Left ventricular dysfunction following rewarming from experimental hypothermia

This study was aimed at elucidating whether ventricular hypothermia-induced dysfunction persisting after rewarming the unsupported in situ dog heart could be characterized as a systolic, diastolic, or combined disturbance. Core temperature of 8 mongrel dogs was gradually lowered to 25 degreesC and returned to 37 degreesC over a period of 328 min. Systolic function was described by maximum rate of increase in left ventricular (LV) pressure (dP/dtmax), relative segment shortening (SS%), stroke volume (SV), and the load-independent contractility index, preload recruitable stroke work (PRSW). Diastolic function was described by the isovolumic relaxation constant (tau) and the LV wall stiffness constant (Kp). Compared with prehypothermic control, a significant decrease in LV functional variables was measured at 25 degreesC: dP/dtmax 2,180 +/- 158 vs. 760 +/- 78 mmHg/s, SS% 20.1 +/- 1.2 vs. 13.3 +/- 1.0%, SV 11.7 +/- 0.7 vs. 8.5 +/- 0.7 ml, PRSW 90.5 +/- 7.7 vs. 29.1 +/- 5.9 J/m. 10(-2), Kp 0.78 +/- 0.10 vs. 0.28 +/- 0.03 mm-1, and tau 78.5 +/- 3.7 vs. 25.8 +/- 1.6 ms. After rewarming, the significant depression of LV systolic variables observed at 25 degreesC persisted: dP/dtmax 1,241 +/- 108 mmHg/s, SS% 10.2 +/- 0.8 J, SV 7.3 +/- 0.4 ml, and PRSW 52.1 +/- 3.6 m. 10(-2), whereas the diastolic values of Kp and tau returned to control. Thus hypothermia induced a significant depression of both systolic and diastolic LV variables. After rewarming, diastolic LV function was restored, in contrast to the persistently depressed LV systolic function. These observations indicate that cooling induces more long-lasting effects on the excitation-contraction coupling and the actin-myosin interaction than on sarcoplasmic reticulum Ca2+ trapping dysfunction or interstitial fluid content, making posthypothermic LV dysfunction a systolic perturbation.

Equal reduction in infarct size by ethylisopropyl-amiloride pretreatment and ischemic preconditioning in the in situ rabbit heart

Inhibition of Na+/H+ exchange with amiloride analogues has been shown to provide functional protection during ischemia and reperfusion and to reduce infarct size in isolated rat hearts. In rat hearts, treatment with ethylisopropyl-amiloride (EIPA, a selective Na+/H- exchange inhibitor) was additive to the protection afforded by ischemic preconditioning. In addition, such compounds have been demonstrated to reduce infarct size in in situ rabbit hearts. The aim of the present study was to determine to what extent preischemic treatment with EIPA could reduce infarct size in an in situ rabbit model of regional ischemia and reperfusion. We also wished to determine if this effect was additive to the infarct reducing effect of ischemic preconditioning. Anaesthetized, open chest rabbits, were subjected to 45 min of regional ischemia and 150 min of reperfusion. The risk zone was determined by fluorescent particles and infarct size was determined by TTC staining. Four groups were investigated: control, ischemic preconditioned (IP) (5 min of ischemia followed by 10 min reperfusion), EIPA (0.65 mg/kg iv given preischemically) and EIPA + IP. The main results expressed as percent infarction of the risk zone +/- S.E.M. for the different groups were: control 59.2+/-3.3% (n = 6), IP 16.3+/-2.1% (n = 6) (p < 0.001 vs. control), EIPA 16.9+/-4.1% (n = 5) (p < 0.001 vs. control), EIPA + IP 22.5 +/-9.5% (n = 6) (p < 0.001 vs. control).

IN CONCLUSION

EIPA, when administered prior to ischemia, caused a reduction in infarct size in the in situ rabbit heart which was similar to that seen with ischemic preconditioning, however, the effect was not additive to ischemic preconditioning.

Mechanism of hypoxic preconditioning in guinea pig papillary muscles

Brief ischemia or hypoxia has been found to protect the heart against subsequent long-lasting ischemia and to improve contractile dysfunction as well to reduce cell necrosis and the incidence of lethal arrhythmias. This phenomenon, termed preconditioning (PC) has been demonstrated in different species. However, little is known about PC in guinea pigs. Moreover, electrophysiological changes underlying protection have not been studied so far in conjuntion with force recovery in a setting of PC. The aim of the study was to study PC in a guinea pig papillary muscle, using recovery of contractility after long hypoxic challenge as the main end-point of protection, and to investigate concomitant electrophysiological alterations. In guinea pig papillary muscle preparations contracting isometrically (paced at 2 Hz), transmembrane action potentials (AP) and developed force (DF) were recorded by conventional microelectrode technique and a force transducer. In addition, effective refractory periods (ERP) were determined. Hypoxia was induced by superfusion with 100% N2 (pO2 < 5 kPa) and pacing at 3,3 Hz. In the control group, long hypoxia lasted for 45 min and was followed by 30 min reoxygenation. In the PC group, muscles were subjected to 5 min hypoxia followed by 10 min recovery prior to sustained hypoxia/reoxygenation.

RESULTS

Long hypoxia induced a similar depression of DF in both, PC and control groups. However, a loss of contractile activity occured earlier in the PC group. AP duration and ERP decreased faster and were significantly shorter after PC. Upon reoxygenation, preconditioned muscles showed significantly better recovery of function (DF 86% of prehypoxic value vs. 36% in controls; p < 0,05). AP and ERP were completely restored in both, PC and control groups. Guinea pig papillary muscle can be preconditioned with a brief hypoxic challenge against contractile dysfunction upon long-lasting hypoxia/reoxygenation. Shortening of AP and loss of contractility occured more quickly during hypoxia and may participate in the protective effect of preconditioning. Possible mechanisms might involve facilitated opening of K(ATP)-dependent channels.

Changes in myocardial ultrastructure induced by cooling as well as rewarming

The aim of the present study was to investigate if hypothermia and rewarming, without accompanying cardiac ischaemia or cardioplegia, causes myocardial damage. Anaesthetized rats were subjected to a cooling procedure (4 h at 15-13 degrees C) where spontaneous cardiac electromechanical activity was maintained, followed by rewarming. Control rats, hypothermic rats and posthypothermic rats were perfusion-fixed, the hearts removed and the ventricles examined using an electron microscope. Based on morphometric methodology volume fractions as well as absolute volumes of cellular and subcellular components of the ventricles were assessed. In hypothermic hearts capillary volume fraction was significantly decreased, which was probably due to a decrease in perfusion pressure. The cytosolic volume increased in both absolute values and as a fraction of the myocyte: from 25 +/- 11 in controls to 43 +/- 8 microliters and from 0.067 +/- 0.023 to 0.102 +/- 0.013, respectively. There was a corresponding relative decrease in the volume fraction of myofilaments from 0.598 +/- 0.030 to 0.548 +/- 0.024. In posthypothermic hearts significant tissue swelling was apparent, dominated by a significant increase in myocyte volume from 372 +/- 66 in controls to 522 +/- 166 microliters. Similar changes were measured in mitochondrial and cytosolic volumes. In conclusion, the myocardial ultrastructure was altered during hypothermia as well as after rewarming. Posthypothermic myocardium showed generalized cellular swelling and areas of cellular necrosis.

Lipid peroxidation, arachidonic acid and products of the lipoxygenase pathway in ischaemic preconditioning of rat heart

OBJECTIVE

Preconditioning with brief intermittent periods of ischaemia is known to provide protection against ischaemic injury. It has been suggested that myocardial ischaemia also activates phospholipase A2, which releases arachidonic acid from phospholipids. In the present study the possible role of phospholipid peroxidation, arachidonic acid and products of the lipoxygenase pathway in cellular mechanisms of ischaemic preconditioning was examined.

METHODS

Isolated, buffer-perfused rat hearts were freeze-clamped at the end of preconditioning (a cycle of 5 min global ischaemia +5 min reperfusion) and at the end of 30 min global ischaemia and analysed for non-esterified fatty acids and fatty acids in the 2-position of phospholipid. In a separate set of experiments, hearts pretreated with a lipoxygenase inhibitor, nordihydroguaiaretic acid (NDGA), were subjected to 30 min regional ischaemia and 120 min reperfusion. Infarct size was determined by tetrazolium staining and the ischaemic risk zone with fluorescent particles.

RESULTS

Myocardial levels of arachidonic as well as of linoleic and docosahexaenoic acid were significantly elevated by preconditioning. Also, the level of peroxidized polyunsaturated fatty acids (measured as hydroxy conjugated dienes) in myocardial phospholipid was significantly increased: 101.4 +/- 16.8 nmol/g versus 51.2 +/- 7.3 nmol/g tissue dw in the control group, p < 0.05. Pre-treatment of hearts with 5 microM NDGA blocked the infarct limiting effects of preconditioning: infarct size was 37.4 +/- 6.4% of risk zone in control, 9.0 +/- 0.9% in the preconditioning group and 27.7 +/- 3.8% in the preconditioning + NDGA group (p < 0.05 vs. i.p., n.s. vs. control).

CONCLUSION

Our findings provide evidence for the involvement of phospholipase A2 and lipoxygenase derived lipid second messengers in ischaemic preconditioning of the isolated rat heart.

Low concentrations of hydrogen peroxide improve post-ischaemic metabolic and functional recovery in isolated perfused rat hearts

The aim of the present study was to test the hypothesis that low concentrations of hydrogen peroxide (H2O2) have a beneficial effect on post-ischaemic myocardial recovery. Functional and metabolic measurements were performed in isolated buffer-perfused rat hearts exposed to 30 min perfusion with 0 (control group A), 25, 50, 100 or 200 microM H2O2 or 30 min global ischaemia followed by 30 min reperfusion with 0 (control group B), 25, 50 or 100 microM H2O2. Catalase (200 U/ml) was added as scavenger during reperfusion with 25 microM H2O2. Non-ischaemic perfusion: All concentrations of H2O2 induced an immediate vasodilatation, which was maintained in the 50 microM group, but it was followed by vasoconstriction in the 100 and 200 microM group. Left ventricular developed pressure (LVDP) was significantly increased at the end of perfusion in the 50 microM group compared to the control group. Exposure to 100 and 200 microM H2O2 significantly decreased LVDP and increased end-diastolic pressure. ATP was reduced in the 100 microM group. Post-ischaemic perfusion: Exposure to 25 microM H2O2 caused improved coronary flow during the first 20 min of reperfusion compared to the control group (accumulated coronary flow; 235.5 +/- 10.8 v 172.7 +/- 8.6 ml). LVDP was significantly higher in the 25 microM group compared to the control (59.8 +/- 10.2 v 22.1 +/- 7.3 mmHg), and end-diastolic pressure was significantly lower (32.1 +/- 19.6 v 78.8 +/- 2.2 mmHg) at the end of reperfusion. Improved recovery was not observed in the group exposed to 25 microM H2O2 plus catalase. Treatment with 25 microM H2O2 caused significantly improved recovery of tissue ATP and creatine phosphate. In conclusion, the present study showed that exposure to 25 microM H2O2 improved post-ischaemic recovery in hearts subjected to global ischaemia.

Reversible ultrastructural alterations in the myocytic mitochondria of isolated rat hearts induced by oxygen radicals

The present study focuses on reversible mitochondrial ultrastructural alterations in myocardial myocytes that correspond or accompany reversible metabolic depression observed after oxygen radical exposure. The myocytic mitochondrial membranes and matrix of isolated Langendorff-perfused rat hearts were examined by semiquantitative morphometry using the electron micrograph as unit. The hearts were exposed to either standard perfusion (group A), 10 min of oxygen radicals together with superoxide dismutase and catalase followed by 35 min of recovery (group B), 10 min of oxygen radicals alone (group C), or 10 min of oxygen radicals followed by 35 min of recovery (group D). Mitochondrial ultrastructural alterations were detected in only a few micrographs in groups A and B. The frequency of micrographs with mitochondrial ultrastructural alterations was 69% in group C and 62% in group D. In the group exposed to 10 min of oxygen radicals without recovery (group C) condensed pentalaminar membranous profiles arranged in parallel, interpreted to be closely adhering cristae, were detected in the intracristal compartment of myocytic mitochondria in 50% of the micrographs. The cristal adhesions were associated with other mitochondrial ultrastructural changes. Cristal adhesions were not present in group A or B, and were rarely found in the group exposed to 10 min of oxygen radicals followed by 35 min of recovery (group D). Thus, the cristal adhesions appear to be reversible alterations caused by exposure to oxygen radicals.

Mepacrine protects the isolated rat heart during hypoxia and reoxygenation--but not by inhibition of phospholipase A2

Mepacrine (quinacrine) has in a number of studies been shown to protect the heart from ischemic injury, a protection commonly claimed to be due to inhibition of phospholipase A2. The aim of the present study was to investigate the effect of mepacrine 1 microM on isolated, buffer perfused rat hearts subjected to 60 min hypoxia and 30 min reoxygenation. We also wanted to clarify whether any cardioprotective effect was due to inhibition of phospholipase A2 or to other effects of the drug. Mepacrine led to a substantial fall in left ventricular developed pressure (LVDP) and coronary flow (CF) during normoxic perfusion. Treated hearts showed less increase in LVEDP and less fall in LVDP during the hypoxic period, and significantly fewer hearts stopped beating compared to untreated controls. Release of CK during hypoxia and reoxygenation was reduced in treated hearts compared to controls (19.9 +/- 3.8 vs. 73.1 +/- 13.3 IU, p < 0.05). Lipid analyses of the myocardium showed a significant increase in the total amount of non esterified fatty acids (NEFA) in both untreated and mepacrine treated hypoxic hearts compared to normoxic controls, but to a significantly lower level in the mepacrine treated hearts. However, contribution of polyunsaturated NEFAs to total NEFAs did not differ between the groups. Also, neither total amount of fatty acids nor amount of polyunsaturated fatty acids obtained from the 2-position of the phospholipid fraction differed between the treated and untreated groups. In an enzyme assay, mepacrine 1 microM did not inhibit phospholipase A2 activity. We conclude that in our model mepacrine protects the heart from hypoxic injury, but probably by another mechanism than inhibition of phospholipase A2 induced membrane damage.

Potassium channel blocker dofetilide does not abolish ischaemic preconditioning

Ischaemic preconditioning (IP) is a powerful mechanism for infarct reduction. Enhanced K+ conductance and shortening of action potential duration in the early phase of the sustained ischaemic episode have been proposed as important factors in the IP mechanism for infarct reduction. We have investigated whether the potassium channel-blocking class III anti-arrhythmic agent dofetilide could abolish IP in an in situ rabbit heart infarct model. Dofetilide is a specific blocker of the delayed rectifier potassium channel and thus lengthens the action potential duration by reducing potassium conductance during repolarization. Anaesthetized, open-chest rabbits were subjected to 30 min of regional ischaemia and 180 min of reperfusion. The ischaemic risk zone was determined by fluorescent particles, and infarct size was determined by TTC staining. Three groups were investigated: control, ischaemic preconditioned (IP) and IP plus dofetilide-treated (IPdof). The preconditioning protocol was 5 min regional ischaemia and 10 min reperfusion. The IPdof group underwent the same preconditioning protocol but additionally received dofetilide 20 micrograms kg-1 i.v. during the first 2 min of the first reperfusion period. Compared to pre-drug values dofetilide increased monophasic action potential duration from 149.2 +/- 11.5 ms (n = 4) to 215.8 +/- 12.4 ms, supporting blockade of the delayed rectifier potassium channel. At the same time heart rate was decreased from 255.5 +/- 12.5 to 230.3 +/- 8.2. The results expressed as percent infarction of the risk zone +/- SEM for the different groups were as follows: control (n = 11), 42.4 +/- 7.1; IP (n = 6), 7.6 +/- 4.3 [symbol: see text]; IPdof (n = 7), 12.3 +/- 4.1 [symbol: see text] (*p < or = 0.05 vs. control). These results show that the potassium channel-blocking agent dofetilide given after the preconditioning ischaemia but before the sustained ischaemia does not abolish ischaemic preconditioning.

Preischemic bradykinin and ischaemic preconditioning in functional recovery of the globally ischaemic rat heart

OBJECTIVES

Substantial release of bradykinin has been demonstrated to occur during short periods of myocardial ischaemia in various species. The aim of the present study was to investigate the protective effect of bradykinin in ischaemia and whether bradykinin could be involved in ischaemic preconditioning in the rat heart.

METHODS

Isolated, buffer-perfused hearts were subjected to 30 min of global ischaemia, followed by 30 min of reperfusion. Postischaemic functional recovery was recorded in the following groups: (1) control; (2) treatment with 0.1 microM bradykinin for 10 min before ischaemia (BK); (3) bradykinin treatment combined with pretreatment with the specific bradykinin B2-receptor antagonist, HOE 140; (4) ischaemic preconditioning by 5 min ischaemia +5 min reperfusion prior to sustained ischaemia (i.p.); and (5) ischaemic preconditioning combined with HOE 140 administration.

RESULTS

Postischaemic myocardial function was significantly improved in both BK and i.p. groups (developed pressure 66.9 +/- 6.8 and 67.6 +/- 7.1 mmHg, respectively, vs. 43.1 +/- 5.9 mmHg in controls, P < 0.05). Pretreatment with 1 microM HOE 140 completely abolished the effect of bradykinin, while protection achieved by i.p. was unaltered by this drug. None of the protective interventions was associated with any significant improvement in myocardial adenosine triphosphate, creatine phosphate, glycogen, lactate or glucose tissue levels, detected either at the end of ischaemia or after 30 min of reperfusion.

CONCLUSIONS

Bradykinin, acting via B2-receptors, can protect against postischaemic contractile dysfunction to a similar extent as i.p.. An involvement of B2-receptors in the ischaemic preconditioning phenomenon could, however, not be demonstrated.

Bradykinin protects against infarction but does not mediate ischemic preconditioning in the isolated rat heart

The aim of the study was to test if pre-ischemic treatment with bradykinin can protect against infarction in an isolated rat heart model of regional ischemia and reperfusion, and if any such protection is dependent upon activation of protein kinase C (PKC) or mediated through the nitric oxide (NO) pathway. We also investigated if bradykinin B2 receptor activation, alone or in combination with activation of adenosine receptors and alpha-adrenoceptors, are involved in the infarct size reducing effect of ischemic preconditioning. Buffer-perfused rat hearts were subjected to 30 min regional ischemia and 120 min reperfusion. Risk zone was determined by fluorescent particles and infarct size by tetrazolium staining. Treatment with bradykinin (0.5 mumol/l) prior to ischemia significantly reduced infarct size in percentage of risk zone compared to control experiments (infarct size: 9.6 +/- 1.3% v 41.8 +/- 3.6%, P < 0.001). An inhibitor of NO synthesis, NOARG (100 mumol/l), did not interfere with the bradykinin induced protection (infarct size: 13.3 +/- 2.0%), while chelerythrine (2 mumol/l), an inhibitor of protein kinase C, reversed the effect of bradykinin (infarct size: 30.0 +/- 2.8%). NOARG did not influence infarct size in the control group (infarct size: 40.1 +/- 3.2%). Ischemic preconditioning with three cycles of 5 min global ischemia + 5 min reperfusion offered protection similar to bradykinin (infarct size: 8.4 +/- 2.0%). The bradykinin antagonist HOE 140 (1 mumol/l) reversed the effect of bradykinin (infarct size: 42.5 +/- 3.1%), but did not interfere with ischemic preconditioning (infarct size: 7.7 +/- 1.6%). Similarily, combined blockade of alpha-adrenergic, adenosine and bradykinin B2 receptors with p-benzamine (10 mumol/l). SPT (100 mumol/l) and HOE 140 did not interfere with ischemic preconditioning (infarct size: 7.8 +/- 1.1%). Thus, bradykinin can protect against infarction via protein kinase C, but independently of NO. A role for bradykinin in mediating ischemic preconditioning against infarction could not be demonstrated.

Endothelin-1 can reduce infarct size through protein kinase C and KATP channels in the isolated rat heart

OBJECTIVE

Protection from ischaemic preconditioning (IP) is dependent on activation of protein kinase C (PKC), and preconditionings protection can be mimicked by stimulation of various membrane receptors which are known to activate PKC. It is well known that KATP channel activation is cardioprotective. We tested the hypothesis that preischaemic treatment with endothelin-1 (ET-1) can protect against infarction by a PKC-dependent mechanism and by activating KATP channels.

METHODS

Buffer-perfused isolated rat hearts were subjected to 30 min regional ischaemia and 120 min reperfusion. Risk zone was determined by fluorescent particles, and infarct size by TTC staining.

RESULTS

Treatment with ET-1 in a dose of 1 nM prior to ischaemia significantly reduced infarct size in % of the risk zone compared to the control group (infarct size: 14.1 +/- 2.6 vs. 41.9 +/- 3.4%), while ET-1 0.1 nM did not protect (infarct size: 40.9 +/- 3%). AS the protective dose of ET-1 resulted in a significant reduction of coronary flow, a control group with a similar preischaemic flow-reduction was included (infarct size: 48.1 +/- 4.2%). Both the nonselective ETA/ETB receptor antagonist bosentan (1 microM) and the ET(A)-receptor-selective antagonist BQ 123 (2 microM) abolished protection from ET-1 (infarct size: 43.3 +/- 3.5 and 41.3 +/- 3.3%, respectively), as did the PKC inhibitor chelerythrine (2 microM) (infarct size: 41.1 +/- 5.2%) and the KATP blocker 5-hydroxydecanoate (infarct size: 41.7 +/- 2.9%). None of the ET receptor antagonists bosentan and BQ-123 influenced infarct size alone (infarct size: 42.7 +/- 2.5 and 41.3 +/- 3.3%, respectively). IP, similarly to ET-1, reduced infarct size (infarct size: 6.1 +/- 1.4%), but the nonselective ET receptor antagonist bosentan did not interfere with preconditioning's protection (infarct size: 13.2 +/- 4.3%).

CONCLUSIONS

ET-1 treatment prior to ischaemia can protect against infarction via ETA receptors by a PKC-dependent mechanism and by activating KATP channels, but ET does not mediate IP in the isolated rat heart.

Blockade of the KATP-channel by glibenclamide aggravates ischemic injury, and counteracts ischemic preconditioning

Blocking of the KATP-channel with glibenclamide has been shown to abolish the infarct-reducing effect of ischemic preconditioning in dog and swine. In the rabbit the results have been divergent purportedly related to anaesthesia. The aim of this study was to investigate the importance of the KATP-channel in a rabbit model where anaesthesia was not a confounding factor. Isolated rabbit hearts perfused with a Krebs-Henseleit bicarbonate buffer were subjected to 30 min regional ischemia by ligating a coronary artery, followed by 120 min reperfusion. The preconditioning protocol was 5 min global ischemia and 10 min reperfusion. Glibenclamide (100 microM) was added to the perfusion solution before the preconditioning ischemia and stopped after 5 min regional ischemia. Infarcts were measured with tetrazolium staining and risk zones with fluorescent microspheres. The main results expressed as percent infarction of the risk zone +/- SEM for the different groups are as follows: control (n = 12) 26.8 +/- 3.2, ischemic preconditioning (IP) (n = 9) 7.3 +/- 1.5, (p < 0.05 vs. control), control + glibenclamide (n = 9) 46.9 +/- 7.3 (p < 0.05 vs. control), IP + glibenclamide (n = 10) 38.3 +/- 6.9 (p < 0.05 vs. IP). These results show that glibenclamide treatment aggravates ischemia. Also, under the influence of glibenclamide ischemic preconditioning was no longer effective in reducing infarct size in the isolated perfused rabbit heart.

Reduced infarct size in the rabbit heart in vivo by ethylisopropyl-amiloride. A role for Na+/H+ exchange

Inhibition of Na+/H+ exchange with amiloride analogues has been shown to offer functional protection during ischemia and reperfusion and reduce infarct size in isolated rat hearts and intact pigs. The aim of the present study was to examine if pre- or postischemic treatment with ethylisopropylamiloride (EIPA), a selective Na+/H+ exchange inhibitor, could reduce infarct size in an in situ rabbit model of regional ischemia and reperfusion. Anesthetized, open-chest rabbits were subjected to 30 min of regional ischemia and 180 min of reperfusion. The risk zone was determined by fluorescent particles, and infarct size was determined by TTC staining. Preischemic treatment with EIPA (0.65 mg/kg) significantly reduced infarct size from 45.8 +/- 3.5% of the risk zone in the control group to 10.6 +/- 3.1% (p < 0.01). EIPA-treatment during the first part of the reperfusion period did not reduce infarct size compared to controls (41.9 +/- 3.5%). We conclude that EIPA, when administered prior to ischemia, reduces infarct size in the rabbit heart of in situ, a protection most likely due to inhibition of Na+/H+ exchange.

Changes in blood flow distribution and capillary function after deep hypothermia in rat

The present experiments were carried out in the rat to investigate the peripheral vascular function prior to the development of posthypothermic circulatory collapse. In the first study, mean arterial blood pressure, heart rate, cardiac output, regional blood flow, and plasma volume of hypothermic (4 h, 15-13 degrees C) and rewarmed rats were compared with normothermic controls. In response to hypothermia, arterial blood pressure, heart rate, and cardiac output declined markedly. After rewarming, arterial blood pressure and heart rate recovered fully, whereas cardiac output was only 33 +/- 7% of the control value (p < 0.025). Tissue blood flow was markedly depressed during hypothermia (p < 0.025), except for the abdominal skin. After rewarming, blood flow in skeletal muscle returned to within control levels, whereas blood flow in internal organs remained low (p < 0.025 vs. control). Posthypothermic plasma volume was 77 +/- 3% of control (p < 0.05). In the second study, the transcapillary colloid osmotic pressure gradient (COPp-COPi) was calculated following measurement of colloid osmotic pressure in plasma (COPp) and interstitium (COPi) in prehypothermic, hypothermic, and posthypothermic rats. The posthypothermic value of COPp-COPi was 76 +/- 4% of the prehypothermic value (p < 0.05). In conclusion this study demonstrates that the reduced cardiac output in rewarmed rats is associated with an altered regional blood flow distribution compared with that of normal rats. Capillary integrity also seemed perturbed. Thus, changes in both control and function of the peripheral vasculature are important mechanisms in the development of a posthypothermic circulatory collapse.

Cardiac injury by activated leukocytes: effect of cyclooxygenase and lipoxygenase inhibition evaluated by electron microscopical morphometry

Leukocytes can take part in an inflammatory response in the heart after myocardial infarction or cardio-thoracic surgery. To investigate the injurious mechanism of activated polymorphonuclear leukocytes (PMN), isolated rat hearts were perfused with phorbol 12-myristate 13-acetate (PMA) activated PMN (3 x 10(6)/ml) alone for 10 min, in combination with a mixture of oxygen free radical scavengers (superoxide dismutase+catalase+thiourea) or in combination with ibuprofen (IBU), a cyclooxygenase inhibitor or diethylcarbamazine (DCM), a lipoxygenase inhibitor or BW 755C, a dual inhibitor of cyclooxygenase and lipoxygenase and an oxygen free radical scavenger. After 30 min of recovery, the hearts were perfusion-fixed with glutaraldehyde for electron microscopical examination. Based on examination of 25 micrographs per heart obtained by a random sampling procedure and on morphometric methods, volume fractions (Vv) of mitochondria (mito), altered mitochondria (alt mito), myofilament, and cellular edema were measured as fractions of myocyte volume. The most important finding was that Vv(alt mito/myocyte) was 0.09 +/- 0.16 and 0.02 +/- 0.04 in the hearts receiving PMN+PMA alone and when scavengers were added, respectively, whilst no changes in mitochondrial ultrastructure was observed after addition of IBU, BW 755C or DCM. Vv(mito/myocyte) was for PMN+PMA alone: 0.33 +/- 0.04, +scavengers: 0.29 +/- 0.02 +IBU:0.29 +/- 0.02, +BW 755C: 0.23 +/- 0.03*, +DCM: 0.28 +/- 0.02 (mean +/- S.D., *P < 0.05 compared to PMN+PMA). Capillary wall volume (cap wall) as a fraction of the whole capillary was also quantified. Vv(cap wall/cap) was for PMN+PMA alone: 0.26 +/- 0.06, +scavengers: 0.22 +/- 0.03, +IBU: 0.19 +/- 0.04*, +BW755C: 0.21 +/- 0.03, +DCM: 0.15 +/- 0.04* (*P < 0.05). These results further strengthen the notion that activated PMN are intravascularly active. In addition to exerting a cardiodepressive effect the present study shows that activated PMN can induce structural changes in the heart through the combined action of oxygen free radicals and arachidonic acid metabolites.

Experimental hypothermia and rewarming: changes in mechanical function and metabolism of rat hearts

Rewarming from accidental hypothermia is associated with fatal circulatory derangements. To investigate potential pathophysiological mechanisms involved, we examined heart function and metabolism in a rat model rewarmed after 4 h at 15-13 degrees C. Hypothermia resulted in a significant reduction of left ventricular (LV) systolic pressure, cardiac output, and heart rate, whereas stroke volume increased. The maximum rate of LV pressure rise decreased to 191 +/- 28 mmHg/s from a control value of 9,060 +/- 500 mmHg/s. Myocardial tissue content of ATP, ADP, and glycogen was significantly reduced, whereas lactate content remained unchanged. After rewarming, heart rate returned to control value, whereas LV systolic pressure, cardiac output, and stroke volume all remained significantly depressed. The posthypothermic maximum rate of LV pressure rise was 5,966 +/- 1.643 mmHg/s. The posthypothermic myocardial lactate content was significantly increased (to 13.3 +/- 3.2 nmol/mg from control value of 5.7 +/- 1.9 nmol/mg), and ATP and glycogen remained significantly lowered. Creatine phosphate or energy charge did not change significantly during the experiment. The finding of deteriorated myocardial mechanical function and a shift in energy metabolism shows that the heart could be an important target during hypothermia and rewarming in vivo, thus contributing to the development of a posthypothermic circulatory collapse.

Hydrogen peroxide as a protective agent during reperfusion. A study in the isolated perfused rabbit heart subjected to regional ischemia

In spite of extensive research during the last decade it has not been possible to prove that endogenously generated hydrogen peroxide or any reduced oxygen species reaches sufficient concentration during reperfusion after myocardial ischemia to contribute significantly to irreversible cell injury. In an attempt to further test this hypothesis we subjected isolated perfused rabbit hearts to 30 min regional ischemia followed by reperfusion and supplied hydrogen peroxide in low levels with or without catalase during the first 30 min of reperfusion and thereafter continued the reperfusion for a total of 120 min. Five different groups were studied: controls, and hearts supplied with 2 microM H2O2, 1 microM H2O2, 1 microM H2O2 + catalase (IU/l) or catalase alone in the initial part of the reperfusion. At the end of 120 min reperfusion, area at risk was measured with fluorescent particles and infarct zone size with tetrazolium staining. The results were: in the control group 32 +/- 5.0% of the risk zone infarcted, in the 2 microM H2O2 group 16.3 +/- 5.6% and in the 1 microM H2O2 group 6.9 +/- 0.8% (P < 0.05 compared to control). The reduction in infarct size was not present when catalase was added to the hydrogen peroxide-containing solution (26.4 +/- 4.5) or if catalase was present alone (22.9 +/- 1.8% infarction). In conclusion, hydrogen peroxide, 1 microM, protected the heart during reperfusion and reduced the amount of cell death after 120 min of reperfusion. The study demonstrated reduction or delay in infarction based only on treatment in the reperfusion period. The mechanism behind this protection remains to be determined.

Ultrastructure of reperfused skeletal muscle: the effect of oxygen radical scavenger enzymes

The aim of the present study was (1) to examine changes in ultrastructure and subcellular volume fractions in reperfused skeletal muscle and (2) to examine the effect of pre- plus postischaemic treatment with oxygen radical scavenger enzymes. Experiments were performed in 8 pentobarbital anesthetized dogs. The gracilis muscle was denervated and vascularly isolated on both lower limbs, except for the main artery and vein. A catheter was advanced through the distal end of the femoral artery for supply of superoxide dismutase (SOD) and catalase (each enzyme 12,000 U/10 min prior to ischaemia and 36,000 U/30 min at reperfusion) through the muscle artery to the muscle on one side. A catheter in the muscle vein secured the collection of venous blood to avoid a supply of SOD and catalase to the other muscle. Ischaemia was induced on both sides by clamping the branch of the femoral artery supplying the muscle for 4 h, and the muscle was thereafter reperfused for 1 h. Ultrastructural examination (morphometric by point counting and semiquantification) of the reperfused tissue revealed mitochondrial damage, margination of nuclear chromatin, lipid droplets, filamental disruption, as well as oedema in both endothelial and myofibrillar cells. In addition some influence of treatment with SOD and catalase could be seen, mainly in reduction of the number of micrographs with cellular oedema and especially oedema in endothelial cells.

Ischaemic preconditioning is protein kinase C dependent but not through stimulation of alpha adrenergic or adenosine receptors in the isolated rat heart

OBJECTIVE

The aim was (1) to clarify whether alpha adrenoceptor and adenosine receptor stimulation is involved in the anti-infarct effect of ischaemic preconditioning in the rat heart, and (2) to test the hypothesis that signal transduction through membrane bound protein kinase C is essential for the protection.

METHODS

Isolated, buffer perfused rat hearts were subjected to 30 min of regional ischaemia and 120 min of reperfusion. The risk zone was determined by fluorescent particles, and infarct size was determined by staining with triphenyltetrazolium chloride.

RESULTS

Ischaemic preconditioning with three cycles of 5 min ischaemia plus 5 min of reperfusion significantly reduced infarct size as compared to non-preconditioned group [4.5(SEM 0.6)% of the risk zone v 45.5(5.7)%, P < 0.001]. Blockade of alpha adrenoceptors alone and simultaneous blockade of alpha adrenoceptors with phenoxybenzamine (10 microM) and adenosine receptors with sulphophenyltheophylline (100 microM) did not prevent the protective effect of ischaemic preconditioning [infarct size = 2.4(0.4) and 5.6(1.9)% respectively, NS v the non-treated preconditioned group]. Blocking either the membrane binding of protein kinase C with polymyxin B (1 microM) or direct inhibition of protein kinase C activity with chelerythrine (2 microM) completely abolished the infarct size reducing effect of ischaemic preconditioning [32.4(3.3)% and 48.2(4.0)% respectively, P < 0.005 v non-treated preconditioned group: NS v the non-preconditioned group].

CONCLUSIONS

In the rat heart infarct model the protective effect of ischaemic preconditioning is not mediated through stimulation of alpha adrenoceptors alone or the combined stimulation of alpha adrenergic and adenosine receptors, and it is dependent on activation of membrane bound protein kinase C.