The effect of chronic cardiac denervation on infarct size following acute coronary occlusion

Cardiac innervation in acute myocardial ischaemia/reperfusion injury and cardioprotection

Acute myocardial infarction (AMI) and the heart failure (HF) that often complicates this condition, are among the leading causes of death and disability worldwide. To reduce myocardial infarct (MI) size and prevent heart failure, novel therapies are required to protect the heart against the detrimental effects of acute ischaemia/reperfusion injury (IRI). In this regard, targeting cardiac innervation may provide a novel therapeutic strategy for cardioprotection. A number of cardiac neural pathways mediate the beneficial effects of cardioprotective strategies such as ischaemic preconditioning and remote ischaemic conditioning, and nerve stimulation may therefore provide a novel therapeutic strategy for cardioprotection. In this article, we provide an overview of cardiac innervation and its impact on acute myocardial IRI, the role of extrinsic and intrinsic cardiac neural pathways in cardioprotection, and highlight peripheral and central nerve stimulation as a cardioprotective strategy with therapeutic potential for reducing MI size and preventing HF following AMI. This article is part of a Cardiovascular Research Spotlight Issue entitled 'Cardioprotection Beyond the Cardiomyocyte', and emerged as part of the discussions of the European Union (EU)-CARDIOPROTECTION Cooperation in Science and Technology (COST) Action, CA16225.

Noninvasive low-frequency electromagnetic stimulation of the left stellate ganglion reduces myocardial infarction-induced ventricular arrhythmia

Noninvasive magnetic stimulation has been widely used in autonomic disorders in the past few decades, but few studies has been done in cardiac diseases. Recently, studies showed that low-frequency electromagnetic field (LF-EMF) might suppress atrial fibrillation by mediating the cardiac autonomic nervous system. In the present study, the effect of LF-EMF stimulation of left stellate ganglion (LSG) on LSG neural activity and ventricular arrhythmia has been studied in an acute myocardium infarction canine model. It is shown that LF-EMF stimulation leads to a reduction both in the neural activity of LSG and in the incidence of ventricular arrhythmia. The obtained results suggested that inhibition of the LSG neural activity might be the causal of the reduction of ventricular arrhythmia since previous studies have shown that LSG hyperactivity may facilitate the incidence of ventricular arrhythmia. LF-EMF stimulation might be a novel noninvasive substitute for the existing implant device-based electrical stimulation or sympathectomy in the treatment of cardiac disorders.

Influence of cardiac decentralization on cardioprotection

The role of cardiac nerves on development of myocardial tissue injury after acute coronary occlusion remains controversial. We investigated whether acute cardiac decentralization (surgical) modulates coronary flow reserve and myocardial protection in preconditioned dogs subject to ischemia-reperfusion. Experiments were conducted on four groups of anesthetised, open-chest dogs (n = 32): 1- controls (CTR, intact cardiac nerves), 2- ischemic preconditioning (PC; 4 cycles of 5-min IR), 3- cardiac decentralization (CD) and 4- CD+PC; all dogs underwent 60-min coronary occlusion and 180-min reperfusion. Coronary blood flow and reactive hyperemic responses were assessed using a blood volume flow probe. Infarct size (tetrazolium staining) was related to anatomic area at risk and coronary collateral blood flow (microspheres) in the anatomic area at risk. Post-ischemic reactive hyperemia and repayment-to-debt ratio responses were significantly reduced for all experimental groups; however, arterial perfusion pressure was not affected. Infarct size was reduced in CD dogs (18.6±4.3; p = 0.001, data are mean±1SD) compared to 25.2±5.5% in CTR dogs and was less in PC dogs as expected (13.5±3.2 vs. 25.2±5.5%; p = 0.001); after acute CD, PC protection was conserved (11.6±3.4 vs. 18.6±4.3%; p = 0.02). In conclusion, our findings provide strong evidence that myocardial protection against ischemic injury can be preserved independent of extrinsic cardiac nerve inputs.

Role of the autonomic nervous system in cardioprotection by remote preconditioning in isoflurane-anaesthetized dogs

AIMS

Remote ischaemic preconditioning (rIPC) protects cardiac and non-cardiac tissues against ischaemic injury. Although there is increased demand to investigate its potential clinical applicability, fundamental mechanisms responsible for rIPC-mediated protection remain unresolved. We examined in isoflurane-anaesthetized dogs whether an intact cardiac nervous system was necessary to mediate rIPC protection against ischaemic injury.

METHODS AND RESULTS

Dogs were randomly allocated to six groups: 1, control (CON, no-rIPC); 2, rIPC (4 × 5 min renal artery occlusion/reperfusion); 3, autonomic ganglionic blockade with hexamethonium (HEX, no-rIPC; 20 mg/kg iv); 4, HEX + rIPC; 5, cardiac decentralization by surgical ablation of extracardiac nerves (DCN, no-rIPC); and 6, DCN + rIPC. All dogs underwent 60 min coronary occlusion and 180 min reperfusion; cardiac haemodynamic parameters were monitored. Regional blood flow (microspheres) in the heart and kidneys was assessed. Necrotic tissue was visualized using triphenyltetrazolium staining and related to anatomic risk zone size (area at risk; P = NS between groups) and coronary collateral blood flow. Infarct size (% AAR) was 29 ± 5 (mean ± 1 SD) in CON and 15 ± 4 in rIPC dogs (P = 0.001 vs. CON); 24 ± 3 in HEX vs. 12 ± 2 in HEX + rIPC (P = 0.001 vs. HEX); and 20 ± 2 in DCN vs. 12 ± 4 in DCN + rIPC (P = 0.001 vs. DCN). In CON dogs, infarct size was inversely related to coronary collateral flow; this relation was shifted downwards in all groups pre-treated with rIPC.

CONCLUSION

We report robust myocardial protection by rIPC against ischaemic injury in canines that was not abrogated by either pharmacological or surgical decentralization of cardiac nerves.

Coronary vasoregulation in health and disease

The present article reviews pertinent contributions from the Coronary Physiology Research Group at the Quebec Heart Institute to the understanding of coronary physiology in health and disease. Mechanisms that contribute to regulation of coronary blood flow and its distribution across the ventricular wall are discussed. Data from animal studies of ischemia-reperfusion injury are also presented and discussed in the context of current concepts regarding postischemic myocardial protection strategies. Future research directions regarding the cardiac nervous system and its importance in the regulation of coronary blood flow, cardiac function and myocyte injury during acute myocardial infarction are also discussed.

Ischemia-Induced Norepinephrine Release, but not Norepinephrine-Derived Free Radicals, Contributes to Myocardial Ischemia - Reperfusion Injury

BACKGROUND

Norepinephrine (NE)-derived free radicals may contribute to myocyte injury after ischemia -reperfusion, so the influence of sympathetic denervation on myocardial ischemia - reperfusion injury was investigated in the present study.

METHODS AND RESULTS

Cardiac sympathetic denervation was produced in Wistar rats by a solution of 10% phenol 1 week before ischemia. Atenolol (0.5 mg/kg) was intravenously administered 10 min before the coronary occlusion. The left coronary artery was occluded for 30 min and thereafter reperfused. Cardiac interstitial fluid was collected by a microdialysis probe and free radicals in dialysate were determined by electron paramagnetic resonance (EPR) spin trapping, using 5,5-dimethyl-1-pyrroline-N-oxide as a spin trap. The ratio of infarct size to the ischemic area at risk (I/R) was decreased in both the phenol and atenolol groups compared with control (28.5+/-11.3, 31.8+/-10.7 vs 50.6+/-14.7%, p<0.05). During the coronary occlusion, concentrations of interstitial NE increased markedly in the control and atenolol groups, but was unchanged in the phenol group. EPR signal intensity (relative value to internal standard) was maximal at 1 h after reperfusion and was similar in the phenol and control groups (0.32+/-0.15 vs 0.45+/-0.19).

CONCLUSIONS

Cardiac denervation protected myocyte against ischemia-reperfusion injury through decreasing direct NE toxicity, but not through decreasing NE-derived free radicals.

Cardiac nerves affect myocardial stunning through reactive oxygen and nitric oxide mechanisms

The goal of this study was to investigate the role of cardiac nerves on the response to 90-minute coronary artery stenosis (CAS), which reduced coronary blood flow by 40% for 90 minutes, and subsequent myocardial stunning after reperfusion in chronically instrumented conscious pigs. In pigs with regional cardiac denervation (CD), myocardial stunning was intensified, ie, at 12 hours reperfusion wall thickening (WT) was depressed more, P<0.05, in CD (-46+/-5%) as compared with intact pigs (-31+/-3%) and remained depressed in CD at 24 hours reperfusion (-45+/-6%). Although the TTC technique was negative for infarct, histopathological analysis revealed patchy necrosis present in 11+/-2% of the area at risk. In intact pigs, WT had essentially recovered at 24 hours without infarct. In CD pigs treated with either an antioxidant, N-2-mercaptopropionyl glycine (MPG, 100 mg/kg per hour) or systemic nitric oxide synthase inhibition using N(omega)-nitro-L-arginine (L-NA, 30 mg/kg for 3 days), recovery of wall thickening was similar to that in pigs with intact nerves and without evidence of infarct. Immunohistochemistry analysis for 3-nitrotyrosine in tissue after CAS and 1 hour reperfusion demonstrated enhanced peroxynitrite-related protein nitration in pigs with regional CD compared with pigs with intact cardiac nerves, and pigs with regional CD and MPG or L-NA. Thus, reperfusion after myocardial ischemia in the setting of CD results in enhanced stunning and development of infarct. The underlying mechanism appears to involve nitric oxide and reactive oxygen species.

Significance of cardiac innervation on spontaneous ventricular arrhythmias elicited by left stellate ganglion stimulation in dogs 4 days after myocardial infarction: comparison of two experimental models

The effects of cardiac sympathetic overactivity on spontaneous arrhythmias and transmural left ventricular effective refractory period (LVERP) were assessed by left stellate stimulation (LSS) in 16 anesthetized dogs. The experiments were performed 4 days after proximal occlusion of the left anterior descending (LAD) coronary artery produced by either ligation (9 dogs) or embolization with histoacryl (7 dogs). The innervation of left ventricular myocardium was studied by light and electron microscopies. Synaptophysin (SYN)- and neuropeptide Y (NPY)-immunoreactive nerve fibers and terminals were thereby detected. In dogs subjected to ligation, LSS elicited negligible arrhythmias in spite of a decrease in LVERP by 6.9 +/- 2.2% (mean +/- SD, p < 0.001). However, dogs with intravascular occlusion were more susceptible to LSS, as indicated by development of sustained ventricular rhythms. In these animals, the LVERP decreased with LSS by 14.6 +/- 3.4% (p < 0.001). The innervation of the anterior left ventricular wall distal to the place of occlusion revealed a higher reduction of SYN- and NPY-immunoreactive nerves in infarcted myocardium and a more heterogeneous distribution of nerves in undamaged regions after ligation, compared to intravascular occlusion. Ultrastructurally, nerve terminals containing small agranular and large dense-core vesicles were found innervating ischemically damaged myocardiocytes. Our findings indicate a higher preservation of nerves in infarcted and noninfarcted myocardium of animals subjected to embolic occlusion of the LAD. Because LSS apparently elicited more arrhythmias in these animals, we suggested a proarrhythmic effect of intact myocardial innervation after infarction.

Effects of selective cardiac denervation on collateral blood flow after coronary artery occlusion in conscious dogs

The extent to which cardiac nerves regulate responses to myocardial ischemia remains controversial. Our data in conscious dogs indicate that neither selective posterior left ventricular (LV) wall denervation nor selective ventricular denervation, leaving the atria intact, modifies the effects of coronary artery occlusion (for 24 h) on regional myocardial function and infarct size as compared to normally innervated dogs. Since hemodynamic changes were similar among the three groups after coronary artery occlusion, it is possible to speculate that responses of collateral blood flow to the ischemic zone were also not modified by chronic selective cardiac denervation. To address this, individual samples were selected and included in either the infarcted (TTC negative) or salvaged (TTC positive) group. The infarcted and salvaged samples were paired according to blood flow levels of 0.1-0.2, 0.2-0.3, or 0.3-0.4 ml/min/g at either 5 min, 1 h, 3 h, or 6 h after coronary artery occlusion. The results demonstrated similar patterns of myocardial blood flow in tissue samples within the area at risk after coronary artery occlusion in the animals, regardless of whether the ischemic zone was innervated or denervated. While blood flow rose in ischemic tissue that ultimately was salvaged, and tended not to rise over the 24 h monitoring period in tissue samples that became necrotic, no differences could be discerned on the basis of intact or absent innervation of the ischemic zone. Thus, chronic absence of cardiac nerves does not affect regulation of ischemic zone blood flow following coronary artery occlusion in conscious dogs.

Improved distribution of regional oxygenation in denervated ischemic dog myocardium

The role of the adrenergic nervous system in the response to coronary artery occlusion has been examined using surgical and chemical denervation techniques. Experiments were conducted on four groups of dogs (n = 18): 1) untreated controls; 2) intrapericardial denervation immediately prior to coronary ligation; 3) surgical denervation 2 weeks prior to the experiment; and 4) chemical sympathectomy 5 days prior to the experiment with 6-hydroxydopamine (50 mg/kg). Small artery and vein O2 saturations were obtained microspectrophotometrically and combined with radioactive microsphere blood flow determinations to calculate regional myocardial O2 consumption in open chest dogs. Denervation significantly reduced the preocclusion heart rate from 165 +/- 16 beats/min in the control to 114 +/- 13 in the chronic surgically denervated and to 137 +/- 15 in the chemically sympathectomized groups. After 2 hours of occlusion, the O2 consumption and flow were similar in the nonischemic area except for lower values in the surgically denervated group. Total coronary blood flow and O2 consumption in the occluded regions were not significantly affected by chronic denervation. However, significant elimination of areas with low venous O2 saturation (less than 20%) were found in the ischemic myocardium of the chronically denervated groups as compared with the control or with the acutely denervated dogs. The mean venous O2 saturation was found to be significantly higher in all regions of these two groups as compared with the control. The O2 extraction was also lowered. Thus, chronic denervation reduced microregional heterogeneity of oxygenation in the ischemic myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of beta blockers on cardiac neural discharge associated with coronary occlusion in the cat

The effect of timolol on postganglionic cardiac sympathetic neural discharge, blood pressure, heart rate, and rhythm changes associated with acute coronary occlusion of the left anterior descending artery was examined and compared with the effects of the beta blockers practolol and metoprolol. Timolol (5 mg/kg, IV) was infused 15 minutes prior to coronary occlusion in cats anesthetized with alpha-chloralose. Control heart rate fell from 129 +/- 10 to 106 +/- 2 one minute prior to coronary occlusion and remained at 106 +/- 2 beats/minute in the minute prior to arrhythmia. Control blood pressure fell from 126 +/- 20 to 91 +/- 19 and stabilized at 99 +/- 19 mm Hg one minute prior to coronary occlusion. Mean time to arrhythmia and death was 4.7 +/- 2.3 and 68.0 +/- 51.0 minutes (P greater than .05 vs no drug), respectively. Three cats died and two were sacrificed six hours after coronary occlusion. Blood pressure fell to 86 +/- 20 mm Hg two minutes after coronary occlusion, rose to 95 +/- 23 mm Hg at ten minutes, and remained there for ten minutes. Timolol did not alter postganglionic cardiac sympathetic neural discharge prior to coronary occlusion. Two minutes after coronary occlusion, mean postganglionic cardiac sympathetic neural discharge was 128 +/- 27 and increased to 139 +/- 36 impulses/second (% control) 4 minutes after coronary occlusion. A similar trend was found for the data recorded in 15 nerves (eight cats) in which coronary occlusion was initiated without timolol. The data suggest that a difference exists among beta blockers because prior to coronary occlusion, the cardioselective drugs metoprolol (1, 5, and 10 mg/kg, IV) and practolol (8 mg/kg, IV) depressed postganglionic cardiac sympathetic neural discharge whereas noncardioselective timolol did not. Because all three beta blockers increased the times to arrhythmia and death (although the increase was significant only after metoprolol and practolol), the acute protective mechanism does not appear to be due primarily to a depression of spontaneous sympathetic neural discharge.

Adverse effects of chronic cardiac denervation in conscious dogs with myocardial ischemia

The extent to which total chronic cardiac denervation protects the ischemic myocardium was investigated in conscious dogs. The major hemodynamic difference after coronary artery occlusion was that left ventricular end-diastolic pressure rose significantly more, P less than 0.01, in the denervated group (12 +/- 1.5 mm Hg) than in the normal group (4.4 +/- 1.4 mm Hg). Blood flow (radioactive microspheres) in the ischemic endo- and epicardium fell to similar levels at 3-5 minutes after coronary occlusion, but was significantly less (P less than 0.01) in denervated dogs at 3 hours after occlusion in the endo- (0.05 +/- 0.01) and epicardium (0.30 +/- 0.02 ml/min per g), than in the endo- (0.13 +/- 0.03) and epicardium (0.42 +/- 0.05 ml/min per g) in the normal group. A subgroup of normal dogs was also studied, with left ventricular end-diastolic pressure increased by volume loading to levels similar to those observed in the denervated group after coronary occlusion; in these dogs, blood flow was similar to that in the other two groups 3-5 minutes after coronary artery occlusion, but, at 3 hours, was significantly more depressed (P less than 0.01) than that observed in normal dogs without volume loading in both endo- (0.03 +/- 0.01) and epicardial (0.25 +/- 0.03 ml/min per g) layers. Infarct size, as a fraction of the area at risk, was significantly greater (P less than 0.05) in the denervated group (60 +/- 4.3%) and in the subgroup of normal dogs with elevated left ventricular end-diastolic pressure (73 +/- 5.8%), compared with the normal group without volume loading (37 +/- 8.1%). Thus, in conscious dogs, total chronic cardiac denervation exerts an adverse effect on infarct size which may be related to the sustained elevation in left ventricular end-diastolic pressure and consequent impairment of collateral perfusion.

Interruption of sympathetic and vagal-mediated afferent responses by transmural myocardial infarction

We have demonstrated previously that sympathetic and vagal afferents travel in an apical-to-basal course in the heart, and can be stimulated selectively with epicardial applications of bradykinin and nicotine, respectively. In this study we tested the hypothesis that transmural myocardial infarction interrupts sympathetic and vagal afferent fibers traveling through the infarction and produces regions of afferent denervation in areas apical to the infarction. In open-chest, chloralose-anesthetized dogs, transmural myocardial infarction was created by embolizing a diagonal branch of the left anterior descending coronary artery with a vinyl latex solution that was injected directly into the artery and hardened rapidly. The transmural nature of the infarction was verified by the nitro blue tetrazolium staining technique for dehydrogenase enzymes. Epicardial applications of bradykinin (5 micrograms) and nicotine (50 micrograms) were used to stimulate chemically sensitive sympathetic and vagal afferent nerve endings, respectively. Twenty-nine dogs were studied before and 90 min after creation of transmural myocardial infarction. In 20 dogs, epicardial bradykinin applied before production of transmural myocardial infarction produced a maximal pressor response of 13 +/- 3 mm Hg 40 sec after application (p less than .01 vs preapplication values), while topical nicotine produced a maximal depressor response of 14 +/- 2 mm Hg (p less than .01 vs preapplication values) 20 sec after application at all sites tested. Ninety minutes after production of transmural myocardial infarction, epicardial sites basal to the infarction continued to respond normally to both drugs, while sites within the area of infarction and apical to the area (noninfarcted myocardium) no longer showed a pressor response to topical bradykinin or a depressor response to topical nicotine.(ABSTRACT TRUNCATED AT 250 WORDS)

Infarct size restriction in cats by the beta-adrenergic blocker timolol

The beta-adrenergic antagonist, timolol, was previously shown to exert a protective effect in cats subjected to 5 h of myocardial ischemia. The present study was designed to determine the effect of timolol on infarct size in cats 24 h after coronary occlusion. Timolol (25 microgram/kg) or vehicle (0.9% NaCl) was administered 0.5, 5, 10, 15 and 20 h after acute ligation of the left anterior descending coronary artery. There was no significant difference in arterial blood pressure or heart rate in MI cats receiving timolol or vehicle. Timolol markedly decreased S-T segment elevation at 2-12 h (P less than 0.05). Left ventricular weights in MI + vehicle cats (8.5 +/- 0.7 g, n = 6) were similar to timolol-treated MI cats (8.9 +/- 0.4 g, n = 7). However, the percent of the left ventricular myocardium infarcted, determined by nitroblue tetrazolium staining, was significantly less (P less than 0.001) in timolol MI cats compared to saline-treated cats, 9.8 +/- 1.2% (n = 7) vs. 18.9 +/- 1.8% (n = 6), respectively. Hemodynamic or cytoprotective actions of timolol do not appear to explain these results. Rather, the mechanism of infarct size reduction by timolol is probably explained by antagonism of beta-receptor-mediated metabolic effects.

Protective effect of chronic versus acute cardiac denervation on contractile force during coronary occlusion

The effects of acute coronary artery (CA) occlusion on myocardial contractile force were studied in mongrel dogs with (1) chronically denervated hearts (n = 10), (2) acutely denervated hearts (n = 5), and (3) normally innervated hearts (n = 6). Contractile force was measured in ischemic and nonischemic areas using Walton-Brodie strain gauge arches sutured to the epicardium. Coronary occlusion was accomplished by ligating several small branches of the left anterior descending and the circumflex arteries supplying the apical region on the left ventricle. Following occlusion, contractile force in the ischemic area decreased by 66.8% in the control group, by 73.6% in the acutely denervated group, but only by 21.6% (P less than 0.001) in the chronically denervated group. These results demonstrate that chronic cardiac denervation protects from the severe loss of contractile force in the ischemic area. This salutary effect is not seen with acute cardiac denervation.

Neural, hormonal and intrinsic mechanisms of cardiac control during acute coronary occlusion in the intact dog

Three basic mechanisms may be involved in the control of cardiac function during acute coronary occlusion: (1) neural; (2) hormonal (circulating catecholamine); and (3) intrinsic (e.g. Frank--Starling law). The response of intact, sedated (Innovar-Vet, 0.08 cc/kg), chronically instrumented dogs to a 5 min left circumflex coronary occlusion was tested to delineate the relative roles of each of the above mechanisms. First, 6 innervated and 6 cardiac denervated dogs were examined. The major difference between groups was that the occlusion-induced tachycardia was significantly smaller in the denervated dogs than in the normally innervated animals (+10 +/- 7 vs +27 +/- 4/min, respectively, (mean +/- S.D.)). Changes in the first time derivative of left ventricular pressure (d(LVP)/dt) were similar (--898 +/- 556 vs --796 +/- 274 mm Hg/sec, denervated vs innervated). Decreases in stroke volume and mean arterial pressure were also similar in the two groups. The occlusion-induced tachycardia was compared in a second group of denervated dogs (n = 5) before and after administration of propranolol to examine the role of circulating catecholamines, and, by exclusion, to observe the response of the heart per se, independently of extrinsic control factors. The heart rate response was similar in both cases (+8 +/- 4 vs +6 +/- 4/min, unblocked vs blocked). Finally, blood pressure was prevented from falling during coronary occlusion in 3 normally innervated dogs by coupling the femoral artery to a reservoir of saline suspended above the animals. Blunting the input to the baroreceptors in this manner did not significantly change the size of the occlusion-induced tachycardia. We conclude that during acute coronary occlusion in dog: (1) the major role of the cardiac nerves involves modulating changes in the chronotropic state of the heart; (2) changes in d(LVP)/dt result principally from intrinsic phenomena linked to ischemia-induced alterations in myocardial performance; (3) changes in circulating catecholamines play only a minor role in controlling the heart during acute coronary occlusion in denervated dog; and (4) receptors located within the heart figure significantly in the etiology of the occlusion-induced tachycardia.

Protective effects of beta-adrenergic blockade in isolated ischemic hearts

The protective effects of the beta-adrenergic blocking drugs, propranolol and atenolol, were tested in a model of global ischemia and assessed electron microscopically. Cats isolated hearts were perfused retrogradely with arterial blood drawn from donor cats. After a period of equilibration, isolated hearts were rendered globally ischemic for 1 h and subsequently reperfused for another hour. Hearts were then flushed with physiological salt solution followed by perfusion-fixation with cacodylate-buffered glutaraldehyde, containing ionic lanthanum. Lanthanum was included as a probe of myocardial membrane integrity. Left ventricular subendocardial samples were processed and examined electron microscopically. Nontreated hearts, which underwent normothermic ischemia and reperfusion, displayed extensive ultrastructural damage. Nonischemic and donor cat control myocardial tissue appeared normal in all respects. Hearts that received either propranolol or atenolol maintained their ultrastructural integrity, resembling controls. Ionic lanthanum proved to be reliable as a marker of membrane integrity and permeability, as nontreated hearts displayed intracellular deposition of the marker, indicating that deteriorations of membrane integrity occurred. The results suggest that beta-adrenergic blockade may be valuable in preserving myocardium subjected to ischemia and reperfusion.

Effect of reserpine upon the haemodynamic course of recovery following experimental myocardial infarction

Acute haemodynamic consequences of coronary artery ligation were evaluated in twenty-four anaesthetized open-chest dogs, nine of which were pretreated with reserpine. The following parameters were measured before, and at 15-min intervals following ligation for at least five hours : ECG, mean arterial blood pressure, aortic blood flow, left ventricular pressure, heart rate, peripheral resistance (P.R.), end-diastolic pressure, dP/dtmax, and "internal work" (I.W. = heart rate x dP/dtmax). It was found that aortic flow was similar in control and reserpine-pretreated dogs (753 +/- 43 vs. 744 +/- 57 ml/min respectively), even though heart rate, blood pressure and other parameters were significantly higher in the control animals. Furthermore, the controls could be divided into two groups : recoverers (R) and non-recoverers (N), on the basis of late stage haemodynamic differences. The ratio of PR/IW taken within one hour of ligation was significantly higher in the R group (496 +/- 69) and reserpine group (479 +/- 30) than in the N group (242 +/- 28), and could predict course of recovery in each dog studied. It is concluded that the presence of myocardial catecholamines may be deleterious to the ischemic heart when the PR or IW are disproportionately altered.

The myocardium and its vasculature: a histochemical comparison of the normal and chronically sympathectomized dog heart

Using histochemical techniques, the reactivities of selected enzymes and other metabolic components were examined in the myocardium, coronary arteries, and coronary arterioles of normal, two-week-sympathectomized, and sham-operated canine hearts. There were no differences in the histochemistry of coronary arteries in any of the hearts, but important differences were noted in the myocardium and especially in the arterioles. The reactivities of the enzyme glucose-6-phosphate dehydrogenase and the nucleic acids were increased in arterioles of the sympathectomized heart, possibly indicating an increased protein synthesis. The reactivities of succinate dehydrogenase, NAD-isocitrate dehydrogenase, and cytochrome oxidase were reduced in myocardium and arterioles of sympathectomized hearts as well as in arterioles of sham-operated hearts; the changes were greater in the sympathectomized arterioles where there was also observed an increase in reactivity of lactate dehydrogenase. These findings suggest a depression in aerobic metabolic capacity and, in the case of the sympathectomized arteriole, imply a possible shift in adaptation from aerobic to anaerobic metabolism.

A histochemical study of the microvasculature in the left and right cardiac ventricles of the dog

Subepicardial and subendocardial arteries and arterioles in both the left and right normal canine ventricle were examined histochemically to determine their metabolic profiles. Aerobic metabolic capacity was assessed by determining the reactivities of the enzymes cytochrome oxidase, succinate dehydrogenase and NAD-isocitrate dehydrogenase. Glucose-6-phosphate dehydrogenase was examined to assess activity of the hexose-monophosphate-shunt. The substrate glycogen was determined as an evaluation of anaerobic metabolic capacity, while the amounts of deoxyribonucleic and ribonucleic acid were assessed as an indication of protein synthesis. Results of the present investigation indicate that despite known hemodynamic differences, the metabolic profile of the coronary vasculature is similar in all regions of ventricular myocardium. Reactivities of the enzymes succinate and NAD-isocitrate dehydrogenase and cytochrome oxidase are greater in smooth muscle of arterioles than in arteries. This suggests that arteriolar smooth muscle has a higher capacity for aerobic metabolism than does arterial smooth muscle. The greater reactivity of glycogen in arterial, than in arteriolar smooth muscle, suggests that arterial muscle is more adapted for anaerobic metabolism. Deoxyribonucleic and ribonucleic acids demonstrate a low reactivity in both arteries and arterioles from all regions of ventricular myocardium which conforms to the opinion that under normal conditions, coronary vasculature is quite stable with little cell proliferation. Glucose-6-phosphate dehydrogenase shows little reactivity in all myocardial vessels with implies a low capacity for nucleic acid and protein synthesis.