Effects of anesthesia and sleep on circulatory response to carotid sinus nerve stimulation

Aortic Body Chemoreceptors Regulate Coronary Blood Flow in Conscious Control and Hypertensive Sheep

BACKGROUND

Peripheral arterial chemoreceptors monitor the chemical composition of arterial blood and include both the carotid and aortic bodies (ABs). While the role of the carotid bodies has been extensively studied, the physiological role of the ABs remains relatively under-studied, and its role in hypertension is unexplored. We hypothesized that activation of the ABs would increase coronary blood flow in the normotensive state and that this would be mediated by the parasympathetic nerves to the heart. In addition, we determined whether the coronary blood flow response to stimulation of the ABs was altered in an ovine model of renovascular hypertension.

METHODS

Experiments were conducted in conscious and anesthetized ewes instrumented to record arterial pressure, coronary blood flow, and cardiac output. Two groups of animals were studied, one made hypertensive using a 2 kidney one clip model (n=6) and a sham-clipped normotensive group (n=6).

RESULTS

Activation of the ABs in the normotensive animals resulted in a significant increase in coronary blood flow, mediated, in part by a cholinergic mechanism since it was attenuated by atropine infusion. Activation of the ABs in the hypertensive animals also increased coronary blood flow (P<0.05), which was not different from the normotensive group. Interestingly, the coronary vasodilation in the hypertensive animals was not altered by blockade of muscarinic receptors but was attenuated after propranolol infusion.

CONCLUSIONS

Taken together, these data suggest that the ABs play an important role in modulating coronary blood flow and that their effector mechanism is altered in hypertension.

Plasticity of cardiovascular chemoreflexes after prolonged unilateral carotid body denervation: implications for its therapeutic use

Unilateral carotid body denervation has been proposed as treatment for sympathetic-related human diseases such as systolic heart failure, hypertension, obstructive sleep apnea, and cardiometabolic diseases. The long-term therapeutic effects of carotid body removal will be maintained if the remnant "buffer nerves," that is, the contralateral carotid nerve and the aortic nerves that innervate second-order neurons at the solitary tract nuclei (NTS), do not modify their contributions to the cardiovascular chemoreflexes. Here, we studied the cardiovascular chemoreflexes 1 mo after unilateral carotid body denervation either by excision of the petrosal ganglion (petrosal ganglionectomy, which eliminates central carotid afferents) or exeresis of a segment of one carotid nerve (carotid neurectomy, which preserves central afferents). Cardiovascular chemoreflexes were induced by intravenous (iv) injections of sodium cyanide in pentobarbitone-anesthetized adult cats. After 1 mo of unilateral petrosal ganglionectomy, without significant changes in basal arterial pressure, the contribution of the contralateral carotid nerve to the chemoreflex increases in arterial pressure was enhanced without changes in the contribution provided by the aortic nerves. By contrast, after 1 mo of unilateral carotid neurectomy, the contribution of remnant buffer nerves to cardiovascular chemoreflexes remained unmodified. These results indicate that a carotid nerve interruption involving denervation of second-order chemosensory neurons at the NTS will trigger cardiovascular chemoreflex plasticity on the contralateral carotid pathway. Then, unilateral carotid body denervation as therapeutic tool should consider the maintenance of the integrity of carotid central chemoafferents to prevent plasticity on remnant buffer nerves.NEW & NOTEWORTHY Unilateral carotid body denervation has been proposed as treatment for sympathetic hyperactivity-related human disorders. Its therapeutic effectiveness for maintaining a persistent decrease in the sympathetic outflow activity will depend on the absence of compensatory chemoreflex plasticity in the remnant carotid and aortic afferents. Here, we suggest that the integrity of central afferents after carotid body denervation is essential to prevent the emergence of plastic functional changes on the contralateral "intact" carotid nerve.

The Effect of Chronic Activation of the Novel Endocannabinoid Receptor GPR18 on Myocardial Function and Blood Pressure in Conscious Rats

Although acute activation of the novel endocannabinoid receptor GPR18 causes hypotension, there are no reports on GPR18 expression in the heart or its chronic modulation of cardiovascular function. In this study, after demonstrating GPR18 expression in the heart, we show that chronic (2 weeks) GPR18 activation with its agonist abnormal cannabidiol (abn-cbd; 100 µg·kg·d; i.p) produced hypotension, suppressed the cardiac sympathetic dominance, and improved left ventricular (LV) function (increased the contractility index dp/dtmax and reduced LV end-diastolic pressure, LVEDP) in conscious rats. Ex vivo studies revealed increased: (1) cardiac and plasma adiponectin (ADN) levels; (2) vascular (aortic) endothelial nitric oxide synthase (eNOS) expression, (3) vascular and serum nitric oxide (NO) levels; (4) myocardial and plasma cyclic guanosine monophosphate (cGMP) levels; (5) phosphorylation of myocardial protein kinase B (Akt) and extracellular signal regulated kinase 1/2 (ERK1/2) along with reduced myocardial reactive oxygen species (ROS) in abn-cbd treated rats. These biochemical responses contributed to the hemodynamic responses and were GPR18-mediated because concurrent treatment with the competitive GPR18 antagonist (O-1918) abrogated the abn-cbd-evoked hemodynamic and biochemical responses. The current findings present new evidence for a salutary cardiovascular role for GPR18, mediated, at least partly, via elevation in the levels of adiponectin.

Carotid sinus nerve electrical stimulation in conscious rats attenuates systemic inflammation via chemoreceptor activation

Recent studies demonstrated a critical functional connection between the autonomic (sympathetic and parasympathetic) nervous and the immune systems. The carotid sinus nerve (CSN) conveys electrical signals from the chemoreceptors of the carotid bifurcation to the central nervous system where the stimuli are processed to activate sympathetic and parasympathetic efferent signals. Here, we reported that chemoreflex activation via electrical CSN stimulation, in conscious rats, controls the innate immune response to lipopolysaccharide attenuating the plasma levels of inflammatory cytokines such as tumor necrosis factor (TNF), interleukin 1β (IL-1β) and interleukin 6 (IL-6). By contrast, the chemoreflex stimulation increases the plasma levels of anti-inflammatory cytokine interleukin 10 (IL-10). This chemoreflex anti-inflammatory network was abrogated by carotid chemoreceptor denervation and by pharmacological blockade of either sympathetic - propranolol - or parasympathetic - methylatropine – signals. The chemoreflex stimulation as well as the surgical and pharmacological procedures were confirmed by real-time recording of hemodynamic parameters [pulsatile arterial pressure (PAP) and heart rate (HR)]. These results reveal, in conscious animals, a novel mechanism of neuromodulation mediated by the carotid chemoreceptors and involving both the sympathetic and parasympathetic systems.

Baroreflexes of the rat. VI. Sleep and responses to aortic nerve stimulation in the dmNTS

The sensitivity of the baroreflex determines its stability and effectiveness in controlling blood pressure (BP). Sleep and arousal are reported to affect baroreflex sensitivity, but the findings are not consistent across studies. After statistically correcting the effect of sleep on the baselines in chronically neuromuscular-blocked (NMB) rats, we found that sleep affects BP and heart period (HP) baroreflex gain similarly. This finding is consistent with baroreflex modulation of HP and BP before divergence of the sympathetic and parasympathetic pathways. Therefore, we hypothesized that the gain modulation occurs in the dorsal medial nucleus of the solitary tract (dmNTS). The present study used long-term dmNTS recordings in NMB rats and single-pulse aortic depressor nerve stimulation. Under these conditions, the magnitude of A-fiber evoked responses (ERs), recorded from second- or higher-order dmNTS baroreflex neurons, was reliably augmented during high-amplitude low-frequency EEG activity (slow-wave sleep) and reduced during low-amplitude high-frequency EEG activity (arousal; DeltaER = 11%, t = 9.49, P < 0.001, degrees of freedom = 1,016). This result has methodological implications for techniques that use changes in HP to estimate baroreflex BP gain and general implications for understanding the relationship between sleep and cardiovascular control.

Autonomic neuropathy precedes cardiovascular dysfunction in rats with diabetes

BACKGROUND

Our team previously demonstrated arterial stiffening and cardiac hypertrophy in type 2 diabetic rats at 8 but not 4 weeks after being administered streptozotocin (STZ) and nicotinamide (NA). The present study focused on investigating the effects of type 2 diabetes on cardiac autonomic nerve function in the STZ- and NA-treated animals, using modern spectral estimation technique.

DESIGN

An autoregressive process was performed to each detrended signal of heart rate and systolic blood pressure measured in the 4- and 8-week STZ-NA rats with anaesthesia. The power of low-frequency and high-frequency oscillations was automatically quantified with each spectral peak by computing the residuals. The closed-loop baroreflex gain was estimated using the square root of the ratio between heart rate and systolic blood pressure powers in the low-frequency band.

RESULTS

Compared with the age-matched controls, both the 4- and 8-week STZ-NA diabetic rats had significantly decreased low-frequency oscillations of heart rate but not systolic blood pressure variability, showing a decline in baroreflex gain (0.451 +/- 0.060 and 0.484 +/- 0.056 vs. 1.196 +/- 0.064 ms mmHg(-1), P < 0.05). On the other hand, the low frequency-high frequency power ratio of the heart period was also diminished in the two diabetic groups, indicating a shift in sympatho-vagal balance of the heart control (0.472 +/- 0.109 and 0.504 +/- 0.090 vs. 1.857 +/- 0.336, P < 0.05).

CONCLUSIONS

The cardiac autonomic dysfunction in the absence of any significant changes in vascular dynamics, 4 but not 8 weeks after induction of type 2 diabetes, suggests that the diabetic autonomic neuropathy may precede arterial stiffening and cardiac hypertrophy in the STZ- and NA-treated rats.

Muscle mechanoreflex augments arterial baroreflex-mediated dynamic sympathetic response to carotid sinus pressure

Although the muscle mechanoreflex is one of the pressor reflexes during exercise, its interaction with dynamic characteristics of the arterial baroreflex remains to be quantitatively analyzed. In anesthetized, vagotomized, and aortic-denervated rabbits ( n = 7), we randomly perturbed isolated carotid sinus pressure (CSP) using binary white noise while recording renal sympathetic nerve activity (SNA) and arterial pressure (AP). We estimated the transfer functions of the baroreflex neural arc (CSP to SNA) and peripheral arc (SNA to AP) under conditions of control and muscle stretch of the hindlimb (5 kg of tension). The muscle stretch increased the dynamic gain of the neural arc while maintaining the derivative characteristics [gain at 0.01 Hz: 1.0 ± 0.2 vs. 1.4 ± 0.6 arbitrary units (au)/mmHg, gain at 1 Hz: 1.7 ± 0.6 vs. 2.7 ± 1.4 au/mmHg; P < 0.05, control vs. stretch]. In contrast, muscle stretch did not affect the peripheral arc. In the time domain, muscle stretch augmented the steady-state response at 50 s (−1.1 ± 0.3 vs. −1.7 ± 0.7 au; P < 0.05, control vs. stretch) and negative peak response (−2.1 ± 0.5 vs. −3.1 ± 1.5 au; P < 0.05, control vs. stretch) in the SNA step response. A simulation experiment using the results indicated that the muscle mechanoreflex would accelerate the closed-loop AP regulation via the arterial baroreflex.

Baroreflexes of the rat. III. Open-loop gain and electroencephalographic arousal

In early studies of humans, baroreflex sensitivity was found to be higher during sleep; however, subsequent observations in several species, including humans, have been at variance with the original reports. Sleep and arousal are behavioral states, and it is difficult to accurately and repeatedly measure baroreflex sensitivity in behaving animals. However, pharmacologically immobilized (neuromuscularly blocked) rats have apparently normal sleep-wakefulness cycles, and baroreflex gain can be measured directly in this preparation. Using the delta band of the EEG (EEGδ) as an index of sleep and arousal and open-loop aortic depressor nerve (ADN) stimulation as a baroreflex input, we found that blood pressure (BP) level depended on arousal ( r = -0.416; P < 0.0001), and BP baroreflex gain depended on BP level ( r = 0.496; P < 0.0001), but that BP baroreflex gain was independent of arousal ( r = 0.001; NS). Heart period (HP) was different; although HP level depended on arousal ( r = 0.352; P < 0.0001), HP baroreflex gain did not depend on HP level ( r = 0.029; NS), and HP baroreflex gain increased with arousal ( r = 0.315; P < 0.0001). A partial-correlations analysis showed that the presence of the relationship between BP level and BP baroreflex gain probably attenuated the relationship between arousal and BP gain. The results are consistent 1) with physiological findings showing that arousal attenuates afferent transmission through the nucleus of the solitary tract and enhances sympathoinhibition at the rostral ventrolateral medulla; and 2) with observations in humans and animals showing increased cardiac baroreflex sensitivity during sleep, but little if any effect of sleep on BP baroreflex sensitivity. The findings are relevant to all methods of baroreflex gain estimation that use HP as the index of baroreflex activation.

Sleep-related changes in baroreflex sensitivity and cardiovascular autonomic modulation

OBJECTIVE

We examined the effects of the various sleep stages on baroreflex sensitivity (BRS), and heart rate and blood pressure (BP) variability, and tested the hypothesis that there is a different behavior of the baroreflex control of the sinus node in response to hypertensive and hypotensive stimuli and in relation to different cycles of the overnight sleep.

DESIGN

Polygraphic sleep recordings were performed in 10 healthy males. The BP and the RR interval were continuously recorded during sleep.

METHODS

BRS was calculated by the sequences method. Autoregressive power spectral analysis was used to investigate the RR-interval and BP variabilities.

RESULTS

During rapid eye movement (REM) sleep BRS significantly increased in response to hypertensive stimuli in comparison with non-rapid eye movement (NREM) sleep and the awake state, whereas it did not change in response to hypotensive stimuli. In the first sleep cycle, BRS significantly increased during NREM in comparison with wakefulness, whereas during REM BRS in response to hypertensive stimuli did not show significant changes as compared with the awake state and/or with NREM. During REM occurring in the sleep cycle before morning awakening, BRS showed a significant increase in response to hypertensive stimuli in comparison with both NREM and the awake state.

CONCLUSIONS

During sleep, arterial baroreflex modulation of the sinus node is different in response to hypotensive and hypertensive stimuli particularly during REM. Furthermore, baroreflex control of the sinus node shows a non-uniform behavior during REM occurring in different nocturnal sleep cycles. These findings suggest that the arterial baroreflex is more effective in buffering the increased sympathetic activation associated with REM at the end of sleep than in the early night.

Linear and nonlinear analysis of heart rate variability during propofol anesthesia for short-duration procedures in children

OBJECTIVE

To determine whether heart rate variability metrics provide an accurate method of monitoring depth of anesthesia, assessing the response to painful stimuli, and assessing neuroautonomic regulation of cardiac activity in children receiving propofol anesthesia for short-duration procedures.

DESIGN

Prospective, case series.

SETTING

Sixteen-bed pediatric intensive care unit, oncology unit, and endoscopy suite in a tertiary care children's hospital and ophthalmology examination rooms in an associated eye institute.

PATIENTS

Thirty-three pediatric patients undergoing propofol anesthesia for short procedures.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Heart rate variability metrics studied included mean, SD, low- and high-frequency power, detrended fluctuation analysis (represented by correlation coefficient, alpha), and approximate entropy. Compared with the initial anesthetized state, we found increased heart rate SD (3.17 +/- 1.31 vs. 7.05 +/- 0.26 bpm, p <.0001), heart rate low-frequency power (3.69 +/- 0.36 vs. 4.48 +/- 0.41 bpm(2)/Hz, p <.0001), heart rate low-/high-frequency ratio (1.47 +/- 0.26 vs. 1.26 +/- 0.24, p =.001), and heart rate alpha (1.12 +/- 0.24 vs. 1.35 +/- 0.21, p <.0001) during painful procedure. Mean heart rate (105.8 +/- 13.4 vs. 101.5 +/- 12.4 bpm, p =.005) and heart rate approximate entropy decreased with painful procedure (0.75 +/- 0.19 vs. 0.53 + 0.16, p <.001), whereas there was no significant change in heart rate high-frequency power (3.04 +/- 0.63 vs. 3.16 +/- 0.71 bpm(2)/Hz, p =.26).

CONCLUSIONS

We conclude that power spectral analysis of heart rate variability may be an accurate and clinically useful measure of depth of propofol anesthesia. We speculate that high-frequency heart rate power during propofol anesthesia correlates with depth of anesthesia, whereas low-frequency power allows for assessment of the patient's sympathetic response to pain.

Analysis of afferent, central, and efferent components of the baroreceptor reflex in mice

Studies of genetically modified mice provide a powerful approach to investigate consequences of altered gene expression in physiological and pathological states. The goal of the present study was to characterize afferent, central, and efferent components of the baroreceptor reflex in anesthetized Webster 4 mice. Baroreflex and baroreceptor afferent functions were characterized by measuring changes in renal sympathetic nerve activity (RSNA) and aortic depressor nerve activity (ADNA) in response to nitroprusside- and phenylephrine-induced changes in arterial pressure. The data were fit to a sigmoidal logistic function curve. Baroreflex diastolic pressure threshold (P(th)), the pressure at 50% inhibition of RSNA (P(mid)), and baroreflex gain (maximum slope) averaged 74 +/- 5 mmHg, 101 +/- 3 mmHg, and 2.30 +/- 0.54%/mmHg, respectively (n = 6). The P(th), P(mid), and gain for the diastolic pressure-ADNA relation (baroreceptor afferents) were similar to that observed for the overall reflex averaging 79 +/- 9 mmHg, 101 +/- 4 mmHg, and 2.92 +/- 0.53%/mmHg, respectively (n = 5). The central nervous system mediation of the baroreflex and the chronotropic responsiveness of the heart to vagal efferent activity were independently assessed by recording responses to electrical stimulation of the left ADN and the peripheral end of the right vagus nerve, respectively. Both ADN and vagal efferent stimulation induced frequency-dependent decreases in heart rate and arterial pressure. The heart rate response to ADN stimulation was nearly abolished in mice anesthetized with pentobarbital sodium (n = 4) compared with mice anesthetized with ketamine-acepromazine (n = 4), whereas the response to vagal efferent stimulation was equivalent under both types of anesthesia. Application of these techniques to studies of genetically manipulated mice can be used to identify molecular mechanisms of baroreflex function and to localize altered function to afferent, central, or efferent sites.

Cholinesterase affects dynamic transduction properties from vagal stimulation to heart rate

Recent investigations in our laboratory using a Gaussian white noise technique showed that the transfer function representing the dynamic properties of transduction from vagus nerve activity to heart rate had characteristics of a first-order low-pass filter. However, the physiological determinants of those characteristics remain to be elucidated. In this study, we stimulated the vagus nerve according to a Gaussian white noise pattern to estimate the transfer function from vagal stimulation to the heart rate response in anesthetized rabbits and examined how changes in acetylcholine kinetics affected the transfer function. We found that although increases in the mean frequency of vagal stimulation from 5 to 10 Hz did not change the characteristics of the transfer function, administration of neostigmine (30 microg . kg-1 . h-1 iv), a cholinesterase inhibitor, increased the dynamic gain from 8.19 +/- 3.66 to 11.7 +/- 4.88 beats . min-1 . Hz-1 (P < 0.05), decreased the corner frequency from 0.12 +/- 0.05 to 0.04 +/- 0.01 Hz (P < 0.01), and increased the lag time from 0.17 +/- 0.12 to 0.27 +/- 0.08 s (P < 0.05). These results suggest that the rate of acetylcholine degradation at the neuroeffector junction, rather than the amount of available acetylcholine, plays a key role in determining the dynamic properties of transduction from vagus nerve activity to heart rate.

Cardiac function in patients with treated hypertension during aortic aneurysm repair

OBJECTIVES

To investigate the impact of arterial hypertension on cardiac function during aortic cross-clamping and declamping.

DESIGN

Prospective study.

SETTING

University hospital.

PARTICIPANTS

Twenty treated hypertensive males with slight left ventricular hypertrophy and 10 normotensive controls undergoing elective repair of an abdominal aortic aneurysm.

INTERVENTIONS

Using transesophageal echocardiography, the mitral inflow profile was evaluated during aortic cross-clamping and declamping.

MEASUREMENTS AND MAIN RESULTS

During the clamping period, the ratio of peak atrial to peak early filling velocity (PA/PE) was significantly higher in the hypertensive patients. One minute after aortic cross-clamping, mean arterial pressure (MAP) and pulmonary artery occlusion pressure significantly increased in the hypertensive patients, whereas they did not change in the normotensive group. Cardiac index and heart rate significantly decreased after cross-clamping, and increased after clamp release in both groups. PA/PE significantly dropped in both groups after aortic declamping, and returned to baseline values thereafter. MAP also decreased significantly in both groups after clamp release, but the fall of MAP tended to be more pronounced in the hypertensive patients.

CONCLUSIONS

In the treated hypertensive patients, more pronounced hemodynamic and echocardiographic responses to aortic cross-clamping probably mirror the altered diastolic left ventricular function in these patients. With respect to intraoperative management, however, the treated hypertensive patients did not react grossly differently from the normotensive controls.

Endothelin-1 produces heterogeneous regional haemodynamic effects in conscious rabbits

Blood flow in the renal artery, superior mesenteric artery and infra-renal abdominal aorta of conscious rabbits was measured by Doppler ultrasound. Arterial pressure, heart rate and blood flow responses were assessed following 0.2 and 0.8 nmol/kg intravenous endothelin-1. The effects of the following antagonists on these responses were examined: phentolamine, propranolol, scopolamine, captopril, nifedipine, indomethacin, the V1-vasopressin receptor antagonist d(CH2)5Tyr(Me)AVP and the competitive nitric oxide (NO) synthase inhibitor NG-nitro L-arginine (NOLA). Hindlimb resistance and arterial pressure responded in two phases, initial vasodilatation followed by vasoconstriction. Renal and mesenteric vasoconstriction occurred without initial vasodilatation. Following 0.2 nmol/kg endothelin-1, arterial pressure decreased by 18.5 +/- 0.8 mmHg, then increased by 25.2 +/- 1.7 mmHg (n = 27). Heart rate changed reciprocally. Renal resistance increased by 533 +/- 73% (n = 12). Mesenteric resistance increased by 420 +/- 34%. Hindlimb resistance decreased 54 +/- 2% (n = 12, all P < 0.01) then increased slightly (P < 0.05). All changes were greater at 0.8 nmol/kg, particularly the hindlimb vasoconstriction. The only antagonist to alter significantly these responses was NOLA, which in the hindlimb attenuated the vasodilatation and accentuated the vasoconstriction. We conclude that most of the haemodynamic effects of endothelin-1 are direct, but that NO generated by NO synthase causes part of the hindlimb vasodilatation, and that endothelin-1-induced vasoconstriction is attenuated by release of NO.

Direct evidence that the hydroxyl radical plays a pathogenetic role in myocardial "stunning" in the conscious dog and demonstration that stunning can be markedly attenuated without subsequent adverse effects

Recent studies suggest that the hydroxyl radical (.OH) plays a pathogenetic role in postischemic ventricular dysfunction (myocardial "stunning"). This concept, however, is predicated exclusively on results obtained in anesthetized open-chest preparations, which are subject to the confounding influence of many unphysiological conditions and in which both myocardial stunning and free radical generation are greatly exaggerated. The lack of supporting evidence in more physiological animal models represents a major limitation of the .OH hypothesis of stunning. Furthermore, concern has been raised that myocardial stunning may be a period of "rest" necessary for full recovery, so that attenuation of the early phase of stunning by antioxidant therapy may have subsequent detrimental effects on the resting function and/or on the return of myocardial contractile reserve. To address these issues, in phase 1 of this study conscious unsedated dogs undergoing a 15-minute coronary artery occlusion received an intravenous infusion of normal saline (n = 22), of the .OH scavenger N-2-mercaptopropionyl glycine (MPG, n = 17), or of the iron chelator desferrioxamine (DF, n = 14). Compared with control dogs, the dogs treated with MPG or DF exhibited significantly greater postischemic wall thickening throughout the first 6 hours of reperfusion; the total deficit of wall thickening during this time interval was reduced 50% by MPG and 50% by DF. The magnitude of this beneficial effect was a function of the severity of ischemia, so that the dogs with the lowest collateral flows had the greatest improvement of wall thickening. The accelerated recovery produced by MPG and DF in the first 6 hours was not followed by any deterioration of resting wall thickening at 24 or 48 hours. Furthermore, in dogs treated with MPG or DF, the increase in wall thickening elicited by maximal inotropic stimulation (isoproterenol or dopamine) was similar before stunning and shortly after resting wall thickening had normalized (24 or 48 hours after reflow); thus, despite the fact that most of the early postischemic dysfunction had been eliminated by antioxidant therapy, there was no subsequent impairment of either resting function or contractile reserve. In phase 2, production of free radicals (measured with the spin trap alpha-phenyl N-tert-butyl nitrone) was markedly (> 80%) inhibited by the same doses of MPG and DF that attenuated stunning in phase 1.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of pentobarbital on inotropic state of isolated canine left ventricle

Although pentobarbital has been found to depress myocardial function, the magnitude of its direct effects on ventricular contraction at anesthetic concentrations has not been well quantified. The direct effects of pentobarbital on left ventricular function were measured by employing an isolated canine heart preparation with a blood oxygenator. Seven hearts were perfused with blood, dextran, and perfluorochemical artificial blood. Ventricular function was evaluated using the slope of the end-systolic pressure-volume relationship (Ees) and the maximal rate of pressure development (dP/dtmax) in ventricles contracting isovolumically in control, after a low dose (13 micrograms/ml), and after a high dose (48 micrograms/ml) of pentobarbital. These concentrations represent one-half and two times the typical value (25 micrograms/ml) found to produce anesthesia in canines (assessed by tail clamp or blink reflex). The low dose of pentobarbital did not produce clear-cut depression in contractile function. The high dose of pentobarbital produced significant reductions of Ees, and dP/dtmax: Ees decreased 29%, from a control of 4.30 +/- 0.84 to 3.05 +/- 0.49 mmHg/ml and dP/dtmax decreased 24%, from a control of 909 +/- 148 to 695 +/- 173 mmHg/s. Thus, the threshold for the direct depressant effect of pentobarbital on ventricular function falls within the range of half to double the typically-reported anesthetic concentrations.

Demonstration of free radical generation in the "stunned" myocardium in the conscious dog and identification of major differences between conscious and open-chest dogs

Conscious dogs undergoing a 15-min coronary occlusion were given alpha-phenyl N-tert-butyl nitrone (PBN) and the local coronary venous plasma was analyzed by electron paramagnetic resonance spectroscopy. A prolonged myocardial release of PBN radical adducts was observed, which exhibited a burst in the initial minutes of reflow (peaking at 3 min) and then abated but continued for 1-3 h after reperfusion. Computer simulation revealed the presence of at least two PBN adducts (aN = 15.2 G and a beta H = 6.0 G; aN = 14.6 G and a beta H = 3.0 G), both consistent with the trapping of secondary carbon-centered radicals. No appreciable PBN adduct production was observed when collateral flow exceeded 30-40% of nonischemic flow, indicating that a flow reduction of at least 60% is necessary to trigger free radical reactions. There was a direct relationship between the magnitude of PBN adduct production and the severity of contractile dysfunction (r = 0.77), suggesting that the radicals generated upon reperfusion play a causal role in the subsequent stunning. The total release of PBN adducts after 3 h of reperfusion following a 15-min coronary occlusion was found to be approximately five times greater in open-chest compared with conscious dogs; at the same time, the recovery of wall thickening was markedly less in open-chest dogs. This study represents the first application of spin trapping to a conscious animal model of myocardial ischemia. The results demonstrate (a) that free radicals are generated in the stunned myocardium in the absence of the artificial or abnormal conditions associated with previously used models (isolated hearts, open-chest preparations), and (b) that both the severity of postischemic dysfunction and the magnitude of the attendant free radical production are greatly exaggerated in the open-chest dog, implying that previous conclusions derived from this model may not be applicable to conscious animals or to humans. This investigation also provides a method to measure free radicals in awake animals.

Transient coronary stenosis associated with arrhythmia and ischemia 24 hours after reperfusion

We examined the electrophysiologic sequelae of transient reductions in coronary blood flow (CBF) in conscious dogs. Animals were instrumented to measure arterial pressure, heart rate, repetitive extrasystole threshold (RET), and CBF before coronary stenosis, during 90 minutes of a 50% reduction in CBF, and at 1, 24, 48, and 72 hours after reperfusion. RET was decreased at 24 and 48 hours but returned to baseline by 72 hours after reperfusion. Frequent ventricular ectopic activity (VEA), absent during the control period, was noted during stenosis and at 1, 24, and 48 hours after reperfusion. beta-Adrenergic blockade (metoprolol) normalized the decreased RET at 24 hours. Addition of alpha 1-adrenergic blockade (prazosin) abolished VEA. Endocardial blood flow in the posterior papillary muscle of the left ventricle was reduced by 52 +/- 9% during stenosis, was similar to baseline levels by 1 hour after reperfusion, and decreased by 39 +/- 5% at 24 hours. We conclude that episodes of modest coronary artery stenosis may be followed by electrophysiologic changes indicative of increased vulnerability to malignant ventricular arrhythmias.

Effects of three anesthetic induction techniques on heart rate variability

STUDY OBJECTIVE

To investigate the effects of different clinical induction techniques on heart rate variability (HRV).

DESIGN

Two studies are reported. Study 1 prospectively compared the effects of two induction techniques (etomidate vs. thiopental sodium) known to have widely disparate effects on cardiovascular reflexes. Study 2 specifically investigated whether the vagotonic effects of sufentanil cause an increase in vagally mediated HRV.

SETTING

Elective surgery in a university-affiliated hospital.

PATIENTS

Study 1: 18 ASA physical status I patients having minor surgery; Study 2: 10 ASA physical status III and IV patients having cardiac surgery.

INTERVENTIONS

In Study 1, anesthesia was induced with either etomidate 0.3 mg/kg or thiopental sodium 4 mg/kg with 60% nitrous oxide in oxygen. In Study 2, anesthesia was induced with a sufentanil infusion (total dose 2.9 +/- 0.2 micrograms/kg).

MEASUREMENTS AND MAIN RESULTS

The electrocardiogram-derived heart rate signal was subjected to power spectral analysis (similar to electroencephalographic analysis) to obtain measurements of (1) absolute HRV power [units of (beats per minute)2] within defined frequency ranges (HRVLO = power between 0 and 0.125 Hz; HRVHI = power between 0.126 and 0.5 Hz; HRVTOT = HRVLO + HRVHI) and (2) normalized HRV power (the percentage of total power) within these same frequency ranges [e.g., %HRVHI = (HRVHI/HRVTOT) x 100%]. In Study 1, both techniques caused large reductions in HRVTOT. The reduction caused by the thiopental sodium technique (-89% +/- 2%) significantly exceeded that caused by the etomidate technique (-58% +/- 13%, p less than 0.02). In Study 2, sufentanil decreased absolute power measurements of vagally mediated HRV (-69 +/- 12 change in HRVHI) but increased corresponding normalized measurements of vagally mediated HRV (90% +/- 30% increase in %HRVHI).

CONCLUSIONS

In Study 1, the greater reduction in HRV with the thiopental sodium technique provides evidence that the depressant effects of anesthetics on HRV are related in part to their effects on cardiovascular reflexes. However, the significant depression in HRV caused by the etomidate technique suggests that mechanisms other than baroreflex depression (e.g., impaired consciousness) also are important in these depressant effects. In Study 2, the decrease in HRVHI caused by sufentanil documents that absolute power measurements of vagally mediated HRV are not correlated with changes in parasympathetic tone during a potent opioid induction. This lack of a correlation may result from the decrease in total HRV observed with loss of consciousness. The increase in %HRVHI suggests that normalized measurements of HRV may still provide an index of changes in sympathetic-parasympathetic balance, even when total HRV is decreased following anesthetic administration.

Intracoronary veratrine attenuates carotid baroreceptor reflex regulation of blood pressure in conscious dogs

1. The effect of activation of left ventricular cardiac receptors on carotid baroreflex control of blood pressure, heart rate, cardiac output, and total peripheral resistance was determined in conscious dogs. Previous studies in conscious subjects assessed only the effect on baroreflex control of heart rate. 2. Dogs with denervated aortic baroreceptors were equipped with aortic flow probes, cardiac pacing electrodes, and catheters in the aorta, vena cava, and left circumflex coronary artery. Both carotid sinus regions were prepared for reversible vascular isolation. 3. Left ventricular receptors were stimulated by an infusion of veratrine (0.1-1.0 micrograms kg-1 min-1) into the left circumflex coronary artery. 4. Veratrine infusion decreased control blood pressure only 10 +/- 2 mmHg, but it decreased the range of baroreflex control of blood pressure by 50% and decreased maximum baroreflex gain by 42%. Both the cardiac output and total peripheral resistance components of the baroreflex were attenuated. 5. Baroreflex control of blood pressure was unaffected by intravenous veratrine or by intracoronary infusion of vehicle. 6. Intracoronary veratrine had no effect after autonomic ganglionic blockade. 7. When cardiac output was kept nearly constant (by beta-adrenergic and cholinergic receptor blockade or by beta-blockade and cardiac pacing), intracoronary veratrine still attenuated baroreflex control of blood pressure and total peripheral resistance. Veratrine impaired the ability of the baroreflex to utilize alpha-adrenergic mechanisms to control total peripheral resistance. 8. We conclude that activation of ventricular receptors attenuates baroreflex regulation of blood pressure in conscious dogs through an attenuation of baroreflex control of both cardiac output and total peripheral resistance.

The effect of pentobarbital anesthesia on the autonomic nervous system control of heart rate during baroreceptor activation

This study determined the influence of pentobarbital anesthesia on the autonomic nervous system control of baroreceptor mediated reflex bradycardia in the rat. Reflex bradycardia was elicited by phenylephrine-induced hypertension in conscious and pentobarbital anesthetized (PA) rats before and after sympathetic blockade with the beta-1 receptor antagonist atenolol or parasympathetic blockade with the peripherally acting muscarinic receptor antagonist methyl-atropine. Reflex bradycardia was significantly decreased by pentobarbital anesthesia. Cardiosympathetic blockade produced equivalent relative decreases in baroreflex gain in conscious and PA rats. In contrast, parasympathetic nervous system blockade with methyl-atropine produced relatively less inhibition of baroreflex gain in the PA rat compared with the conscious rat. These results suggest that pentobarbital anesthesia decreases baroreflex gain by inhibiting vagally mediated reflex bradycardia.

Postischemic myocardial "stunning". Identification of major differences between the open-chest and the conscious dog and evaluation of the oxygen radical hypothesis in the conscious dog

Recent studies suggest that oxygen-derived free radicals contribute to the pathogenesis of postischemic myocardial dysfunction (myocardial "stunning"). This concept, however, is predicated exclusively on results obtained in open-chest preparations, which are subject to the confounding influence of many unphysiological conditions. The lack of supporting evidence in more physiological animal models represents a major persisting limitation of the oxy-radical hypothesis of myocardial stunning. The goal of this study was to address two fundamental (and related) questions: 1) Does the open-chest animal model alter the phenomenon of myocardial stunning? 2) If so, how valid are the concepts, derived from such a model, regarding the pathogenetic role of oxy-radicals? In part 1 of the study, myocardial stunning after a 15-minute coronary occlusion was compared in 30 pentobarbital-anesthetized open-chest dogs and in 19 conscious dogs. For any given level of collateral flow during occlusion, the recovery of systolic wall thickening after reperfusion was markedly less in open-chest animals. In an additional group of five open-chest dogs, a close inverse relation was noted between body temperature and postischemic wall thickening, indicating that the recovery of the stunned myocardium in acute experiments may vary markedly depending on how temperature is controlled. Because of these major differences between open-chest and conscious dogs, the oxy-radical hypothesis needs to be tested in the latter model. Thus, in part 2 of the study, conscious unsedated dogs undergoing a 15-minute coronary occlusion were randomized to an intravenous infusion of either saline (19 coronary occlusions) or superoxide dismutase (SOD) plus catalase (CAT) (21 coronary occlusions). Despite the fact that the plasma levels of SOD and CAT declined rapidly after reperfusion, postischemic wall thickening was significantly greater in treated compared with control dogs throughout the first 6 hours of reflow. Thus, a brief (60-minute) infusion of SOD and CAT produced a sustained improvement of recovery of contractility. The magnitude of this beneficial effect was a function of the severity of ischemia: the lower the collateral perfusion, the greater the improvement effected by the enzymes. The accelerated recovery produced by SOD and CAT was not followed by any deterioration of contractility, suggesting that postischemic dysfunction is not a teleologically "protective" phenomenon. In conclusion, the severity of myocardial stunning is greatly exaggerated by the unphysiological conditions present in the barbiturate-anesthetized open-chest dog.(ABSTRACT TRUNCATED AT 400 WORDS)

Decreases in repetitive extrasystole threshold in the conscious pig with myocardial infarct were reversed by tyrosine

Reports indicate that the administration of tyrosine, the precursor amino acid for catecholaminergic neurotransmitters, may be beneficial under conditions of physiologic stress. We studied the effects of tyrosine on vulnerability to ventricular arrhythmia in conscious pigs with healing myocardial infarcts, and sham operated (intact) pigs. Mean arterial pressure and heart rate were measured via chronically implanted aortic catheters. The repetitive extrasystole threshold (defined as the energy in milliamperes (ma) needed to cause a spontaneous ventricular beat following a premature beat induced by an electrical impulse), was measured via a bipolar pacing catheter placed during instrumentation surgery in the apex of the right ventricle. One week after infarct, the myocardial infarct group was studied before and ninety minutes after the administration of tyrosine (8 mg/kg iv). Before tyrosine, the myocardial infarct group had a significantly lower repetitive extrasystole threshold (12 +/- 1 ma) compared to the intact group (19 +/- 2 ma). Ninety minutes after tyrosine, the repetitive extrasystole threshold in the myocardial infarct group was 17 +/- 1 ma. The availability of tyrosine did not alter the repetitive extrasystole threshold in the intact group. Thus, vulnerability to ventricular arrhythmia was enhanced in pigs with recent myocardial infarction. Tyrosine, which can be nutritionally manipulated, may reduce myocardial vulnerability to arrhythmia after infarct.

Autoimmune preganglionic sympathectomy induced by acetylcholinesterase antibodies

Systemic injection of monoclonal antibodies to neural acetylcholinesterase in adult rats caused a syndrome with permanent, complement-mediated destruction of presynaptic fibers in sympathetic ganglia and adrenal medulla. Ptosis, hypotension, bradycardia, and postural syncope ensued. In sympathetic ganglia, acetylcholinesterase activity disappeared from neuropil but not from nerve cell bodies. Choline acetyltransferase activity and ultrastructurally defined synapses were also lost. Electrical stimulation of presynaptic fibers to the superior cervical ganglion ceased to evoke end-organ responses. On the other hand, direct ganglionic stimulation remained effective, and the postganglionic adrenergic system appeared intact. Motor performance and the choline acetyltransferase content of skeletal muscle were preserved, as was parasympathetic (vagal) function. This model of selective cholinergic autoimmunity represents another tool for autonomic physiology and may be relevant to the pathogenesis of human dysautonomias.

Left ventricular effects of nicorandil in comparison with nitroglycerin in chronic conscious dogs

Nicorandil is a new coronary vasodilator possessing beneficial properties. However, its detailed cardiovascular effects, especially on left ventricular (LV) preload, have not yet been fully elucidated. The purpose of this study was to assess the effects of nicorandil on LV hemodynamics in conscious dogs and to examine the mechanisms of its antiischemic action. Nine mongrel dogs were instrumented for instantaneous and continuous measurements of LV diameters, and aortic and LV pressures. The effects of nicorandil (0.2 mg/kg, intravenously) were compared with those of nitroglycerin (15 micrograms/kg, intravenously) in conscious dogs. Mean aortic pressure decreased similarly with both nicorandil and nitroglycerin (-20.1 +/- 3.1% vs. 21.6 +/- 2.8%, ns). Heart rate was elevated with both drugs. Both nicorandil and nitroglycerin significantly decreased LV systolic pressure to the same extent (-11.3 +/- 0.5% vs. -10.5 +/- 11.6%, ns). LV max dp/dt was not significantly changed by either drug. Although both nicorandil and nitroglycerin significantly increased fractional shortening, nicorandil had a greater effect on fractional shortening than nitroglycerin (20.0 +/- 3.0% vs. 10.2 +/- 2.3%, p less than 0.05). In this study, nicorandil, administered intravenously, had a salutary effect on cardiac function. LV end-diastolic diameter decreased with nicorandil and nitroglycerin (-6.5 +/- 1.5% vs. -12.6 +/- 2.6%, p less than 0.01), respectively. In conclusion, nicorandil decreased LV end-diastolic diameter in conscious dogs, indicating a decrease in venous return and preload of the heart. In addition, nicorandil decreased LV afterload to the same extent as nitroglycerin. The decrease in preload and afterload on the heart is thought to be one of the mechanisms of the antiischemic action of nicorandil.

Selective dysfunction of the vagal component of the baroreflex following cerebral ischemia: protection by ifenprodil and flunarizine

Baroreflex sensitivity assessed from the phenylephrine-induced reflex bradycardia was significantly decreased following 5 min global incomplete cerebral ischemia in pentobarbitalized dogs. Although bilateral vagotomy in the cervical region decreased baroreflex sensitivity by about 50% in sham-operated animals, it hardly affected the baroreflex in animals subjected to ischemia. The extent of the decrease in the influence of vagotomy on the baroreflex was dependent on the severity of ischemia in the dorsal medulla oblongata. In animals vagotomized before ischemia, no significant decrease in baroreflex sensitivity was observed following ischemia. Pretreatment with ifenprodil or flunarizine, 1 mg/kg i.v., 5 min prior to ischemia prevented the post-ischemic decrease in baroreflex sensitivity. Vagotomy decreased baroreflex sensitivity during the reperfusion period in these treated animals. These results suggest that the post-ischemic attenuation of reflex bradycardia may be due to a selective dysfunction of the vagal component of baroreflex, which can be prevented by the cerebroprotective agents.

Inducible monomorphic sustained ventricular tachycardia in the conscious pig

Sustained monomorphic ventricular tachycardia (VT) is of clinical importance but has not been readily modeled in conscious animals. Eleven pigs had myocardial infarction induced by pulling snares previously placed around the left anterior descending (LAD) coronary artery. Six days after occlusion, bipolar pacing catheters were inserted in the right ventricular apex for induction of VT. Testing was repeated in conscious pigs on 6 out of 8 to 19 days after infarction. Monomorphic VT was induced in each animal during each session, using three to four extrastimuli. VT was terminated by burst pacing in 74% of trials; average VT rate was 362 +/- 26 beats/min. VT was prevented in four of eight animals by procainamide and in five of eight animals by magnesium, but was not prevented by lidocaine or metoprolol. The model may be useful in the study of potentially malignant ventricular tachyarrhythmias, important prodromes for sudden death.

Oxygen consumption and coronary reactivity in postischemic myocardium

Coronary vascular responses in regions of reversible postischemic myocardial contractile dysfunction (stunned myocardium) were examined in chronically instrumented, awake dogs. Left anterior descending coronary artery blood flow and oxygen extraction, aortic and left ventricular pressures, and regional myocardial segment shortening were determined. Regional myocardial blood flow was measured with microspheres. Coronary reactive hyperemia and vasodilator reserve, and regional myocardial oxygen consumption were determined. Three sequential 10-minute left anterior descending coronary artery occlusions separated by 30-minute reperfusion periods resulted in progressive postischemic dysfunction so that 1 hour after the final coronary artery occlusion, myocardial segment shortening was reduced to 37% of baseline. Despite this decrease in contractile function, left anterior descending artery flow (19.6 +/- 2.6 vs. 18.4 +/- 3.0 ml/min), myocardial blood flow and the transmural distribution of flow measured with microspheres, and regional myocardial oxygen consumption were unchanged. Although the coronary vasodilator reserve in response to adenosine was unaltered (63 +/- 9 vs. 70 +/- 15 ml/min), the reactive hyperemia response to a 10-second coronary occlusion was decreased in intensity (debt repayment ratio = 474 +/- 78% vs. 322 +/- 74%; p less than 0.05) and duration (57 +/- 9.1 vs. 35 +/- 4.5 seconds; p less than 0.05), while the peak flow response was unchanged (57 +/- 6.8 vs. 60 +/- 7.1 ml/min). Thus, in the intact awake animal postischemic myocardial contractile dysfunction was not associated with decreased myocardial oxygen consumption and did not impair the normal relation between coronary blood flow and myocardial oxygen utilization. Although coronary vessels showed a normal ability to vasodilate in response to adenosine, coronary reactive hyperemia was reduced.

Direct observations of the effects of baroreceptor stimulation on skeletal muscle circulation of the rat

1. In anaesthetized rats, supramaximal baroreceptor stimulation by carotid sinus inflation evoked a reflex fall in arterial pressure and an increase in vascular conductance and flow of muscles of the hindquarters. 2. Simultaneously, main arteries, primary and secondary arterioles (13-90 microns, internal diameter (i.d.)) and terminal arterioles (7-13 microns) of the spinotrapezius muscle all showed a diameter increase that reached a peak as arterial pressure neared its zenith; terminal arterioles then showed a diameter decrease to below control level. These responses were abolished by local application of phentolamine or guanethidine to the spinotrapezius, or by crushing the paravascular nerve supply. 3. It is suggested that the diameter increases were mediated by reflex inhibition of sympathetic tone, while the secondary diameter decrease in terminal arterioles was induced by a fall in local concentrations of vasodilator metabolites, caused by an increase in muscle blood flow. 4. But after sympathetic blockade 25% of all arterial vessels showed a diameter increase beginning as the fall in arterial pressure neared its zenith. These responses may be attributed to vasodilator metabolites accumulating as a consequence of a reduction in muscle blood flow secondary to the reflex reduction in perfusion pressure. 5. No venous vessels, from venules (9-18 microns) to main veins (65-130 microns) that drain the muscle, showed a diameter change in response to baroreceptor stimulation, in accord with evidence that they have no sympathetic supply. 6. These results accord with and can provide explanation for changes in blood flow, regional blood volume and capillary filtration evoked by baroreceptor stimulation in studies on whole-limb muscle. They support suggestions that active changes in capacity of venous vessels of muscle play a minor role in the baroreceptor reflex; both active and passive changes in vascular capacity may be due to large veins outside of muscle proper.

Direct observations of effects of baroreceptor stimulation on mesenteric circulation of the rat

1. In anaesthetized rats, supramaximal baroreceptor stimulation by carotid sinus inflation evoked a reflex fall in systemic arterial pressure and an increase in gross mesenteric vascular conductance, but no significant change in gross mesenteric blood flow. 2. Principal arteries (80-350 microns internal diameter (i.d.)) which supply the intestine and mesentery, small arteries (30-40 microns) and some terminal arterioles (18-30 microns) of the mesentery showed a diameter increase beginning with the fall in arterial pressure, but remaining terminal arterioles and precapillary arterioles (10-18 microns) showed a diameter increase beginning when arterial pressure neared its lowest level. 3. Application of phentolamine to the mesentery abolished diameter increases that began with the reflex fall in arterial pressure. Thereafter, 50% of small arteries and terminal and precapillary arterioles showed diameter increases that began when the fall in arterial pressure neared its zenith. 4. It is proposed that the proximal arterial vessels, which are known to be sympathetically innervated, predominantly showed dilatation mediated by reflex inhibition of sympathetic tone, while more distal arterioles showed myogenic dilatation secondary to the fall in systemic arterial pressure. 5. Small veins (30-50 microns) of the mesentery and principal veins (100-560 microns) that drain mesentery and intestine also showed a diameter increase beginning with the reflex fall in systemic arterial pressure. Since they are known to have a sympathetic noradrenergic supply, and in the absence of changes in mesenteric blood flow likely to cause passive changes in venous diameter, they apparently showed dilatation due to inhibition of sympathetic tone. 6. The above responses are all compatible with, and so suggest underlying mechanisms for, changes in blood flow, regional blood volume, and capillary filtration evoked by baroreceptor stimulation in studies of whole mesenteric circulation, i.e. intestine as well as mesentery.

Effects of atrial natriuretic peptide on coronary vascular resistance in the intact awake dog

Atrial natriuretic peptide has been reported to cause vasoconstriction, vasodilation or no change of coronary vascular resistance in isolated perfused hearts or in open chest animal models. Because general anesthesia and acute surgical trauma may perturb baseline coronary hemodynamics and alter responses to experimental interventions, this study examined the effects of human atrial natriuretic peptide (arginine-102-tyrosine-126) and rat atriopeptin II (serine-103-arginine-125) on the coronary circulation of unsedated, awake dogs. Studies were performed in 12 chronically instrumented animals in which a surgically implanted electromagnetic flow probe and intracoronary catheter allowed measurement of left circumflex coronary blood flow during intraarterial administration of the atrial natriuretic peptides. Bolus doses of both human atrial natriuretic peptide and rat atriopeptin II produced dose-dependent coronary vasodilation; the threshold for coronary vasodilation was 0.2 micrograms/kg body weight for both agents. Coronary vasodilation produced by human atrial natriuretic peptide was not antagonized by adenosine receptor blockade or by cyclooxygenase inhibition with indomethacin. Thus, atrial natriuretic peptides produced dose-dependent coronary vasodilation in intact awake dogs that was not dependent on adenosine-mediated or prostaglandin-mediated mechanisms.

Enhanced responsiveness to carotid baroreceptor unloading in conscious dogs during development of perinephritic hypertension

The effects of unloading the carotid sinus baroreceptors before and during the development of perinephritic hypertension were studied in conscious dogs instrumented with aortic catheters to measure arterial pressure and heart rate, and electromagnetic flow probes to measure cardiac output and calculate total peripheral resistance. Prior to hypertension, bilateral carotid occlusion (BCO) increased mean arterial pressure by 38 +/- 2 from 101 +/- 2 mm Hg and total peripheral resistance by 19 +/- 2 from 46 +/- 3 mm Hg/l/min, while cardiac output and heart rate did not change from 2,299 +/- 128 ml/min and 84 +/- 4 beats/min, respectively. At 2 weeks after renal wrapping, there were significant increases in baseline mean arterial pressure, cardiac output, and total peripheral resistance and decreases in heart rate; BCO increased mean arterial pressure by 59 +/- 5 from 130 +/- 4 mm Hg, heart rate by 36 +/- 5 beats/min from 69 +/- 3 beats/min, and cardiac output by 458 +/- 103 from 2,711 +/- 239 ml/min. By 4 weeks after renal wrapping, heart rate and mean arterial pressure responses to BCO were approaching baseline levels. After beta-adrenergic receptor blockade, responses to BCO of mean arterial pressure, cardiac output, and heart rate were no longer significantly enhanced during the development of hypertension. Thus, in conscious dogs, reflex pressor responses to baroreceptor unloading via BCO were enhanced during the development of hypertension but no longer present 3 weeks later. The augmented mean arterial pressor responses to BCO were mediated by increases in cardiac output and heart rate, which in turn, appeared to be controlled by beta-adrenergic receptor mechanisms.

Autonomic nervous system control of heart rate during baroreceptor activation in conscious and anesthetized rats

Heart rate and blood pressure were recorded in conscious, freely behaving rats through a catheter in the tail artery during administration of nitroprusside or phenylephrine through a catheter in the jugular vein. The sympathetic and parasympathetic components were distinguished by treating the rats with atenolol or methyl-atropine. Reflex bradycardia induced by all doses of phenylephrine was almost totally blocked blocked following methyl-atropine treatment. Reflex tachycardia induced by small to moderate doses of nitroprusside was attenuated to an equal extent following atropine or atenolol treatment. A similar experimental schedule was followed with a separate group of rats to determine the effects of pentobarbital and urethane anesthesia on the baroreceptor reflex. Both pentobarbital and urethane equally attenuated the tachycardia response to a decrease in blood pressure. However, pentobarbital anesthesia resulted in a greater attenuation of the bradycardia response to an increase in blood pressure than did urethane anesthesia. These data support the conclusion that the parasympathetic nervous system is primarily responsible for baroreceptor reflex-induced bradycardia in conscious rats. The sympathetic and parasympathetic systems contribute equally to control baroreceptor reflex-induced tachycardia except in extreme acute hypotension when the tachycardia is predominantly due to the activation of sympathetic nerves. The findings of the second experiment indicate that pentobarbital and urethane affect sympathetic systems differently.

Microcirculatory responses to carotid sinus nerve stimulation at various ambient O2 tension in the rabbit tenuissimus muscle

To quantify the integrated effects of local and central control mechanisms through tissue metabolites and the autonomic nervous system on the peripheral vascular beds, microcirculatory responses to the carotid sinus nerve stimulation at various levels of ambient oxygen tension (PO2) were measured in the rabbit tenuissimus muscle suffused with oxygenated Tyrode solution, using a microscope-TV system. The statistical analysis of the experimental data exhibited that both capillary red cell velocity and perfused capillary density at the control state were significantly decreased as PO2 was elevated (P less than 0.01) and that the stimulation also significantly augmented their values (P less than 0.01) except for the peak velocity data. Regression analysis indicated that both the velocity and density responses to PO2 changes during stimulation were less sensitive than those at the control state. For instance, the vasodilating effect of stimulation on density at PO2 20 mm Hg was enhanced by about four-fold at 80 mm Hg, although the effect on velocity was increased only by 16% with the same PO2 change. From these results, it was concluded that the microcirculatory changes due to the arteriolar smooth muscle contraction evoked by unit sympathetic discharge was significantly influenced by the ambient PO2 level. Such synergistic interaction of the local and central control mechanisms like a series-coupled gain control system was suspected to play an important role in the overall regulation of the microcirculation.

Cardiovascular effects and plasma levels of denopamine (TA-064), a new positive inotropic agent, in chronically instrumented dogs

Cardiovascular effects of denopamine (TA-064), a new positive inotropic agent, in chronically instrumented dogs were investigated following intravenous and oral administration. In the conscious state, denopamine (0.5-4 micrograms/kg/min, i.v. infusion) increased LV dp/dtmax, cardiac output, stroke volume in a dose-dependent manner, and it decreased left ventricular end-diastolic pressure, total peripheral resistance and PQ interval. Denopamine produced an increase in LV dp/dtmax by 90% at a rate of 4 micrograms/kg/min with slightly increasing systemic blood pressure and heart rate. In the same dogs after anesthetizing with pentobarbital, denopamine in the same dose range showed more marked positive inotropic effects and less changes in cardiac output and total peripheral resistance than in conscious dogs. Other cardiovascular effects of denopamine were qualitatively similar to conscious dogs. These effects of denopamine were diminished by treatment with propranolol. Oral administration of denopamine to conscious dogs (0.1-0.4 mg/kg) produced a rise in LV dp/dtmax dose-dependently, and denopamine at a dose of 0.4 mg/kg increased LV dp/dtmax by 66%, this effect lasting for 7 hr. Heart rate and blood pressure were not affected significantly. Effects on other cardiovascular parameters were changed in the same direction as intravenous administration. The increase in LV dp/dtmax corresponded well with the changes in plasma levels of denopamine in conscious dogs by both intravenous and oral administration. Denopamine showed a selective positive inotropic effect in chronically instrumented dogs, and its positive inotropic action was more marked in the myocardium depressed with an anesthetizing dose of pentobarbital than in the conscious state.

Autonomic mechanisms regulating myocardial contractility in conscious animals

In this review some of the issues and controversies involved in the neural control of the myocardial inotropic response to stress have been discussed. For example, it is surprising that either direct or reflex activation of the sympathetic nerves induces a relatively small increase (20-40%) in the left ventricular inotropic state when compared with the three-five-fold increase associated with maximal dynamic exercise. Studies contrasting the levels of circulating catecholamines with the left ventricular inotropic responses induced by hemorrhage, exercise and exogenously administered catecholamines, suggest that the catecholamine concentration at the synaptic cleft is the primary determinant of the left ventricular inotropic response. Although parasympathetic neural activation alone appears to have little direct influence on the left ventricular inotropic state and central nervous system integration of the autonomic nervous system usually insures there is a reciprocal relationship between sympathetic and parasympathetic neural activity, the potential for parasympathetic inhibition of the response to sympathetic or sympathomimetic augmentation of the intropic response exists. The importance of sympathetic-parasympathetic nervous system interaction in physiologic and pathologic conditions has yet to be defined. It is this type of knowledge of the interactions of reflex pathways which will be critical to the full understanding of autonomic reflex control of myocardial performance under physiologic and pathologic conditions.

Acute increases in blood pressure cause arousal from sleep in lambs

We did experiments to determine whether or not acute increases in blood pressure would cause arousal from sleep. Nine lambs were studied 3-4 days after a balloon-tipped catheter was inserted via the femoral artery into the descending aorta and electrodes for the following recordings were implanted: electrocorticogram, electro-oculogram, electrocardiogram and diaphragm electromyogram. Balloon inflation increased mean arterial blood pressure 27.9 +/- 6.3 mm Hg (mean +/- 1 S.D.) during quiet sleep (QS) and 24.4 +/- 8.9 mm Hg during active sleep (AS). Behavioral arousal occurred from both sleep states but was significantly (P less than 0.05) delayed during active sleep (21.3 +/- 6.7 s) compared to quiet sleep (10.0 +/- 3.3 s). This occurred despite there being a greater percent change in R-R interval for a percent change in mean blood pressure during active sleep (2.68 +/- 1.08) than during quiet sleep (1.55 +/- 0.62). The increases in blood pressure did not produce any significant changes in respiratory rate of integrated diaphragm activity. Thus, acute increases in blood pressure are capable of causing arousal from sleep. This finding should be considered when one discusses the possible mechanisms of arousal from sleep during rapidly developing hypoxemia.

Time delays in the human baroreceptor reflex

In 11 normotensive subjects with coronary artery disease, low intensity electrical stimulation of the carotid sinus nerves (CSN) was triggered by the R-wave in the electrocardiogram with an adjustable delay. The latent period was estimated between the start of CSN stimulation and the onset of the reflex-PP-interval prolongation and, during right atrial pacing, the onset of the reflex fall of diastolic arterial pressure and prolongation of the AV-interval. The latency to the reverse changes was determined after switching CSN stimulation off. The PP-interval changes started after a latency of 0.5-0.6 s. This latency was independent of the respiratory phase and it was independent of the directional change of the afferent activity. AV-interval changes started after about 1 s. When heart rate was fixed, arterial pressure changes started after 2-3 s. It is estimated that central processing of baroreceptor afferent activity may require 0.25 s in the human.

Haemodynamics and oxygen consumption in the dog during high epidural block with special reference to the splanchnic region

High epidural block (Th I-IV) with bupivacaine was carried out in 16 dogs. Mean arterial blood pressure decreased to 52% of control value owing to nearly equal decreases in systemic vascular resistance and cardiac output. Portal venous blood flow decreased from 25.8 +/- 8.6 to 16.7 +/- 7.2 ml/kg b.w. X min-1 following epidural block, while hepatic arterial blood flow remained unchanged at 9.1 +/- 3.1 ml/kg b.w. X min-1 owing to a reduction in hepatic arterial resistance of 51%. Hepatic oxygen uptake was maintained during the epidural block through increased oxygen extraction. However, total oxygen uptake decreased by 18% and, in spite of this, arteriovenous oxygen content difference increased by 25%, indicating circulatory depression.

Renal sympathetic baroreflex during normoxia and during hypoxia in conscious and in anesthetized rabbits

The responses of renal sympathetic nerve activity (RSNA) to changes in mean arterial pressure (MAP) during normoxia and hypoxia was studied in conscious rabbits and during anesthesia with pentobarbitone (PB) by determining the RSNA baroreflex curves. In conscious rabbits, the gain in RSNA response was greater and the range of MAP between minimum and maximum levels of RSNA was narrower than in anesthetized rabbits. The renal sympathetic baroreflex was augmented by hypoxia, indicating a central excitatory interaction between the effects of baro- and chemoreceptor stimulation. However, hypoxia produced no significant change in median blood pressure. During anesthesia with PB, resting MAP was decreased, median blood pressure was lowered, and renal sympathetic baroreflexes were less pronounced. Renal sympathetic baroreflex was augmented by hypoxia, and there was a significant increase in median blood pressure. These results provide direct evidence of an inhibitory effect of PB on the response of RSNA to baro- and chemoreceptor stimulation.

Pentobarbital-induced changes in vagal tone and reflex vagal activity in rabbits

The effects of pentobarbital on heart rate, reflexly mediated vagal activity and the automaticity (intrinsic rate) of the sinoatrial (SA) node were investigated in rabbits. When administered in three cumulative doses (10 mg/kg i.v. each) at 15 min intervals, each dose produced transient hypotension which was not modified by prior muscarinic receptor blockade (MRB), beta-receptor blockade (BRB), or combined MRB and BRB. Subanesthetic doses (10 mg/kg) of pentobarbital produced tachycardia in normal rabbits, as well as in rabbits with prior BRB. After pentobarbital, MRB failed to increase the heart rate, indicating total loss of resting vagal tone. Failure to induce tachycardia in rabbits with prior MRB indicated the absence of sympathetic stimulation in pentobarbital-induced tachycardia. In rabbits with prior BRB, pentobarbital enhanced the hypotensive effects of acetylcholine and nitroglycerin and abolished nitroglycerin-induced tachycardia without significant effects on the magnitude of reflex bradycardia produced by norepinephrine. Pentobarbital had minimal, transient depressant effects on the intrinsic rate of the SA node. These results indicate that pentobarbital produces total loss of resting vagal tone without major impairment of reflex vagal activation.

Effects of prazosin on coronary and left ventricular dynamics in conscious dogs

The left ventricular (LV) and coronary vascular effects of prazosin, a drug that reduces peripheral vascular resistance by blocking postsynaptic alpha receptors, were examined in conscious dogs. Prazosin, 0.07 mg/kg/min i.v. for 7 minutes, induced sustained hypotensive effects for more than 12 hours. The peak effects occurred 30-45 minutes after administration. Prazosin increased heart rate by 28 +/- 9%, did not change mean coronary blood flow significantly, decreased mean arterial pressure by 15 +/- 4%, LV end-diastolic diameter by 10 +/- 2%, LV end-systolic diameter by 8 +/- 2%, late diastolic coronary resistance by 22 +/- 7%, and LV dP/dt by 9 +/- 4%. These effects of prazosin were not altered substantially by maintaining heart rate constant with electrical pacing or by pretreatment with beta-adrenergic blockade. However, after chronic reserpine treatment, prazosin did not reduce either mean arterial pressure or late diastolic coronary resistance. The alpha-blocking properties of the drug were established when prazosin attenuated pressure responses to phenylephrine, norepinephrine and bilateral carotid occlusion. Thus, in conscious dogs with heart rate constant, prazosin, by blocking alpha-adrenergic receptors, induces prolonged coronary vasodilation associated with reductions in the major determinants of myocardial oxygen consumption, e.g., arterial and LV pressures, LV end-diastolic diameter and LV dP/dt. However, the coronary vasodilation was not intense enough to increase coronary blood flow above control levels.

Arterial pressure and heart rate changes during natural sleep in rat

Mean arterial pressure and heart rate data during quiet wakefulness and phases of sleep in conscious rat are sampled by a computer at a rate of 100/sec. Average values and variability expressed as standard deviation are computed for each recording session. Mean arterial pressure and heart rate and their variability decrease from quiet wakefulness to synchronized sleep. During desynchronized sleep, mean arterial pressure increases to the level of quiet wakefulness, and is more variable than during synchronized sleep. Heart rate is lower and more uniform during sleep than during quiet wakefulness, and there is no difference between synchronized and desynchronized sleep except that a greater variability occurs during desynchronized sleep. The study shows that characteristic and specific cardiovascular changes accompany the phases of sleep and that a hierarchy of arterial pressure is present during the resting behavior in rat.

Correlation of blood temperature fluctuations with blood pressure waves

Constant blood temperature in the pulmonary artery is assumed when the thermal dilution method is used for cardiac output determination. In some cases, however, slow temperature fluctuations (2-6 cycles per min.) occur in arterial and venous blood and interfere in the measurement. Those thermal fluctuations were investigated in the pulmonary artery and venae cavae of dogs. The temperature variations were found to be correlated with blood pressure waves: an increase of blood pressure was accompanied by an increase in the blood temperature in the pulmonary artery and a decrease in the blood temperature in the venae cavae. Therefore, measurement of the temperature of the pulmonary artery relative to that of the venae cavae does not rule out those fluctuations, and will not improve the thermal dilution method.