Does protein kinase C play a pivotal role in the mechanisms of ischemic preconditioning?

This communication reviews the evidence for the pivotal role of protein kinase C in ischemic myocardial preconditioning. It is believed that several intracellular signalling pathways via receptor-coupled phospholipase C and its "cross-talk" with phospholipase D converge to activation of protein kinase C isotypes which is followed by phosphorylation of until now (a number of) unknown target proteins which produce the protective state of ischemic preconditioning. After briefly introducing the general biochemical properties of protein kinase C, its isotypes and the limitations of the methodology used to investigate the role of protein kinase C, studies are discussed in which pharmacological inhibition and activation and (immunore) activity and/or isotypes measurements of protein kinase C isotypes were applied to assess the role of activation of protein kinase C in ischemic myocardial preconditioning. It is concluded that definitive proof for the involvement of protein kinase C in preconditioning requires future studies which must focus on the isotype(s) of protein kinase C that are activated, the duration of action, cellular translocation sites and the identity and stability (of covalently bound phosphate) of phosphorylated substrate proteins.

Influence of data processing on cyclic variation of integrated backscatter and wall thickness in stunned porcine myocardium

This study was performed to investigate the relationship between the cyclic variation of integrated backscatter and myocardial wall thickening in stunned myocardium. Different definitions of cyclic variation were evaluated to be able to compare with other studies. Ultrasound data were acquired from 10 open-chested Yorkshire pigs (25-33 kg) at baseline, during regional ischemia and during 30 min of stunning, using a broadband ultrasound transducer (3-7 MHz) sutured directly upon the left ventricular myocardial wall. Cyclic variation of integrated backscatter and myocardial wall thickening were calculated using three definitions obtained from the literature. Independent of the definition, cyclic variation of wall thickness and integrated backscatter were blunted during acute ischemia and returned transiently to or above baseline during the first minute of reperfusion, followed by a gradual decrease to a level under baseline during stunning. An early return of the cyclic variation of the integrated backscatter was not observed in pigs, independent of the data processing used. The relationship between integrated backscatter and wall thickness was maintained.

Myocardial protection by brief ischemia in noncardiac tissue

BACKGROUND

Brief coronary artery occlusions (CAOs) protect both the artery's own perfusion territory ("myocardial preconditioning") and adjacent "virgin" myocardium. Whether ischemia in remote organs protects myocardium is unknown. We examined whether brief occlusion of the anterior mesenteric artery (MAO) or left renal artery (RAO) protects against myocardial infarction.

METHODS AND RESULTS

Area at risk (AR) and infarcted area (IA) were determined in anesthetized rats after 180 minutes of reperfusion following a 60-minute CAO. At normothermia (body temperature, 36.5 degrees C to 37.5 degrees C), IA/AR was 68 +/- 2% (mean +/- SEM, n = 11) in control rats and 50 +/- 3% (n = 9, P < .001) in rats preconditioned by 15-minute CAO 10 minutes before 60-minute CAO. A 15-minute MAO was equally protective (IA/AR = 50 +/- 3%, n = 10, P < .001), whereas 15-minute RAO failed to limit IA/AR (72 +/- 5%, n = 8). Hypothermia (body temperature, 30 degrees C to 31 degrees C) did not affect IA/AR (67 +/- 3%, n = 11) in control animals but enhanced protection by 15-minute CAO (IA/AR = 22 +/- 3%, n = 8), whereas protection by 15-minute MAO (IA/AR = 44 +/- 5%, n = 11, P < .001) was minimally enhanced. Hypothermia unmasked protection by 15-minute RAO (IA/AR = 46 +/- 6%, n = 9, P < .01). Hexamethonium (20 mg/kg IV) did not alter protection by 15-minute CAO, but it abolished protection by 15-minute MAO. When MAO was sustained throughout the study, cardioprotection was absent.

CONCLUSIONS

Brief ischemia in "remote" organs protects myocardium against infarction as effectively as myocardial preconditioning. The mechanism of protection by MAO differs from that of CAO, because ganglion blockade abolished protection by MAO but not by CAO. The neurogenic pathway is activated during reperfusion after 15-minute MAO, because sustained MAO failed to produce cardioprotection.

Cardioprotection by ischemic and nonischemic myocardial stress and ischemia in remote organs. Implications for the concept of ischemic preconditioning

Ischemic preconditioning studies employ one or more brief total coronary artery occlusions separated by complete reperfusion to limit infarct size during a subsequent prolonged coronary artery occlusion. We now present evidence that in anesthetized pigs a partial coronary artery occlusion without intervening reperfusion between the partial and prolonged total occlusions can also precondition the myocardium provided that the reduction in coronary blood flow is sufficiently severe. Thus infarct size was reduced after a 60 min total coronary artery occlusion when the total occlusion was preceded by a partial coronary occlusion that reduced coronary blood flow by 70% but not when the flow reduction was only 30%. In this two-stage coronary occlusion model the degree of protection appears greater in the epicardial than in the endocardial half. In view of evidence that brief occlusions of a coronary artery also protect myocardium outside its perfusion territory, we subsequently investigated whether ischemia in remote organs can protect myocardium. Because of reports that development of infarct size may be temperature dependent, we also investigated whether the cardioprotection by remote organ ischemia was temperature dependent. In anesthetized rats a 15 min coronary artery occlusion was more effective in reducing infarct size produced by a subsequent 60 min total coronary artery occlusion when the experiments were performed at a body core temperature of 30-31 degrees C than at 36-37 degrees C, while infarct size of animals which were subjected to only the 60 min total coronary artery occlusion was the same for the two body core temperatures. In rats with a body core temperature of 36-37 degrees C a 15 min mesenteric artery occlusion, but not a 15 min renal artery occlusion, reduced infarct size produced by a subsequent 60 min coronary artery occlusion. When the experiments were performed at 30-31 degrees C both the mesenteric and renal artery occlusions were protective. These observations indicate the local myocardial ischemia is not required to protect the myocardium during a prolonged coronary occlusion. We further investigated whether myocardium could also be protected by a cardiac stimulus which does not produce ischemia at all. For this purpose we electrically paced the left ventricle of anesthetized pigs to produce heart rates of 200 bpm (which did not lead to ischemia as assessed by a number of functional and biochemical variables) and found that 30 min of ventricular pacing reduced myocardial infarct size produced by a subsequent 60 min coronary artery occlusion. The protection by ventricular pacing involved activation of K+ATP channels as pretreatment with glibenclamide abolished the protection by ventricular pacing. We conclude that a number of distinctly different stimuli can protect the myocardium suggesting that ischemic myocardial preconditioning could be just one feature of a more general protection phenomenon.

The effect of a thiadiazinone derived Ca2+ sensitizer on the responsiveness of Mg(2+)-ATPase to Ca2+ in myofibrils isolated from stunned and nonstunned porcine and human myocardium

Previously, we showed, in an in situ porcine model, that the thiadiazinone derivative [+]EMD 60263, a putative Ca2+ sensitizer with minimal phosphodiesterase III inhibitory properties, increased contractility more profoundly in stunned than in nonstunned myocardium. The aim of the present investigation was to study the mechanism of action by determining the in vitro effects of [+]EMD 60263 on the Ca2+ responsiveness of the Mg(2+)-dependent ATPases of myofibrils and sarcoplasmic reticulum membrane vesicles, isolated from normal ventricle of swine and hypertrophic septum of cardiomyopathic patients. Contamination of the myofibrils with sarcoplasmic reticulum membranes was excluded by testing the effect of the sarcoplasmic reticulum Ca(2+)-pumping ATPase inhibitor thapsigargin. The plasma concentrations at which [+]EMD 60263 exerted its inotropic effect in the in situ porcine model were found to be submicromolar. [+]EMD 60263 stimulated concentration-dependently (1-10 microM) the submaximally activated Mg(2+)-ATPases (at pCa 6.1) of pig heart myofibrils. [+]EMD 60263 (10 microM) shifted the pCa50 of porcine myofibrillar Ca(2+)-stimulated, Mg(2+)-dependent ATPase from 6.00 +/- 0.05 to 6.67 +/- 0.05, whereas the [-]enantiomer EMD 60264 had no significant effect. Although the effect was much less at 1 and 3 microM, [+]EMD 60263 (10 microM) also stimulated maximal myofibrillar Mg(2+)-ATPase activity. The Hill coefficient, reflecting the steepness of the fitted pCa/Mg(2+)-ATPase curve at half-maximal activation, was not affected by [+]EMD 60263 (10 microM). [+]EMD 60263 (10 microM) had no effect on sarcoplasmic reticulum Ca(2+)-stimulated, Mg(2+)-dependent ATPase from swine heart. The thiadiazinone derivative [+]EMD 57033 (10 microM), but not its [-]enantiomer EMD 57439, had similar, although less potent, effects on pig heart myofibrillar Mg(2+)-ATPase activity as compared to [+]EMD 60263. [+]EMD 60263 (3 microM) produced a significantly larger leftward shift of the pCa2+/Mg(2+)-ATPase activity curve of myofibrils isolated from the stunned compared to the adjacent nonstunned myocardium (Delta pCa50s caused by the presence of [+]EMD 60263 amounted to +0.57 +/- 0.04 and +0.42 +/- 0.05, respectively) in the in situ porcine model. The effects of [+]EMD 60263 on myofibrillar Mg(2+)-ATPase of hypertrophic human heart were identical to those observed with porcine heart myofibrils. The results indicate that the positive inotropic action of [+]EMD 60263 observed in the in situ porcine model of stunned myocardium may be primarily due to myofilament sensitization to Ca2+, and that this compound may have a similar action on diseased human myocardium.

Coordinated expression of heme oxygenase-1 and ubiquitin in the porcine heart subjected to ischemia and reperfusion

Heme oxygenase (HO) isozymes, HO-1 and HO-2 catalyze the cleavage of heme b to form the antioxidant biliverdin IXa, iron and the putative cellular messenger carbon monoxide (CO). Heat and stress have been reported to induce the expression of HO-1, in analogy to ubiquitin, a protein of 8 kDa involved in ATP dependent proteolysis. Earlier, we have shown in anesthetized pigs that brief periods of coronary artery occlusion followed by reperfusion produce prolonged regional cardiac dysfunction (stunning) associated with altered expression of a number of genes. In the present study, we report on a coordinated expression pattern of HO-1 and ubiquitin in the same porcine model in which the left anterior descending coronary artery (LAD) was occluded for 10 min and reperfused for 30 min (group I) and after a second occlusion of 10 min, reperfused for either 30 min (group II) or 90 min (group III) or 210 min (group IV). Myocardial tissue from LAD (stunned) and left circumflex coronary artery (LCx, control) perfused regions were collected in liquid nitrogen and analysed by Northern and dot blot hybridization techniques. We demonstrated a basal myocardial expression of multiple mRNAs (monomer and polymers) encoding ubiquitin and a single mRNA species (1.8 kb) encoding HO-1. However, the expression of both genes was drastically enhanced in the stunned myocardium as compared to the control in groups II and III with maximum mRNAs levels in group II. These results suggest that the myocardial adaptive response to ischemia involves the coordinated induction of HO-1 and ubiquitin, which may be indicative for the existence of a pathophysiologically important defense mechanism whereby, both degradation of denatured cellular proteins and generation of biologically active products of heme metabolism are accelerated.

Assessment of the role of the renin-angiotensin system in cardiac contractility utilizing the renin inhibitor remikiren

1. The role of the renin-angiotensin system in the regulation of myocardial contractility is still debated. In order to investigate whether renin inhibition affects myocardial contractility and whether this action depends on intracardiac rather than circulating angiotensin II, the regional myocardial effects of systemic (i.v.) and intracoronary (i.c.) infusions of the renin inhibitor remikiren, were compared and related to the effects on systemic haemodynamics and circulating angiotensin II in open-chest anaesthetized pigs (25-30 kg). The specificity of the remikiren-induced effects was tested (1) by studying its i.c. effects after administration of the AT1-receptor antagonist L-158,809 and (2) by measuring its effects on contractile force of porcine isolated cardiac trabeculae. 2. Consecutive 10 min i.v. infusions of remikiren were given at 2, 5, 10 and 20 mg min-1. Mean arterial pressure (MAP), cardiac output (CO), heart rate (HR), systemic vascular resistance (SVR), myocardial oxygen consumption (MVO2) and left ventricular (LV) dP/dtmax were not affected by remikiren at 2 and 5 mg min-1, and were lowered at higher doses. At the highest dose, MAP decreased by 48%, CO by 13%, HR by 14%, SVR by 40%, MVO2 by 28% and LV dp/dtmax by 52% (mean values; P < 0.05 for difference from baseline, n = 5). The decrease in MVO2 was accompanied by a decrease in myocardial work (MAP x CO), but the larger decline in work (55% vs. 28%; P < 0.05) implies a reduced myocardial efficiency ((MAP x CO)/MVO2). 3. Consecutive 10 min i.c. infusions of remikiren were given at 0.2, 0.5, 1, 2, 5 and 10 mg min-1. MAP, CO, MVO2 and LV dP/dtmax were not affected by remikiren at 0.2, 0.5 and 1 mg min-1, and were reduced at higher doses. At the highest dose, MAP decreased by 31%, CO by 26%, MVO2 by 46% and LV dP/dtmax by 43% (mean values; P < 0.05 for difference from baseline, n = 6). HR and SVR did not change at any dose. 4. Thirty minutes after a 10 min i.v. infusion of the AT1 receptor antagonist, L-158,809 at 1 mg min-1, consecutive 10 min i.c. infusions (n = 5) of remikiren at 2, 5 and 10 mg min-1 no longer affected CO and MVO2, and decreased LV dP/dtmax by maximally 27% (P < 0.05) and MAP by 14% (P < 0.05), which was less than without AT1-receptor blockade (P < 0.05). HR and SVR remained unaffected. 5. Plasma renin activity and angiotensin I and II were reduced to levels at or below the detection limit at doses of remikiren that were not high enough to affect systemic haemodynamics or regional myocardial function, both after i.v. and i.c. infusion. 6. Remikiren (10(-10) to 10(-4) M) did not affect contractile force of porcine isolated cardiac trabeculae precontracted with noradrenaline. In trabeculae that were not precontracted no decrease in baseline contractility was observed with remikiren in concentrations up to 10(-5) M, whereas at 10(-4) M baseline contractility decreased by 19% (P < 0.05). 7. Results show that with remikiren i.v., at the doses we used, blood pressure was lowered primarily by vasodilation and with remikiren i.c. by cardiac depression. The blood levels of remikiren required for its vasodilator action are lower than the levels affecting cardiac contractile function. A decrease in circulating angiotensin II does not appear to be the sole explanation for these haemodynamic responses. Data support the contention that myocardial contractility is increased by renin-dependent angiotensin II formation in the heart.

Reduction in thrombotic events with heparin-coated Palmaz-Schatz stents in normal porcine coronary arteries

BACKGROUND

The use of stents improves the result after balloon coronary angioplasty. Thrombogenicity of stents is, however, a concern. In the present study, we compared stents with an antithrombotic coating with regular stents.

METHODS AND RESULTS

Regular stents were placed in coronary arteries of pigs receiving no aspirin (group 1; n = 8) or aspirin over 4 weeks (group 2, n = 10) or 12 weeks (group 3, n = 9). Stents coated with heparin (antithrombin III uptake, 5 pmol/stent) were placed in 7 pigs that did not receive aspirin (group 4). The other animals received aspirin and coated stents with a heparin activity of 12 pmol antithrombin III/stent (group 5, n = 10) or 20 pmol/stent (group 6, n = 10; group 7, n = 10). Quantitative arteriography was performed at implantation and after 4 (groups 1, 2, and 4 through 6) or 12 weeks (groups 3 and 7). In an additional 5 animals, five regular and five coated stents (20 pmol/stent) were placed and explanted after 5 days for examination of the early responses to the implants. Thrombotic occlusion of the regular stent occurred in 9 of 27 in groups 1 through 3. However, in 0 of 30 of the animals receiving high-activity heparin-coated stents (groups 5 through 7), thrombotic stent occlusion was observed (P < .001). Histological analysis at 4 weeks showed that the neointima in group 6 was thicker compared with its control group 2 (259 +/- 104 and 117 +/- 36 microns, P < .01), but at 12 weeks the thickness was similar (152 +/- 61 and 198 +/- 49 microns, respectively). Comparison at 5 days suggested delayed endothelialization of the coating.

CONCLUSIONS

High-activity heparin coating of stents eliminates subacute thrombosis in porcine coronary arteries.

The relationship between myocardial integrated backscatter, perfusion pressure and wall thickness during isovolumic contraction: an isolated pig heart study

To investigate the independent effect of myocardial wall thickness and myocardial perfusion pressure on integrated backscatter, experiments were designed in which integrated backscatter of normally perfused myocardial tissue was measured while changes in wall thickness during the cardiac cycle were reduced to a minimum. In nine blood-perfused isolated pig hearts, perfusion pressure was uncoupled from left ventricular pressure generation (Langendorff method) and isovolumic contraction and relaxation were realized by inserting a noncompressible water-filled balloon into the left ventricle. In a first experiment, at constant perfusion pressure (85 mmHg), the integrated backscatter (3-7 MHz), the myocardial wall thickness and the left ventricular pressure were determined simultaneously at various balloon volumes (5-25 mL). A quasistatic increase of balloon volume by 50% resulted in an average decrease of wall thickness of 6.5% (p < 0.01) and a mean increase in the integrated backscatter level of 1.1 dB (p < 0.01). Integrated backscatter levels increased statistically significant by 0.14 +/- 0.014 dB per percent decrease of wall thickness. Measurements of percentage end-systolic myocardial wall thickening ranged from -10% to +10%, mean 0.15 +/- 4.5% (NS from zero); whereas cyclic variation of integrated backscatter ranged from -3.9 to +3.9 dB, mean 0.19 +/- 1.5 dB (NS from zero). In a second experiment, at a constant midrange balloon volume, the same parameters were determined simultaneously at various perfusion pressures (20-120 mmHg). An increase in perfusion pressure by 50% resulted in a small but statistically significant increase of 1.5% in myocardial wall thickness, which could be explained by an increase of intravascular volume. The integrated backscatter levels did not change statistically significantly. Measurements of percentage end-systolic myocardial wall thickening ranged from -8.9 to +7.8%, mean 0.13 +/- 4.0% (NS from zero); whereas cyclic variation of integrated backscatter ranged from -1.8 to +4.2 dB, mean 0.37 +/- 1.3 dB (NS from zero). The magnitude of cyclic variation of integrated backscatter of myocardial tissue in a contractile state is reduced if myocardial muscle is prevented from normal thickening. In addition, changes in intravascular volume during the cardiac cycle have a negligible influence on the absolute backscatter level or its cyclic variation. We conclude, if only wall thickness and perfusion pressure are involved, that integrated backscatter is mainly determined by myocardial wall thickness.

Rapid ventricular pacing produces myocardial protection by nonischemic activation of KATP+ channels

BACKGROUND

Rapid ventricular pacing reduces the incidence of ventricular arrhythmias during a subsequent sustained period of ischemia and reperfusion. We investigated whether rapid ventricular pacing also limits myocardial infarction and determined the role of KATP+ channels in the protection afforded by ventricular pacing.

METHODS AND RESULTS

Myocardial infarction was produced by a 60-minute coronary artery occlusion in open chest pigs. Infarct size of pigs subjected to 10 minutes of ventricular pacing at 200 beats per minute followed by 15 minutes of normal sinus rhythm before the occlusion (79 +/- 3% of the area at risk, mean +/- SEM) was not different from control infarct size (84 +/- 2%). Thirty-minute pacing followed by 15-minute sinus rhythm resulted in modest reductions in infarct size (71 +/- 2%, P<.05 versus control). Thirty minutes of pacing immediately preceding the occlusion without intervening sinus rhythm resulted in considerable limitation of infarct size (63 +/- 4%, P<.05), which was abolished by pretreatment with the KATP+ channel blocker glibenclamide (78 +/- 4%, P=NS). KATP+ channel activation did not appear to involve ischemia: (1) myocardial endocardial/epicardial blood flow ratio was 1.07 +/- 0.08, (2) phosphocreatine and ATP levels and arterial-coronary venous differences in pH and PCO2 were unchanged, (3) end-systolic segment length did not increase and postsystolic shortening was not observed during pacing, and (4) systolic shortening recovered immediately to baseline levels and coronary reactive hyperemia was absent after cessation of pacing. Administration of glibenclamide after 30 minutes of pacing at the onset of 15 minutes of normal sinus rhythm did not attenuate the protection (73 +/- 3%, P<.05 versus control), suggesting the KATP+ channels did not contribute to the moderate degree of protection that was still present 15 minutes after cessation of pacing.

CONCLUSIONS

Rapid ventricular pacing protects the myocardium against infarction via nonischemic KATP+ channel activation. Continued activation of KATP+ channels does not appear mandatory for the protection that is still present 15 minutes after cessation of pacing.

Endocardial and epicardial infarct size after preconditioning by a partial coronary artery occlusion without intervening reperfusion. Importance of the degree and duration of flow reduction

OBJECTIVE

Recently, we reported that a partial coronary artery occlusion immediately preceding a sustained coronary artery occlusion limited infarct size. We now investigated whether the protection by partial coronary artery occlusions (i) depends on the severity and(or) duration of the flow reduction and (ii) varies in the different myocardial layers.

METHODS

In 71 open-chest pigs (eight groups) left ventricular area at risk (AR) and infarct area (IA) were determined for the endocardial (IAendo and ARendo) and epicardial halves (IAepi and ARepi).

RESULTS

In control animals (60 min total coronary artery occlusion (TCO) followed by 120 min reperfusion (Rep)) there were highly linear relations between IA and AR in the endocardium (r = 0.98, P < 0.01) and epicardium (r = 0.97, P < 0.01), which could be described by IAendo = 1.01 ARendo - 4.5 and by IAepi = 0.88ARepi - 3.6, respectively. In animals that underwent a 10 min TCO + 15 min Rep prior to the 60 min TCO + 120 min Rep, IA in both myocardial layers were again highly linearly related with AR, with less steep slopes for both the endocardium (0.63) and epicardium (0.57) (both P < 0.01). Two groups of pigs were subjected to either a 30 or 90 min 70% reduction in coronary blood flow (FR) immediately preceding the 60 min TCO + 120 min Rep, without intervening reperfusion. A 30 min 70% FR decreased IA to the same degree in the endo- and epicardial half. A 90 min 70% FR resulted in protection in the epicardium (P < 0.01) but not in the endocardium, most likely because 90 min 70% FR without 60 min TCO already caused infarction which was more severe in the endo- than in the epicardium (P < 0.01). Endocardial and epicardial IA after either a 30 or 90 min 30% FR prior to the 60 min TCO was not different from that in the control group, indicating that this mild flow reduction failed to limit irreversible damage.

CONCLUSIONS

Thirty or ninety min of severe (70%) but not mild (30%) coronary flow reductions protected against myocardial infarction. The protection by a 70% FR was influenced by the duration of FR as a 30 min 70% FR similarly decreased IA in the endocardial and epicardial halves, while 90 min 70% FR preferentially limited IA in the epicardial half. These findings suggest that perfusion abnormalities immediately preceding an infarction could be an important source of infarct size variability in patients.

Ischaemic preconditioning: is it clinically relevant?

Direct clinical evidence for the classical preconditioning phenomenon, with infarct size limitation as an endpoint, cannot be obtained. However, a number of patient groups have been identified in which adaptation to ischaemia has been demonstrated by enhanced recovery of function or preservation of high energy phosphates in models of repeated ischaemia, such as atrial pacing stress tests, percutaneous transluminal coronary angioplasty and aortic cross-clamping during cardiac surgery. Evidence is accumulating that mechanisms which are operative in experimental ischaemic preconditioning (infarct size limitation) are also operative in these clinical models of repeated reversible ischaemia. Insight into the mechanisms responsible for ischaemic preconditioning could potentially help to develop pharmacological agents which mimic preconditioning. This is especially attractive as several of the ischaemic episodes maybe too short or insufficiently severe to trigger preconditioning. By a synergistic or additive action, the combination of such a stimulus with a low dose of pharmacological agent might result in protective action. If these agents were also to be used for treating cardiovascular conditions, such as the K+ATP channel activator nicorandil for the treatment of angina pectoris, the cardioprotective effect could be a beneficial side effect. The currently available protein kinase C activators are oncogenic, but with the recognition and better understanding of the different subtypes possibly involved in preconditioning, new protein kinase C activators may become available without these side-effects. On the other hand, hearts of patients who regularly experience episodes of ischaemia may be in a more or less permanent state of preconditioning afforded by one of these stimuli or have developed tolerance. In this situation it is likely that (additional) protection by a pharmacological agent cannot be accomplished at that time. It is reassuring, however, that in the animal, preconditioning can be reinstated immediately after the cardioprotection is lost and that it can also be demonstrated in hearts with pathological conditions such as hypertrophy. Finally, in view of the observations that cardioprotection may also be produced by transient ischaemia in other organs, and even by some forms of stress which do not lead to myocardial ischaemia, it could be envisioned that ischaemic preconditioning is only one component of a general form of adaptation.

Insulin-like growth factor II is an experimental stress inducible gene in a porcine model of brief coronary occlusions

OBJECTIVE

Previous observations have shown that myocardium activates many adaptive processes after brief ischaemia. The aim of this study was to determine whether insulin-like growth factors (IGF) as well as their receptors and binding proteins (IGFBP), which control the activity of the IGF, may play an important role during these processes.

METHODS

Ischaemia was induced in anaesthetised open chest pigs by two 10 min occlusions of the left anterior descending coronary artery, separated by 30 min of reperfusion, and followed by reperfusion up to 210 min. Tissue from the ischaemic area and from a non-ischaemic control region of the same heart was examined by means of northern blot, slot blot, and in situ hybridisation.

RESULTS

IGF-I, IGF-II, the type I receptor, the insulin receptor, and IGFBP-2-6 are constitutively expressed in porcine myocardium. In situ hybridisation showed that IGF-I and IGF-II are mainly transcribed by myocytes. Ischaemia/reperfusion led to an early and significant increase in IGF-II mRNA compared to non-sham controls but not in comparison with sham operated animals, which already showed a (not significantly) enhanced IGF-II expression. In each case the IGF-II mRNA levels are equal in the control and the experimental region of the same heart. Whereas IGF-II expression was already increased by experimental stress, IGFBP-5 mRNA was enhanced only by ischaemia/reperfusion. The expression of IGF-I, the receptors, and IGFBP-2, 3, 4, and 6 remained unchanged during the experimental protocol. IGFBP-1 was neither expressed nor induced in our model.

CONCLUSIONS

IGF-II acts like a stress-response gene activated by the experimental conditions (surgery, anaesthesia) and remains induced during following episodes of ischaemia/reperfusion. A possible interaction of IGFBP-5 with other components of the IGF system may contribute to the preconditioning response.

Mechanical efficiency of stunned myocardium is modulated by increased afterload dependency

OBJECTIVE

Oxygen consumption (MVO2) of stunned myocardium is relatively high compared to, and poorly correlated with, systolic contractile function. The aim of this study was to investigate whether an increased afterload dependency, induced by the decreased contractility of the stunned myocardium, contributes to the large variability in the mechanical efficiency data.

METHODS

In 13 anaesthetised open thorax pigs undergoing two cycles of 10 min occlusion of left anterior descending coronary artery and 30 min reperfusion, segment shortening, the slope of end systolic pressure segment length relationship (Ees), external work (EW, derived from the area inside the left ventricular pressure segment length loop), the efficiency of energy conversion (EET, = EW/PLA x 100%, where PLA = total pressure-segment length area), mechanical efficiency (EW/MVO2), and their dependency on left ventricular end systolic pressure (Pes) were determined before and after induction of stunning, and during subsequent inotropic stimulation with dobutamine (1 and 3 micrograms.kg-1.min-1 over 15 min).

RESULTS

The stunning protocol not only caused significant decreases in segment shortening, external work, energy conversion efficiency, and EW/MVO2 but also increased the afterload dependency of these variables. Before stunning an increase in Pes from 100 to 160 mm Hg decreased segment shortening from 18(SEM 1)% to 14(2)% (P > 0.05) and increased external work from 206(18) to 254(32) mm Hg.mm (P < 0.05). After induction of stunning the same increase in Pes caused a decrease in segment shortening from 9.5(1.8)% to -4.6(2.1)% (P < 0.05) and in external work from 149(21) to -11(10) mm Hg.mm (P < 0.05). The afterload dependency of the PLA was not altered by stunning, but the afterload dependency of energy conversion efficiency increased, since efficiency decreased from 67(3)% to 59(5)% as Pes was increased from 100 to 160 mm Hg before stunning, but from 57(5) to -7(5)% after induction of stunning (P < 0.05). Furthermore, the same increase in Pes resulted in an 8% decrease of EW/MVO2 before stunning and 107% after induction of stunning. Infusion of dobutamine not only restored segment shortening, external work, energy conversion efficiency, and EW/MVO2 of the stunned myocardium, but also attenuated their afterload dependency to pre-stunning levels.

CONCLUSIONS

Myocardial stunning increases the afterload dependency of segment shortening, external work, energy conversion efficiency, and mechanical efficiency, which can be attenuated by inotropic stimulation with dobutamine. However, the decrease in left ventricular end systolic pressure, which accompanies the induction of stunning, counteracts the decrease in these variables. These two mechanisms can explain most of the reported scatter in mechanical efficiency.

Nucleotide sequence and expression of the porcine vascular endothelial growth factor

We cloned and sequenced two cDNAs encoding the angiogenic, vascular endothelial growth factor (VEGF) from the porcine heart. Deduced amino acid sequence of the clone pPVE-18 and pPVE-5 predicted 164 (VEGF164), and 120 (VEGF120) residues of VEGF, respectively, with a putative N-terminal signal sequence of 26 amino acids. The porcine VEGF is shorter by one amino acid as compared to human VEGF, but a potential glycosylation site is present at Asn-74. PCR detection, and verification of the identity of the PCR products by Southern hybridization, confirmed wide expression of VEGF in different porcine tissues. Northern blot analysis with a radiolabeled porcine specific VEGF probe, showed one major (3.9 kb) and one minor (1.7 kb) mRNA species expressed in all four chambers of the heart.

Angiotensin II induced expression of transcription factors precedes increase in transforming growth factor-beta 1 mRNA in neonatal cardiac fibroblasts

Angiotensin II (ANG II), a potent vasoconstricting peptide, may act as a growth factor for cardiac muscle cells and induce hypertrophy. We examined the molecular phenotype of neonatal rat cardiac fibroblasts in relation to ANG II by studying the expression pattern of three transcription factors (Egr-1, c-fos and c-jun) and the transforming growth factor-beta 1 (TGF-beta 1). ANG II did not affect cell proliferation and growth of serum deprived neonatal cardiac fibroblasts as predicted from their DNA and protein contents. The expression of Egr-1 and c-fos was induced as early as 15 min that reached maximal levels at 45 min and declined thereafter, whereas c-jun was induced at 45 min and remained elevated up to 2 hrs of ANG II addition. ANG II up-regulated the expression of TGF-beta 1, which became apparent after 1 hr of incubation and reached a plateau between 16-48 hrs. Our results indicate that ANG II transiently stimulates the expression of transcription factors, which may up-regulate TGF-beta 1, that in turn could contribute to the process of myocardial extra-cellular matrix remodeling in hypertrophy.

Involvement of ATP-sensitive potassium channels in preconditioning protection

Single or multiple brief periods of ischemia (preconditioning, PC) have been shown to protect the myocardium from infarction during a subsequent more prolonged ischemic insult. To test the hypothesis that opening of ATP-sensitive potassium channels (KATP) is involved in this mechanism, either bimakalim, a KATP channel opener, or glibenclamide, a KATP channel blocker, were administered to mimic or to block preconditioning protection in barbital-anesthetized pigs. PC was elicited by a single period of 10 min left anterior descending coronary artery (LADCA) occlusion followed by 15 min of reperfusion before the LADCA was reoccluded for 60 min. Instead of PC, bimakalim infusion was started 15 min before the 60 min LADCA occlusion (TCO) and stopped with the onset of ischemia. Glibenclamide was administered either for 10 min prior to the PC protocol, before bimakalim infusion, or before TCO. Regional wall function was quantified with ultrasonic crystals aligned to measure wall thickening (% delta WT). At the end of the protocol, infarct size was determined by incubating myocardium with p-nitrobluetetrazolium. In seven preconditioned pigs, infarct size was 9.9 +/- 5.1% of the risk region compared with 65.9 +/- 6.0% in the seven control pigs subjected to 60 min of ischemia only (p < 0.001). In seven pigs treated with bimakalim, infarct size was reduced to 35.3 +/- 6.6 (p < 0.05 vs. controls). Blocking ATP-sensitive potassium channels with glibenclamide prior to PC abolished its protective effect (infarct size, 62.2 +/- 4.5%; p < 0.001 vs. PC alone). Glibenclamide also antagonized the protective effect of bimakalim (infarct size, 55.2 +/- 4.0%), but did not affect infarct size, when solely administered prior to the prolonged ischemic period (62.2 +/- 4.3%). We conclude that in swine myocardium KATP channels are involved in the protective effect of ischemic preconditioning, since glibenclamide completely abolished the protective effect of preconditioning, while bimakalim could--at least in part--mimic it.

Effect of arterial blood pressure and ventilation gases on cardiac depression induced by coronary air embolism

In this study the time course of cardiac depression after selective intracoronary injection of air bubbles was investigated in six anesthetized pigs (30 +/- 2 kg) with different mixtures of ventilation gases and different mean arterial blood pressures (MAP). Air bubbles of 150 microns diam were injected into the left anterior descending coronary artery (LADCA) in a volume of 2 microliters/kg body wt. In each animal an injection of air bubbles was applied during ventilation with N2-O2 and a MAP of 77 +/- 3 mmHg (N2-O2/low pressure) or 111 +/- 3 mmHg (N2-O2/high pressure) and during ventilation with pure O2 and a MAP of 77 +/- 3 mmHg (O2/low pressure) or 110 +/- 3 mmHg (O2/high pressure). Systemic hemodynamic variables such as left ventricular pressure, its peak first derivatives, and MAP changed < 10% after injection of air bubbles. During N2-O2/low pressure, systolic segment length shortening in the LADCA region (SS-LADCA) decreased from baseline and did not return to baseline within the 10 min after injection of air bubbles. During N2-O2/high pressure and O2/low pressure, SS-LADCA was decreased between 60 and 120 s, whereas for O2/high pressure this period was from 60 to 90 s. By calculating the time integral of the deviation from baseline of SS-LADCA, it could be demonstrated that the depression of regional myocardial function was less severe during O2/high pressure and O2/low pressure than during N2-O2/low pressure. We conclude that, when coronary air embolism occurs during hypertension and during ventilation with pure O2 instead of a normal N2-O2 mixture, the resulting depression of regional myocardial function is reduced.

Loss of elastic recoil in postischemic myocardium induces rightward shift of the systolic pressure-volume relationship

Ischemia-induced systolic dysfunction has been ascribed to changes in cellular excitation-contraction coupling and diastolic dysfunction because of disruption of the extracellular collagen matrix. Therefore, systolic and diastolic pressure-volume relationships and O2 consumption were determined before and after 5 min of global ischemia in isolated blood-perfused porcine hearts. The slope of the systolic pressure-volume relationship was 7.2 +/- 0.6 (SE) mmHg.ml-1.100 g-1 (n = 18) at baseline and did not change during reperfusion, but the systolic volume intercept shifted from 1.0 +/- 0.4 ml/100 g at baseline to 3.7 +/- 1.4, 4.1 +/- 1.1, and 4.2 +/- 0.9 ml/100 g at 15, 30, and 60 min of reperfusion, respectively (all P < 0.05). The diastolic volume intercept was 8.2 +/- 0.7 ml/100 g at baseline and remained unchanged during reperfusion. Therefore, the difference of the systolic-diastolic volume intercepts, an index of elastic recoil forces, was decreased to 57 +/- 8, 49 +/- 7, and 47 +/- 9% of baseline values (P < 0.05). The shift of the systolic pressure-volume relationship was accompanied by a transient decrease of contractile efficiency (slope of O2 consumption-pressure-volume-area relationship) at 15 min of reperfusion (from 43 +/- 6 to 27 +/- 7%). We hypothesize that the rightward shift of the systolic pressure-volume relationship was compatible with a decrease of elastic-restoring forces, probably induced by alterations in the extracellular collagen matrix and/or the cytoskeleton, and thereby our data imply that left ventricular dysfunction of postischemic myocardium does not result solely from disturbances in excitation-contraction coupling.