02) the [K+]o at 4 and 5 minutes after the onset of LAD occlusion. These data show that sympathetic stimulation increased the [K+]o evoked by acute myocardial ischemia, an effect that was not mimicked by the intravenous administration of norepinephrine.

Animals↗

PubMed1991

Sympathetic stimulation-evoked overflow of norepinephrine and neuropeptide Y from the heart.

Neuropeptide Y (NPY) and norepinephrine are released together on sympathetic activation. To compare the time courses of NPY and norepinephrine washout from cardiac tissues, we measured the overflow of NPY-like immunoreactivity (NPY-LI) and norepinephrine in coronary sinus blood before, during, and after 3-minute trains of ansae subclaviae stimulation in 13 anesthetized dogs. We also measured vagally induced cardiac cycle length responses before and after ansae stimulation. Ansae stimulation increased NPY-LI and norepinephrine overflow from the heart in a frequency-dependent manner (p less than 0.02). After stimulation of the ansae at 5 and 10 Hz, the peak norepinephrine overflows decayed by 90% within 2 minutes, but the NPY-LI overflows required 17 +/- 11 and 35 +/- 21 minutes, respectively, to decay by 90%. Cardiac vagal effects were inhibited after 5- and 10-Hz ansae stimulations, and the peak inhibitions decayed by 90% after 19 +/- 7 and 39 +/- 16 minutes, respectively. The 90% decay times of the NPY-LI overflows were longer (p less than 0.003) than those of the norepinephrine overflows but did not differ significantly (p greater than 0.4) from the 90% decay times of the inhibition of vagal effects. We characterized NPY-LI in coronary sinus and arterial plasma by reversed-phase high-performance liquid chromatography. Before ansae stimulation, the main peak of NPY-LI in the plasma had a retention time similar to that of the oxidized human NPY-(1-36) standard. During ansae stimulation, however, there was a substantial increase in the peak of NPY-LI that eluted in a position similar to that of the monoxidized human NPY-(1-36) standard. These data support the hypothesis that neurally released NPY mediates the sympathetically evoked inhibition of vagal effects and indicate that the time course of removal of NPY from the heart differs substantially from that of norepinephrine. Moreover, under basal conditions, most NPY in the circulation is present in the oxidized form or as a fragment of the 36-amino-acid peptide. In contrast, cardiac sympathetic stimulation evokes the overflow of monoxidized NPY-(1-36) into the coronary sinus plasma.

Animals↗

PubMed1990

Sinus and atrioventricular nodal distribution of sympathetic fibers that contain neuropeptide Y.

Neuropeptide Y and norepinephrine are localized in sympathetic nerve terminals throughout the heart. We sought to determine the functional distribution of the neuropeptide Y-containing sympathetic fibers to the sinus and atrioventricular (AV) nodal regions. We recorded cycle length, AV interval, and arterial pressure in 14 anesthetized dogs. We assessed the release of neuropeptide Y from sympathetic nerve terminals by measuring the attenuation of the vagal effects on cycle length and AV interval that occurred after unilateral ansa subclavia stimulation. Three-minute trains of right or left ansa stimulation, each applied at frequencies of 2, 5, and 10 Hz, produced a frequency-dependent inhibition of the vagal effects on cycle length and AV interval. After right ansa stimulation (10 Hz), however, the percent inhibition of the vagal effects on cycle length was 21 +/- 5% greater (p less than 0.001) than the percent inhibition of the vagal effects on AV interval. Conversely, after left ansa stimulation (10 Hz), the percent inhibition of the vagal effects on AV interval was 54 +/- 7% greater (p less than 0.001) than the percent inhibition of the vagal effects on cycle length. The vagal stimulus characteristics (frequency or voltage) did not significantly alter the percent inhibition, nor did the percent inhibition depend on the vagus stimulated (right or left vagus). We conclude that most of the neuropeptide Y-containing sympathetic fibers at the sinus node originate in right-sided ganglia, whereas most of those at the AV node originate in left-sided ganglia.

Adrenergic Fibers↗

PubMed1989

Neuropeptide Y as a putative modulator of the vagal effects on heart rate.

Neuropeptide Y is stored in sympathetic nerve terminals throughout the heart and has direct and indirect effects on cardiac function. Although neuropeptide Y has been shown to be released upon intense (16-30 Hz) cardiac sympathetic stimulation, we sought to determine whether effective quantities of neuropeptide Y were released from cardiac sympathetic neurons under more natural conditions. We recorded arterial pressure and cardiac cycle length in 29 anesthetized dogs. We assessed neuropeptide Y release by measuring the attenuation of vagally induced increases in cardiac cycle length (10 seconds every 2 minutes) after trains of sympathetic stimulation. We examined the effect of constant-frequency sympathetic stimulation (frequencies of 2, 5, 10, and 15 Hz, applied for train durations of 1, 3, and 5 minutes) on vagally induced chronotropic responses. We also determined the effect of varying the pattern of sympathetic stimulation. Both the magnitude and duration of the inhibition of the vagal effects on cardiac cycle length were augmented significantly by increases in the frequency or duration of sympathetic stimulation. In contrast, the inhibition of the vagally induced chronotropic responses was not significantly affected by changes in the pattern of sympathetic stimulation. We also characterized the role of adrenergic receptors. Phentolamine significantly increased the sympathetically mediated inhibition of the vagal effects on cardiac cycle length, but propranolol had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Inhibition of cardiac vagal effects by neurally released and exogenous neuropeptide Y.

Neuropeptide Y (NPY) attenuates vagal effects on cardiac cycle length, presumably by inhibiting the release of acetylcholine from vagal nerve endings. We sought to determine if NPY inhibited the vagal effects on atrioventricular (AV) interval and atrial contraction in a manner similar to its inhibition of the vagal effects on cycle length. In 19 anesthetized dogs we measured the vagal effects on cycle length, AV interval, and atrial contraction before and after 3-minute trains of sympathetic stimulation or before and after exogenous NPY (20 micrograms/kg i.v.). Three minutes after 10-Hz sympathetic stimulation, the vagal effects on cycle length and AV interval were attenuated by 52 +/- 9% and 63 +/- 8%, respectively. Phentolamine significantly augmented this attenuation, but propranolol had no appreciable effect. In the control group of animals or in the group that received phentolamine, the vagal effects on atrial contraction measured before and after sympathetic stimulation were not significantly different. In these two groups, however, the basal atrial contraction was reduced substantially after the cessation of sympathetic stimulation. Propranolol prevented this reduction in atrial contraction. After propranolol, the vagal effects on atrial contraction 3 minutes after sympathetic stimulation were attenuated by 31 +/- 6%. Exogenous NPY had no direct effect on cycle length, AV interval, or atrial contraction, but exogenous NPY did persistently inhibit the vagal effects on each of these cardiac processes. Three minutes after NPY was given, the vagal effects on cycle length, AV interval, and atrial contraction were inhibited by 62 +/- 7%, 69 +/- 5%, and 68 +/- 5%, respectively. We conclude that NPY attenuates the vagal effects on the atrial myocardium and on the sinus and AV nodes. In the absence of beta-blockade, the inhibitory effect of neurally released NPY on the vagally induced decreases in atrial contraction may be masked by the reduction in the atrial contraction that occurs after sympathetic stimulation.

Animals↗

PubMed1988

Phasic influences of vagal stimulation on atrioventricular conduction.

The beat-by-beat changes in atrioventricular (AV) conduction evoked by constant frequency and phase-coupled vagal stimulation were examined both qualitatively and quantitatively in 13 anesthetized dogs. The effects of pacing cycle length and sympathetic activity on the vagally induced phasic changes in AV conduction were also characterized. When the vagal stimulus interval was nearly equal to the pacing cycle length and the vagal stimulus moved progressively through the cardiac cycle, AV interval oscillated in a rhythmic fashion. The rhythmicity of the vagally induced AV interval oscillations was altered substantially by changes in either the vagal stimulus interval or the pacing cycle length. The vagally induced AV interval oscillations were abolished during phase-coupled vagal stimulation; however, the magnitude of the resultant steady-state AV interval depended on the time relative to the phase of the cardiac cycle that the vagal stimulus was delivered. In the presence or absence of sympathetic stimulation, a vagal stimulus falling approximately 200 ms prior to atrial depolarization evoked the greatest prolongation in AV interval, regardless of the pacing cycle length. Additionally, the effects of combined sympathetic and phase-dependent vagal stimulation on the AV interval were additive. These data confirm that the influence of a vagal stimulus on AV interval can be predicted from the phase in the cardiac cycle that the vagal stimulus is delivered. Moreover, this phase dependency of vagal effects evokes marked qualitative variations in AV interval response patterns when either the vagal stimulus interval or the pacing cycle length is altered.

Animals↗

PubMed1987

Atrioventricular nodal accommodation: rate- and time-dependent effects.

To characterize intrinsic rate- and time-dependent properties of the atrioventricular (AV) node, we examined whether AV interval (AVI) would be comparable at identical heart rates (HR) reached using different types of stimulation paradigms. We compared changes in AVI during five consecutive 30-s, 20 beat/min increases in HR from control with AVI changes induced by 30-s single-step protocols to each of the same levels. In addition, HR was maintained at each level for 30, 60, 90, 120, and 150 s to control for the influences of time. Chloralose-anesthetized dogs (n = 16) were autonomically decentralized and instrumented to record electrocardiogram, blood pressure, and multiple intracardiac electrograms. Computer-generated HR steps were begun 20 beats/min above control, while continuously recording AVI. The beat-by-beat changes in AVI within each step were quantitated for all protocols. Differences between AVI during single- and multiple-step protocols were most pronounced during the first 15 s of atrial pacing. Accommodation in AVI (change in AV conduction time associated with an increased but constant heart rate) was evident during both protocols. The degree of accommodation during multiple-step protocols was modulated by the cumulative effects of earlier HR; however, a similar degree of accommodation occurred at higher HR irrespective of protocol used. Finally, the time to onset of loss of 1:1 AV conduction was significantly shorter when HR was increased using a multiple-step protocol. Thus the intrinsic response of the AV node to HR change is dependent on absolute level of HR, duration of rate change, and the potential cumulative effects of any earlier HR steps.

Animals↗

PubMed1987

Reflex regulation of atrioventricular conduction.

We evaluated the time course of baroreflex modulation of atrioventricular (AV) nodal conduction in anesthetized dogs (n = 28). Beat-by-beat changes in heart rate (HR) and AV interval (AVI) evoked by transient alterations in arterial pressure (AP) were recorded in the intact state, after vagotomy, and following stellectomy. Under each experimental condition, alterations in AP induced parallel changes in HR and AVI with maximum HR and AVI responses occurring simultaneously. In three animals, AP alterations elicited pacemaker shifts that markedly altered AVI. Reflex changes in AVI were also examined during atrial pacing. When pacing at a low (120 beats/min) versus a high (190 beats/min) level of heart rate, reductions in AP decreased AVI to a significantly lower absolute value in the intact state, after vagotomy, and following stellectomy. However, under each experimental condition, decreases in AP elicited marked changes in AVI at either level of pacing. We conclude that baroreflex-induced changes in sympathetic and parasympathetic activity influence the sinoatrial and AV nodes simultaneously, predominate over the effects of changes in HR at the AV node, and may induce pacemaker shifts that influence the measurement of AVI.

Animals↗

PubMed1987

Postpacing tachycardia: autonomic involvement.

The cessation of pacing from the sinus node region is followed by a transient sinus tachycardia or postpacing tachycardia (PPT). We sought to characterize autonomic involvement in PPT. We used alpha-chloralose-anesthetized dogs and recorded electrocardiograms, blood pressure, and electrograms from the sinus node, right atrium, right ventricle, and His bundle. Both vagi and both stellate ganglia were transected. PPT developed immediately after either linear or stepped heart rate changes. PPT followed pacing from the rostral but not the caudal region of the sulcus terminalis. Independent manipulation of absolute level of heart rate (+33, +66, and +100 beats/min above control) and duration of atrial pacing (10, 20, and 30 s) revealed that PPT is dependent predominantly on the duration of pacing and less on level of heart rate. During pacing in the control state, a 1:1 atrial capture was maintained. After atropine administration (0.2 mg/kg iv), PPT increased significantly in magnitude, time to peak PPT was shortened significantly, and loss of 1:1 atrial capture during pacing was evident at pacing rates of from 10 to 60 beats above control. In contrast, propranolol significantly attenuated PPT. We conclude that acetylcholine, released during pacing from the sinus node region, suppresses inherent sinus node acceleration induced by the concurrent release of intramural catecholamines. During pacing, acetylcholine release is essential for the maintenance of 1:1 atrial capture and significantly modulates both the latency and magnitude of PPT.

Animals↗

PubMed1986

Human fibroblast chromatin states as effectors of the DNA-binding characteristics of benzo[a]pyrene anti-7,8-dihydrodiol 9,10-epoxide and two nonalkylating DNA-binding molecules.

Pure populations of mitotic or nonmitotic diploid human fibroblasts (greater than 98% pure) were exposed to [3H]benzo [a]pyrene (CAS: 50-32-8) anti-7,8-dihydrodiol 9,10-epoxide: r-7,t-8 dihydroxy-t-9, 10-oxy-7,8,9,10-tetrahydrobenzo [a]pyrene (or anti-diol-epoxide). In addition, metaphase chromosomes, interphase chromatin, or naked DNA was isolated from the pure cell populations and then titrated to saturation with anti-diol-epoxide, chromomycin A3, or 3,8-diamino-5-ethyl-6-phenylphenanthridinium bromide (ethidium bromide). At saturation, anti-diol-epoxide had covalently modified 1.5% of the total deoxyguanosine residues in naked DNA, and this was reduced to 29 and 15% of this level in saturating the available anti-diol-epoxide-binding sites in chromosomes or chromatin, respectively. A similar hierarchy of accessible binding sites (naked DNA greater than chromosomes greater than chromatin) was also observed for the noncovalent interaction of chromomycin A3 or ethidium bromide with the human cell DNA. Deproteinization of the chromosome or chromatin preparations returned the level of drug binding to that seen with naked DNA. The results clarify the association between proteins and DNA in human chromatin and suggest how cell-cycle-dependent changes in DNA-associated proteins or higher-order changes in protein-DNA conformation can act to alter the access of molecules to DNA-binding sites.

7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide↗

PubMed1986

Beat-by-beat modulation of AV conduction. I. Heart rate and respiratory influences.

We examined the integration of heart rate and neural influences at the atrioventricular (AV) node in conscious dogs. Animals were anesthetized and, under sterile conditions, instrumented to chronically record atrial and ventricular electrograms and blood pressure. In the conscious state, electrocardiogram (ECG), respiration, blood pressure, and electrograms were recorded on a beat-by-beat basis, and heart rate and AV interval were plotted graphically as a function of time. Resting animals exhibited both respiratory sinus arrhythmia and marked oscillations in AV conduction time associated with respiration. During inspiration AV interval was shortened, and during expiration AV interval was prolonged. To obviate the effect of cyclic changes in heart rate, atrial pacing was used to increase heart rate over a wide range both abruptly and linearly. Regardless of the pattern of heart rate change, AV interval oscillated at the respiratory frequency at pacing rates 10-100 beats/min above control. Higher levels of atrial pacing resulted in AV conduction patterns that were correlated with changes in blood pressure. Thus in the conscious dog variations in AV conduction time occur on a beat-by-beat basis in conjunction with respiration; oscillatory activity of AV conduction is not dependent on simultaneous changes in heart rate; and during atrial pacing, autonomic neural activity associated with respiration and blood pressure appears to dynamically modulate AV conduction with respiratory effects predominating at low heart rates and blood pressure effects at high heart rates.

Animals↗

PubMed1986

Beat-by-beat modulation of AV conduction. II. Autonomic neural mechanisms.

We examined the mechanism by which autonomic neural activity associated with respiration and blood pressure modulates atrioventricular (AV) conduction in conscious dogs. Mongrel dogs were anesthetized and instrumented under sterile conditions to record atrial and ventricular electrograms and blood pressure. In the conscious state, electrocardiogram (ECG), respiration, blood pressure, and electrograms were recorded continuously, and heart rate and AV interval were plotted graphically as a function of time. To delineate the role(s) of sympathetic and parasympathetic activity, AV conduction was studied during abrupt and linear changes in heart rate after administration of atropine, propranolol, or both. In the basal state and after propranolol, AV interval oscillated with respiration both in the absence of atrial pacing and at pacing rates 10-100 beats/min above control. Following atropine, oscillations in AV interval associated with respiration were abolished; however, linear and abrupt heart rate increases resulted in AV conduction changes that were associated with fluctuations in blood pressure. In contrast, after both atropine and propranolol, alterations in blood pressure or respiration did not influence AV conduction and rate-dependent prolongation of AV conduction occurred. We conclude that in the basal state, AV conduction is influenced predominately by changes in parasympathetic activity which is the major determinant of respiratory-related AV interval oscillations; after atropine, sympathetic activity produces fluctuations in both AV conduction and blood pressure; and intrinsic rate-dependent properties of the AV node are modulated continually by both divisions of the autonomic nervous system.

Animals↗