Volker Thämer

Isoflurane Preconditions Myocardium against Infarction via Release of Free Radicals

Background Isoflurane exerts cardioprotective effects that mimic the ischemic preconditioning phenomenon. Generation of free radicals is implicated in ischemic preconditioning. The authors investigated whether isoflurane-induced preconditioning may involve release of free radicals. Methods Sixty-one &agr;-chloralose–anesthetized rabbits were instrumented for measurement of left ventricular (LV) pressure (tip-manometer), cardiac output (ultrasonic flowprobe), and myocardial infarct size (triphenyltetrazolium staining). All rabbits were subjected to 30 min of occlusion of a major coronary artery and 2 h of subsequent reperfusion. Rabbits of all six groups underwent a treatment period consisting of either no intervention for 35 min (control group, n = 11) or 15 min of isoflurane inhalation (1 minimum alveolar concentration end-tidal concentration) followed by a 10-min washout period (isoflurane group, n = 12). Four additional groups received the radical scavenger N-(2-mercaptoproprionyl)glycine (MPG; 1 mg · kg−1 · min−1) or Mn(III)tetrakis(4-benzoic acid)porphyrine chloride (MnTBAP; 100 &mgr;g · kg−1 · min−1) during the treatment period with (isoflurane + MPG; n = 11; isoflurane + MnTBAP, n = 9) or without isoflurane inhalation (MPG, n = 11; MnTBAP, n = 7). Results Hemodynamic baseline values were not significantly different between groups (LV pressure, 97 ± 17 mmHg [mean ± SD]; cardiac output, 228 ± 61 ml/min). During coronary artery occlusion, LV pressure was reduced to 91 ± 17% of baseline and cardiac output to 94 ± 21%. After 2 h of reperfusion, recovery of LV pressure and cardiac output was not significantly different between groups (LV pressure, 83 ± 20%; cardiac output, 86 ± 23% of baseline). Infarct size was reduced from 49 ± 17% of the area at risk in controls to 29 ± 19% in the isoflurane group (P = 0.04). MPG and MnTBAP themselves had no effect on infarct size (MPG, 50 ± 14%; MnTBAP, 56 ± 15%), but both abolished the preconditioning effect of isoflurane (isoflurane + MPG, 50 ± 24%, P = 0.02; isoflurane + MnTBAP, 55 ± 10%, P = 0.001). Conclusion Isoflurane-induced preconditioning depends on the release of free radicals.

α1- and α2-adrenoceptor-mediated vasoconstriction of large and small canine coronary arteries in vivo

The role of α-adrenoceptor subtypes mediating vasoconstriction of large epicardial and small resistive coronary arteries was investigated in 26 open-chest dogs. The left circumflex coronary artery was perfused at a constant pressure. Large vessel vasomotion was determined by measurement of circumflex coronary arterial diameter (ultrasonic transit-time technique); the vasomotion of the small resistive coronary arteries was determined from calculated end-diastolic circumflex artery resistance. β-Receptors were blocked by propranolol (2 mg/kg i.v.) and both vagal nerves were cut. In eight dogs, the intracoronary administration of the α1-adrenoceptor agonist methoxamine (500 μg) increased calculated large vessel resistance by 18.8 ± 5.7% and end-diastolic resistance by 9.5 ± 0.3%. Intracoronary administration of the α2-adrenoceptor agonist BHT 920 (500 μg) did not affect large vessel resistance, but increased end-diastolic resistance by 37.5 ± 5.6%. In an additional 18 dogs, left cardiac sympathetic nerve stimulation induced an increase in large vessel resistance by 11.1 ± 0.9% and in end-diastolic resistance by 31.8 ± 3.0%. The increase in large vessel resistance was prevented by the α1-adrenoceptor antagonist prazosin (1.2 mg/kg i.v.), which still permitted an increase in end-diastolic resistance by 23.0 ± 3.2%. The increase in end-diastolic resistance was prevented by the α2-adrenoceptor antagonist rauwolscine (0.2 mg/kg i.v.), which still permitted an increase in large vessel resistance by 11.9 ± 2.4%. The calcium antagonist nifedipine (20 μg/kg i.v.) prevented both the increase in large vessel resistance and end-diastolic resistance during sympathetic stimulation. Our experiments indicate that α-adrenoceptor-mediated vasoconstriction of large coronary arteries is mediated exclusively by α1-adrenoceptors, whereas α-adrenergic vasoconstriction of small resistive coronary arteries is mediated in part by α1-, but predominantly by α2-adrenoceptors. The calcium antagonist nifedipine can antagonize the vasoconstriction initiated by α1- as well as by α2-adrenoceptors.

Xenon administration during early reperfusion reduces infarct size after regional ischemia in the rabbit heart in vivo.

The noble gas xenon can be used as an anesthetic gas with many of the properties of the ideal anesthetic. Other volatile anesthetics protect myocardial tissue against reperfusion injury. We investigated the effects of xenon on reperfusion injury after regional myocardial ischemia in the rabbit. Chloralose-anesthetized rabbits were instrumented for measurement of aortic pressure, left ventricular pressure, and cardiac output. Twenty-eight rabbits were subjected to 30 min of occlusion of a major coronary artery followed by 120 min of reperfusion. During the first 15 min of reperfusion, 14 rabbits inhaled 70% xenon/30% oxygen (Xenon), and 14 rabbits inhaled air containing 30% oxygen (Control). Infarct size was determined at the end of the reperfusion period by using triphenyltetrazolium chloride staining. Xenon reduced infarct size from 51% ± 3% of the area at risk in controls to 39% ± 5% (P < 0.05). Infarct size in relation to the area at risk size was smaller in the xenon-treated animals, indicated by a reduced slope of the regression line relating infarct size to the area at risk size (Control: 0.70 ± 0.08, r = 0.93; Xenon: 0.19 ± 0.09, r = 0.49, P < 0.001). In conclusion, inhaled xenon during early reperfusion reduced infarct size after regional ischemia in the rabbit heart in vivo. Implications Xenon might be a suitable volatile anesthetic in an ischemia-reperfusion situation.

Cardiac sympathetic nerve activity and progressive vasoconstriction distal to coronary stenoses: feed-back aggravation of myocardial ischemia

This study tested the hypothesis that the relative ischemia distal to a severe coronary stenosis increases the activity of cardiac sympathetic nerves which in turn results in poststenotic vasoconstriction and an aggravation of ischemia. An acute severe stenosis which reduced coronary blood flow to 50% of control was produced in 23 anesthetized, vagotomized dogs and maintained for 20 min. The activity of postganglionic cardiac sympathetic nerves increased by 23 +/- 4% within 20 min. In parallel, poststenotic coronary resistance increased from 0.48 +/- 0.03 to 0.61 +/- 0.03 mm Hg X min X 100 g/ml resulting in a net lactate production after 15 min. Pretreatment with aspirin (6 mg/kg i.v.; n = 5) was without any influence on these reactions. The selective alpha 2-adrenoceptor antagonist rauwolscine (0.2 mg/kg i.v.; n = 6) and the calcium antagonist nifedipine (10 micrograms/kg i.v.; n = 6) prevented the progressive increase in poststenotic resistance and lactate production, but still permitted an increase in sympathetic activity. Segmental anesthesia of cardiac sympathetic nerves by epidural infiltration of procaine at segments C7-T6 (n = 6) prevented the sympathetic activation, the progressive increase in poststenotic resistance and the resulting myocardial ischemia. Sympathetic activation and a concomitant increase in poststenotic resistance resulting in myocardial ischemia were also found in 6 dogs with intact vagus nerves. These data support the hypothesis of a vicious cycle between poststenotic coronary vasoconstriction and sympathetic activation resulting in severe myocardial ischemia.

Ketamine, but Not S (+)-ketamine, Blocks Ischemic Preconditioning in Rabbit Hearts In Vivo

BackgroundKetamine blocks KATP channels in isolated cells and abolishes the cardioprotective effect of ischemic preconditioning in vitro. The authors investigated the effects of ketamine and S (+)-ketamine on ischemic preconditioning in the rabbit heart in vivo. MethodsIn 46 &agr;-chloralose–anesthetized rabbits, left ventricular pressure (tip manometer), cardiac output (ultrasonic flow probe), and myocardial infarct size (triphenyltetrazolium staining) at the end of the experiment were measured. All rabbits were subjected to 30 min of occlusion of a major coronary artery and 2 h of subsequent reperfusion. The control group underwent the ischemia–reperfusion program without preconditioning. Ischemic preconditioning was elicited by 5-min coronary artery occlusion followed by 10 min of reperfusion before the 30 min period of myocardial ischemia (preconditioning group). To test whether ketamine or S (+)-ketamine blocks the preconditioning-induced cardioprotection, each (10 mg kg−1) was administered 5 min before the preconditioning ischemia. To test any effect of ketamine itself, ketamine was also administered without preconditioning at the corresponding time point. ResultsHemodynamic baseline values were not significantly different between groups [left ventricular pressure, 107 ± 13 mmHg (mean ± SD); cardiac output, 183 ± 28 ml/min]. During coronary artery occlusion, left ventricular pressure was reduced to 83 ± 14% of baseline and cardiac output to 84 ± 19%. After 2 h of reperfusion, functional recovery was not significantly different among groups (left ventricular pressure, 77 ± 19%; cardiac output, 86 ± 18%). Infarct size was reduced from 45 ± 16% of the area at risk in controls to 24 ± 17% in the preconditioning group (P = 0.03). The administration of ketamine had no effect on infarct size in animals without preconditioning (48 ± 18%), but abolished the cardioprotective effects of ischemic preconditioning (45 ± 19%, P = 0.03). S (+)-ketamine did not affect ischemic preconditioning (25 ± 11%, P = 1.0). ConclusionsKetamine, but not S (+)-ketamine blocks the cardioprotective effect of ischemic preconditioning in vivo.

Adenosine A2-receptor activation at reperfusion reduces infarct size and improves myocardial wall function in dog heart

Reestablishment of blood supply to ischemic myocardium leads to biochemical and cellular changes which are believed to reduce the amount of potentially salvageable myocardium (reperfusion injury). In this situation, adenosine is known to have myocardial protective properties. Activation of adenosine A2-receptors may account for most of the beneficial effects of adenosine in reperfusion injury because A2-receptor activation mediates vasodilation, inhibits neutrophil adhesion to vascular endothelium and diminishes generation of free radicals by neutrophils, thus acting on some of the key mechanisms of reperfusion injury such as postischemic vascular dysfunction and neutrophil-mediated damage. Therefore, we investigated the effect of an intracoronary A2-agonist, CGS 21680, on regional postischemic myocardial function (measured as wall thickening) and infarct size [determined by triphenyltetrazolium chloride (TTC) staining]. Fourteen anesthetized open-chest dogs underwent 1-h left anterior descending artery (LAD) occlusion and 6-h reperfusion and were randomly assigned to receive intracoronary CGS 21680 or to serve as control. The drug was infused for 60 min starting 5 min before reperfusion with a concentration of 10 -7 M at a rate of 10 ml/min under anoxic conditions. The infusion was then continued for the first 55 min of reperfusion with 10 -6 M at a rate of 1 ml/min. Intracoronary infusion of CGS 21680 led to significant improvement in regional wall function in postischemic myocardium (p < 0.05 vs. control). Thickening fraction (percentage of baseline) increased from −13.1 ± 13.7% (mean ± SD) during occlusion to 15.3 ± 29.8% at 30 min of reperfusion in the CGS 21680 treatment group and remained at this level throughout the reperfusion period. In the control group, thickening fraction was - 23.7 ± 16.2% during occlusion and did not recover during reperfusion. A significant reduction in infarct size was noted in the treatment group (11.5 ± 7.9% of area at risk) as compared with the control group (28.6 ± 11.4%, p < 0.05). These findings support the hypothesis that the protective effect of adenosine on reperfusion injury is not due to replenishment of the nucleoside pool but rather is induced by stimulation of the adenosine A2-receptor.

Inotropic Effects of Glyceryl Trinitrate and Spontaneous NO Donors in the Dog Heart

BACKGROUND In vitro, NO has a biphasic effect on myocardial inotropy. To determine the inotropic effect of NO in vivo, we investigated the activity of glyceryl trinitrate (GTN) and the NO donors S-nitroso-N-acetyl-D,L-penicillamine (SNAP) and sodium-(2)-1-(N,N-diethyl-amino)-diazen-1-ium-1,2-diolat+ ++ (DEA/NO) in dogs. METHODS AND RESULTS Eight anesthetized open-chest dogs were instrumented for measurement of left ventricular and aortic pressures (tip manometers) and coronary flow (ultrasonic flow probes). Regional myocardial function was assessed by sonomicrometry as systolic wall thickening (sWT), mean systolic thickening velocity (Vs), and regional myocardial stroke work index (RSW). GTN, SNAP, and DEA/NO were infused into the left anterior descending coronary artery (LAD) to achieve defined coronary plasma concentrations of GTN, SNAP (both 10 to 100 micromol/L), and DEA/NO (2 to 20 micromol/L). All drugs increased LAD flow and myocardial contractile function in the LAD-dependent myocardium within the first 120 seconds. The greatest inotropic effect was noted after infusion of DEA/NO (20 micromol/L), which increased sWT by 9.7+/-3.1% from 28.5+/-2.2%, Vs by 10.3+/-3.4% from 9.1+/-1.1 mm/s, and RSW by 7.1+/-2.1% from 200.0+/-22.1 mm Hg x mm (P<.05). At the same time, systemic hemodynamics remained unchanged. Prevention of the flow response to GTN by external narrowing of the LAD did not influence the inotropic effect of GTN. CONCLUSIONS Organic nitrates and NO donors evoke a small but constant positive inotropic effect in vivo that is not caused by coronary vasodilation.

Regional contractile blockade at the onset of reperfusion reduces infarct size in the dog heart

An important mechanism of lethal myocardial reperfusion injury is the development of cellular hypercontracture at the onset of reperfusion. Hypercontracture can lead to cytolysis by mutual mechanical disruption of myocardial cells. 2,3-Butanedione monoxime (BDM) inhibits myofibrillar cross-bridge cycling and may therefore reduce infarct size in ischaemic reperfused myocardium. This study investigated whether a temporary presence of BDM protects against myocardial reperfusion injury in an intact-animal preparation. Anaesthetized open-chest dogs (n=10) underwent 1 h of left anterior descendent artery (LAD) occlusion and received intracoronary BDM (25 mM, n=5) or vehicle (n=5) for 65 min starting with an anoxic local infusion 5 min before reperfusion. Infarct size was assessed by triphenyltetrazolium staining after 6 h reperfusion. The infusion of BDM was accompanied by a transient reduction of left ventricular systolic pressure from 84.3±11.2 mm Hg during occlusion to 66.4±9.9 mm Hg at 30 min reperfusion (mean±SD, P<0.01 vs. control). LAD-flow and regional wall motion in the area at risk showed no difference between groups. Infarct size (% of area at risk) was reduced from 24.4±8.7 (control) to 6.6±2.0% (BDM) (P<0.01). The results demonstrate that development of necrosis in reperfused myocardium can be greatly reduced by temporary presence of the contractile inhibitor BDM at the onset of reperfusion.

Clonidine prevents the sympathetic initiation and aggravation of poststenotic myocardial ischemia

Summary: We studied the effects of clonidine (10 μg/kg i.v.) on the positive feedback between poststenotic myocardial ischemia and cardiac sympathetic nerve activation and on the initiation of poststenotic myocardial ischemia during bilateral carotid occlusion in 12 anesthetized, open-chest dogs. During 20 min of acute severe coronary stenosis, cardiac sympathetic nerve activity increased by 24 ± 6%, poststenotic coronary resistance increased from 0.45 ± 0.05 to 0.61 ± 0.08 mm Hg⋅min⋅100 g/ml, and the poststenotic myocardial lactate consumption of 25 ± 6 μmol/min⋅100 g at 1 min of stenosis was reversed to a net lactate production of 8 ± 3 μmol/min⋅100 g after 20 min of stenosis. An additional 60-s bilateral carotid occlusion increased cardiac sympathetic nerve activity furthermore to 157 ± 7% of control, increased poststenotic coronary resistance to 0.84 ± 0.14 mm Hg⋅min⋅100 g/ml, and increased the net lactate production to 22 ± 6 μmol/min⋅100 g. With heart rate and left ventricular pressure kept constant, clonidine decreased cardiac sympathetic nerve activity to 21 ± 6% of control and prevented any further sympathetic activation during 20 min of coronary stenosis and bilateral carotid occlusion. Conversely, poststenotic coronary resistance and myocardial lactate consumption remained unchanged. Intracoronary clonidine (100 μg) in five of the 12 dogs increased poststenotic coronary resistance from 0.47 ± 0.04 to 0.61 ± 0.05 mm Hg⋅min⋅100 g/ml and reversed the myocardial lactate consumption of 36 ± 4 μmol/min⋅100 g to a net lactate production of 7 ± 5 μmol/min⋅100 g. Our results suggest that clonidine can, by a central nervous action, prevent the sympathetic initiation and aggravation of poststenotic myocardial ischemia and may thus be effective in the treatment of exertional angina.

Thiopentone does not block ischemic preconditioning in the isolated rat heart

PurposeIschemic preconditioning protects the heart against subsequent prolonged ischemia by opening of adenosine triphosphate-sensitive potassium (KATP) channels. Thiopentone blocks KATP channels in isolated cells. Therefore, we investigated the effects of thiopentone on ischemic preconditioning.MethodsIsolated rat hearts (n = 56) were subjected to 30 min of global no-flow ischemia, followed by 60 min of reperfusion. Thirteen hearts underwent the protocol without intervention (control, CON) and in 11 hearts (preconditioning, PC), ischemic preconditioning was elicited by two five-minute periods of ischemia. In three additional groups, hearts received 1 (Thio 1,n = 11), 10(Thio 10,n = 11) or 100 μg·mL−1 (Thio 100,n = 10) thiopentone for five minutes before preconditioning. Left ventricular (LV) developed pressure and creatine kinase (CK) release were measured as variables of myocardial performance and cellular injury, respectively.ResultsRecovery of LV developed pressure was improved by ischemic preconditioning (after 60 min of reperfusion, mean ± SD: PC, 40 ± 19% of baseline) compared with the control group (5 ± 6%,P < 0.0l) and this improvement of myocardial function was not altered by administration of thiopentone (Thio 1, 37 ± 15%; Thio 10, 36 ± 16%; Thio 100, 38 ± 16%,P=0.87–0.99 vs PC). Total CK release over 60 min of reperfusion was reduced by preconditioning (PC, 202 ± 82 U·g−1 dry weight) compared with controls (CON, 383 ± 147 U·g−1,P < 0.0l) and this reduction was not affected by thiopentone (Thio 1, 213 ± 69 U·g−1; Thio 10, 211 ± 98U·g−1; Thio 100, 258 ± 128 U·g−1,P=0.62–1.0vs PC).ConclusionThese results indicate that thiopentone does not block the cardioprotective effects of ischemic preconditioning in an isolated rat heart preparation.RésuméObjectifLe préconditionnement ischémique protège le cœur contre l’ischémie ultérieure prolongée en ouvrant les canaux potassiques sensibles à l’adénosine triphosphate (KATP). Or, le thiopental bloque les canaux KATP dans des cellules isolées. Nous avons donc recherché les effets du thiopental sur le préconditionnement ischémique.MéthodeDes cœurs de rats isolés (n = 56) ont été soumis à 30 min d’ischémie globale à débit nul, puis de 60 min, à une reperfusion. Treize cœurs ont subi le protocole sans intervention (témoin, TEM) et dans onze cœurs (groupe de préconditionnement, PC) le préconditionnement ischémique a été amorcé par deux périodes de cinq minutes d’ischémie. Dans trois groupes additionnels, les cœurs ont reçu 1 (Thio 1, n = 11), 10 (Thio 10, n = 11) ou 100 μg·mL−1 (Thio 100, n = 10) de thiopental pendant cinq minutes avant le préconditionnement. La pression du ventricule gauche (VG) développée et la libération de créatine kinase (CK) ont été mesurées en qualité de variables de la performance myocardique et de la lésion cellulaire, respectivement.RésultatsLa récupération de la pression du VG développée a été améliorée par le préconditionnement ischémique (après 60 min de reperfusion, la moyenne ± l’écart type : PC, 40 ± 19 % de la mesure de base) comparée au groupe témoin (5 ± 6 %, P < 0,01). Cette amélioration de la fonction myocardique n’a pas été modifiée par l’administration de thiopental (Thio 1, 37 ± 15 %; Thio 10, 36 ± 16 %; Thio 100, 38 ± 16%, P = 0,87 - 0,99 vs PC). La libération totale de CK après 60 min de reperfusion a été réduite par le préconditionnement (PC, 202 ± 82 U·g−1 de poids anhydre) comparé au témoin (TEM, 383 ± 147 U·g−1, P < 0,01) et cette réduction n’a pas été affectée par le thiopental (Thio 1, 213 ± 69 U·g−1; Thio 10, 211 ± 98 U·g−1; Thio 100, 258 ± 128 U·g−1, P = 0,62– 1,0 vs PC).ConclusionCes résultats indiquent que le thiopental ne bloque pas les effets cardioprotecteurs du préconditionnement ischémique dans une préparation de cœur de rat isolé.