Attenuation of S-T segment elevation during repetitive coronary occlusions truly reflects the protection of ischemic preconditioning and is not an epiphenomenon

Attenuation of S-T segment elevation between the first and subsequent balloon inflations of a coronary angioplasty procedure has been assumed to indicate a transition to a preconditioned state, but there has been no validation of this assumption. Open-chest rabbits were instrumented with a coronary snare and epicardial electrode. The coronary artery was occluded twice for 5 min with each occlusion followed by 10 min of reflow before a final 30 min occlusion. The evolving S-T elevation was quantitated as the voltage-time integral. For the first coronary occlusion total S-T segment elevation averaged 40.8+/-5.4 mV x min, significantly greater than 26.2+/-4.6 mV x min for the second occlusion (p < 0.001). There was no further change during the initial 5 min of the third occlusion (24.5+/-4.5 mV x min). When the protection of ischemic preconditioning was blocked by intravenous infusion of 8-(p-sulfophenyl)theophylline, an adenosine receptor antagonist, attenuation of S-T segment elevation was no longer apparent. When preconditioning was pharmacologically triggered by tyramine rather than ischemia, there also was no alteration in S-T segment elevation among the 3 occlusions. Therefore, S-T elevation was diminished during the second episode of ischemia only when a transition occurred from non-preconditioned to preconditioned state between occlusions. An attenuated S-T segment is a valid marker for the presence of the preconditioned state.

Signal transduction in ischemic preconditioning

Ischemic preconditioning is a phenomenon in which exposure of the heart to a brief period of ischemia causes it to quickly adapt itself to become resistant to infarction from a subsequent ischemic insult. The mechanism is not fully understood but, at least in the rabbit, it is known to be triggered by occupation of adenosine receptors, opioid receptors, bradykinin receptors and the generation of free radicals during the preconditioning ischemia. All of these are thought to converge on and activate protein kinase C (PKC), which in turn activates a tyrosine kinase. This kinase cascade eventually terminates on some unknown effector, possibly a potassium channel or a cytoskeletal protein, which makes the cells resistant to infarction. If this process can be understood, it should be possible to devise a method for conferring this protection to patients with acute myocardial infarction.

The mechanism of protection from 5 (N-ethyl-N-isopropyl)amiloride differs from that of ischemic preconditioning in rabbit heart

We investigated the effects of 5-(N-ethyl-N-isopropyl)amiloride (EIPA) on infarction in isolated rabbit hearts and cardiomyocytes. Thirty min of regional ischemia caused 29.6 +/- 2.8% of the risk zone to infarct in untreated Krebs buffer-perfused hearts. Treatment with EIPA (1 microM) for 20 min starting either 15 min before ischemia or 15 min after the onset of ischemia significantly reduced infarction to 5.4 +/- 2.0% and 7.0 +/- 1.0%, respectively (p < 0.01 versus untreated hearts). In both cases salvage was very similar to that seen with ischemic preconditioning (PC) (7.1 +/- 1.5% infarction). Unlike the case with ischemic preconditioning, however, protection from EIPA was not blocked by 50 microM polymyxin B, a PKC inhibitor, or 1 microM glibenclamide, a KATP channel blocker. Forty-five min of regional ischemia caused 51.0 +/- 2.9% infarction in untreated hearts. Ischemic preconditioning reduced infarction to 23.4 +/- 3.1% (p < 0.001 versus untreated hearts). In these hearts with longer periods of ischemia pretreatment with EIPA reduced infarction similarly to 28.8 +/- 2.1% (p < 0.01 versus untreated hearts). However, when EIPA was combined with ischemic PC, no further reduction in infarction was seen (23.8 +/- 3.5% infarction). To further elucidate the mechanism of EIPA's cardioprotective effect, this agent was also examined in isolated rabbit cardiomyocytes. Preconditioning caused a delay of about 30 min in the progressive increase in osmotic fragility that occurs during simulated ischemia. In contrast, EIPA had no effect on the time course of ischemia-induced osmotic fragility. Furthermore, EIPA treatment did not alter the salutary effect of ischemic preconditioning when the two were combined in this model. We conclude that Na+/H+ exchange inhibition limits myocardial infarction in the isolated rabbit heart by a mechanism which is quite different from that of ischemic preconditioning. Despite the apparently divergent mechanisms, EIPA's cardioprotective effect could not be added to that of ischemic or metabolic preconditioning in these models.

Cardiac morphology and left ventricular function in normotensive morbidly obese patients with and without congestive heart failure, and effect of weight loss

To assess cardiac morphology and left ventricular (LV) function in normotensive morbidly obese patients with and without congestive heart failure (CHF) we performed a physical examination and obtained a transthoracic echocardiogram and cardiac Doppler studies before and after substantial weight loss in patients whose actual body weight was initially equal to or more than twice their ideal body weight and who were free from systemic hypertension and underlying organic heart disease. There were 24 patients with CHF, 14 of whom were studied after weight loss. There were 50 patients without CHF, 39 of whom were studied after weight loss. Compared to patients without CHF, those with CHF had significantly greater mean LV internal dimension in diastole, LV end-systolic wall stress, LV mass/height index values, left atrial dimension and right ventricular internal dimension values, significantly lower mean LV fractional shortening, and transmitral Doppler E/A ratio values, and significantly longer mean transmitral E-wave deceleration time and duration of morbid obesity than patients without CHF. Substantial weight loss in those with and without CHF produced comparable reductions in mean LV internal dimension in diastole, LV end-systolic wall stress, LV mass/height index, transmitral Doppler E-wave deceleration time, and left atrial dimension, and comparable increases in LV fractional shortening and transmitral Doppler E/A ratio. Linear regression analysis identified duration of morbid obesity as the strongest predictor of CHF (p <0.00000002). Thus, LV mass is greater and LV systolic function and diastolic filling are more impaired in normotensive morbidly obese subjects with CHF than in those without CHF. Duration of morbid obesity is the strongest predictor of CHF among the variables studied. Substantial weight loss produces comparable changes in cardiac morphology and function in those with and without CHF.

Phosphorylation of tyrosine 182 of p38 mitogen-activated protein kinase correlates with the protection of preconditioning in the rabbit heart

p38 mitogen-activated protein kinase (MAPK) is known to be activated after exposure to endotoxin, osmotic and environmental stress, and, most recently, during ischemia/reperfusion. We investigated whether ischemic preconditioning also causes phosphorylation of the activation sites on p38 MAPK. Three groups of isolated rabbit hearts were studied. Control hearts experienced 30 min of ischemia only. The second group was preconditioned with 5 min of global ischemia and 10 min of reperfusion. Group 3 was also ischemically preconditioned, but in the presence of 100 microM 8-(p-sulfophenyl)theophylline (SPT). Transmural left ventricular biopsies were taken before and during the long ischemic period. Western blot analysis with either p38 MAPK or phospho-specific p38 MAPK (Tyr-182) antibodies showed a decreased phosphorylation during ischemia in non-preconditioned hearts, but phosphorylation was enhanced several fold after 10 and 20 min of ischemia in preconditioned hearts. Furthermore, when protection from ischemic preconditioning was blocked by SPT, increased phosphorylation of p38 MAPK during ischemia was not present. Therefore the phosphorylation of p38 MAPK at tyrosine 182, which is required for the kinase's activation, occurred during ischemia only when protection from preconditioning was evident. In a second study, changes in osmotic fragility were measured during simulated ischemia in rabbit cardiomyocytes. Reduced fragility in ischemically preconditioned myocytes could be completely abolished by the specific p38 MAPK inhibitor SB-203580. In contrast, anisomycin, an activator of p38 MAPK and JUN kinase pathways, was found to be as protective as ischemic preconditioning. We conclude that p38 MAPK phosphorylation correlates with preconditioning's protection, and that its activation may be an important step in the signal transduction cascade of ischemic preconditioning.

Role of adenosine receptors in late preconditioning against myocardial stunning in conscious rabbits

Conscious rabbits underwent six 4-min coronary occlusions interspersed with 4-min periods of reperfusion for 2 consecutive days (days 1 and 2 of stage I); 2 wk later, they underwent the same protocol (days 1 and 2 of stage II) except that they received either 8-(p-sulfophenyl)theophylline (SPT) on day 1 (group I, n = 5) or 2-chloro-N6-cyclopentyl-adenosine (CCPA) on the day before day 1 (group II, n = 6). In both groups I and II, on day 1 of stage I, systolic wall thickening (WTh) remained significantly depressed for several hours, indicating myocardial stunning; on day 2, however, the total deficit of WTh was approximately 50% less than on day 1 (P < 0.01), indicating the development of late preconditioning (PC) against myocardial stunning. Despite administration of SPT, in group I the deficit of WTh during stage II was 55% less on day 2 than on day 1 (P < 0.05). Similar results were obtained in three other rabbits treated with PD-115199 on day 1. In group II, pretreatment with CCPA during stage II failed to decrease the deficit of WTh on day 1. This study presents a new conscious rabbit model for studying myocardial stunning that is relatively inexpensive and technically less demanding than larger animal models. In this model, the development of late PC against myocardial stunning is not blocked by nonselective blockade of adenosine receptors with either SPT or PD-115199, nor is it induced by activation of adenosine A1 receptors with CCPA, indicating that adenosine receptors are not involved in the pathogenesis of this phenomenon.

Intravenous co-infusion of adenosine and norepinephrine preconditions the heart without adverse hemodynamic effects

OBJECTIVE

A simple intervention is needed that could protect the heart against infarction during limited-access coronary artery bypass grafting. Adenosine and norepinephrine can precondition the heart with resulting protection, but adverse hemodynamic effects prevent clinical application. Because heart rate, blood pressure, and contractility effects of these two drugs are diametrically opposite, a mixture might be beneficial.

METHODS

A superficial branch of the left coronary artery of rabbits was surrounded with a suture. Infarction was produced in all hearts by a 30-minute coronary artery occlusion. Infarct size after reperfusion was measured and is presented as a percentage of the risk zone. The effect of 5-minute intravenous co-infusion of adenosine (20 mg/kg) and norepinephrine (0.1 mg/kg) 15 minutes before ischemia was examined. In addition, the protective effect of three sequential intravenous bolus injections of adenosine at either 0.2 or 0.4 mg/kg was evaluated.

RESULTS

Thirty minutes of regional ischemia caused infarction of 40% +/- 4% of the risk zone. The combination of adenosine and norepinephrine caused no change in blood pressure but rather protected the heart, with infarction of only 9% +/- 2% of the risk zone (p = 0.0001 vs control). Adenosine-norepinephrine co-infusion still protected the heart when the interval between infusion and ischemia was extended to 60 minutes, but it did not protect with a 120-minute interval. Intravenous bolus injections of adenosine resulted in cardiac slowing and marked hypotension. Boluses of 0.2 mg/kg resulted in a minimal, but significant, reduction in infarct size, whereas the higher dose provided no protection.

CONCLUSION

Adenosine-norepinephrine co-infusion provides a feasible and safe parenteral method for preconditioning the heart.

Protection of ischemic preconditioning is dependent upon a critical timing sequence of protein kinase C activation

The protection of ischemic preconditioning (PC) appears to be triggered by activation of receptors which couple to protein kinase C (PKC) during the brief ischemia. Previous experiments, however, suggest that phosphorylation of PKC's substrates is not required for the myocytes to enter the preconditioned state. Because of the fundamental importance of this observation, the present study was designed to stringently test when phosphorylation must occur during a PC protocol. We used an in vitro rabbit heart which permitted precise control of the timing of exposure to staurosporine (STA), a reversible blocker of PKC's kinase activity. In control hearts a 30-min regional coronary occlusion followed by 2 h of reperfusion resulted in 31.4 +/- 1.5% infarction of the region at risk, and STA (100 nM) had little effect. PC with 5 min of global ischemia and 10 min of reperfusion reduced infarction to 11.4% (P < 0.01 v control). STA starting 5 min before and ending 5 min after the 5-min PC ischemia did not block protection (14.1 +/- 1.7% infarction, P < 0.01 v control). When the PC protocol was changed to 5 min ischemia/20 min reperfusion, STA still could not block protection even though the infusion continued for 15 min after the PC ischemia. However, when a 15-min STA infusion was initiated 5 min before the 30-min ischemic period. PC's protection was totally blocked. Moreover this late infusion of STA continued to block protection even when the PC stimulus was amplified by three cycles of 5-min ischemia/10-min reperfusion. These observations indicate that kinase activity is not required to put the rabbit heart into a preconditioned state suggesting that some process upstream of PKC's kinase is responsible for the triggering and memory of PC.

Volatile anesthetics protect the ischemic rabbit myocardium from infarction

BACKGROUND

The influence of anesthetic agents on the infarction process in the ischemic myocardium is unclear. This study evaluated the effects of three intravenous and three inhalational anesthetic agents on myocardial infarction within a quantified ischemic risk zone in rabbit hearts subjected to a standardized regional ischemia-reperfusion insult.

METHODS

Both in vitro and in situ rabbit models were used to investigate the effects of anesthetic agents on infarct size. In all rabbits the heart was exposed and a coronary artery surrounded with a suture to form a snare for subsequent occlusion. In in situ preparations, both intravenous and inhalational agents were tested, whereas only the latter were used in isolated hearts. Infarct size was determined by triphenyltetrazolium chloride staining. To determine whether an adenosine-mediated protective mechanism was involved, 8-(p-sulfophenyl)theophylline, an adenosine receptor blocker, was added to halothane-treated isolated hearts. Adenosine concentration in the coronary effluent was also measured in isolated hearts exposed to halothane. In other protocols, chelerythrine, a highly selective protein kinase C inhibitor, was administered to both halothane-treated and untreated isolated hearts.

RESULTS

Infarcts in the three in situ groups anesthetized with halothane, enflurane, and isoflurane were about one half as large as infarcts in rabbits that underwent anesthesia with pentobarbital, ketamine-xylazine, or propofol. Volatile anesthetics also protected isolated hearts by a similar amount. Both adenosine receptor blockade and chelerythrine abolished cardioprotection from halothane in isolated hearts. Halothane treatment did not increase adenosine release.

CONCLUSIONS

The volatile anesthetics tested protected the ischemic rabbit heart from infarction, in contrast to the three intravenous agents tested. Protection was independent of the hypotensive effect of the inhalational agents because halothane also protected isolated hearts, in which changing vascular tone is not an issue and coronary perfusion pressure is constant. Cardioprotection by volatile anesthetics depended on both adenosine receptors and protein kinase C, and thus is similar to the mechanism of protection seen with ischemic preconditioning.

Mast cell degranulation does not contribute to ischemic preconditioning in isolated rabbit hearts

Preconditioning the heart with a short period of ischemia makes it resistant to infarction from a subsequent ischemic insult. We have proposed that preconditioning is triggered by the release of endogenous substances including adenosine which activate protein kinase C through receptormediated cell signaling pathways. However, it has also been proposed that the initial brief ischemia may result in mast cell degranulation without significant myocardial damage, making it less likely that the toxic granule contents could be released to irreversibly damage vulnerable myocardial cells during the subsequent prolonged ischemia. To study the role of mast cells in ischemic preconditioning (PC) isolated rabbit hearts were subjected to 30 min of regional ischemia followed by 120 min of reperfusion. Infarct size was measured with triphenyltetrazolium chloride. In control hearts infarction was 31.9 +/- 2.6% of the risk zone. Preconditioning with 5 min of global ischemia and 10 min of reperfusion reduced infarct size to 5.6 +/- 6.1% (p < 0.01). When disodium cromoglycate (DSCG)(10 microM), a mast cell stabilizer, was infused shortly before the long ischemia it did protect the heart (12.8 +/- 2.9% infarction, p < 0.01 vs control) which supports the mast cell theory. However, a mast cell degranulating agent, compound 48/80 (24 mg/L), added to the perfusate prior to the 30 min ischemic period could not mimic PC (39.7 +/- 5.6% infarction). Mast cell granules are rich in histamine, and the latter was assayed in myocardium by immunoassay as a marker of intact granules. In homogenized left ventricle from normal rabbit hearts and those following a standard PC protocol of 5-min global ischemia/10-min reperfusion, histamine contents were 9.3 +/- 1.4 and 8.9 +/- 1.4 ng/g wet tissue, respectively. Compound 48/80 reduced histamine levels to 2.9 +/- 0.6 ng/g (p < 0.05 vs control). Although baseline histamine contents were 10-fold higher in rats, PC also had no effect, but compound 48/80 reduced content by 91%. Therefore, histamine tissue content and presumably mast cell granules were unaffected by a PC protocol which successfully protected ischemic myocardium, while pharmacological myocardial histamine depletion was not associated with protection. Hence, mast cells do not appear to be important in ischemic preconditioning. Although a mast cell stabilizer such as DSCG can protect ischemic myocardium, it may do so by one of its other properties, e.g., membrane stabilization.

Phospholipase D plays a role in ischemic preconditioning in rabbit heart

BACKGROUND

Activation of protein kinase C (PKC) is thought to be a critical step in ischemic preconditioning. Many receptor agonists activate PKC via stimulation of phospholipase C (PLC), which degrades membrane phospholipids to diacylglycerol (DAG), an important PKC cofactor. However, adenosine receptors, critical components of the prototypical preconditioning pathway, are not thought to couple to PLC in the cardiomyocyte. We therefore tested whether ischemic preconditioning or adenosine might instead activate phospholipase D (PLD) to produce DAG.

METHODS AND RESULTS

PLD activity was measured in isolated rabbit hearts. Ischemic injury was evaluated in either isolated rabbit hearts or dispersed myocytes. PLD activity doubled from a control level of 74.8 +/- 10.0 to 140.0 +/- 11.5 mumol.min-1.g-1 (P < .025) after two 5-minute periods of global ischemia separated by 5 minutes of reperfusion. A similar increase was noted after the heart had been exposed to (R)-N6-(2-phenylisopropyl)-adenosine [(R)-PIA] for 20 minutes. When sodium oleate, which activates PLD, was administered to isolated hearts before a 30-minute coronary occlusion, infarct size (15.6 +/- 2.0% of the risk zone) was significantly smaller than in untreated hearts (30.4 +/- 2.2%; P < .01). Exposure to sodium oleate significantly prolonged the rate of isolated myocyte survival during simulated ischemia. Propranolol 100 mumol/L, which blocks DAG production from metabolites produced by PLD catalysis, completely abolished the protective effects of both metabolic preconditioning and (R)-PIA exposure in myocytes.

CONCLUSIONS

We conclude that PLD stimulation is involved in the protection of ischemic preconditioning in the rabbit heart.

Infarct limitation of the second window of protection in a conscious rabbit model

OBJECTIVES

Myocardial protection associated with ischemic preconditioning (PC) wanes within an hour or two. It has recently been observed, however, that a delayed phase of protection appears about 24 h after ischemic PC in anesthetized rabbits and dogs which might be related to synthesis of cytoprotective proteins. We tested whether a second window of protection could be induced in conscious rabbits.

METHODS

Rabbits chronically instrumented with a coronary artery occluder and ECG electrodes experienced a 30-min coronary occlusion followed by 3 h reperfusion. Infarct size was measured with triphenyltetrazolium chloride.

RESULTS

35.7 +/- 2.3% of the risk zone infarcted in control animals. PC with 4 cycles of 5-min coronary occlusion/10-min reperfusion 24 h prior to the 30-min ischemia decreased infarction to 24.1 +/- 1.4% of the risk zone (P < 0.01). During the 30-min occlusion 3 of 7 non-PC rabbits developed ventricular fibrillation, while this arrhythmia did not occur in the 7 PC animals (P < 0.1). Myocardial hsp70 content in PC rabbits was twice that in controls. Collateral blood flow was not different in the two groups.

CONCLUSIONS

A second window of protection exists in conscious rabbits which minimizes both infarction and arrhythmias, and cytoprotective protein content is increased in the myocardium of protected animals.

Attenuated purine production during subsequent ischemia in preconditioned rabbit myocardium is unrelated to the mechanism of protection

Preconditioned hearts release less purines during ischemia than virgin hearts. This study tested whether this reduced purine production is related to the mechanism of protection by ischemic preconditioning. Coronary effluent from isolated rabbit hearts was collected and purine (adenosine + inosine + hypoxanthine) levels were measured. All hearts underwent two cycles of 5 min global ischemia, each followed by 10 min reperfusion. In the first minute of reflow after the first ischemic period untreated hearts released 155 +/- 14 nmol purines per g wet weight, but only 104 +/- 16 nmol/g following the second bout of ischemia (P < 0.05). Thus, preconditioned hearts released less purines during ischemia. When 8-(p-sulfophenyl)theophylline (100 microM), which prevents the infarct size-limiting effect of ischemic preconditioning by blocking adenosine receptors was present in the perfusate, the pattern of purine release was not altered (151 +/- 13 nmol/g during the first minute after the first 5-min ischemic episode dropping to 117 +/- 6 nmol/g after the second ischemic period P < 0.05). Furthermore, pharmacological preconditioning with 5 min exposure of the heart to either adenosine (10 microM) or phenylephrine (0.1 microM) 15 min prior to the first ischemia did not affect purine release during early reperfusion after either the first (144 +/- 16 nmol/g and 153 +/- 12 nmol/g, respectively) or second (84 +/- 12 nmol/g and 111 +/- 9 nmol/g, respectively) bout of ischemia. Since the attenuated purine release was apparently unaffected by the presence or absence of a protected state, we conclude that this pattern is unrelated to the mechanism by which preconditioning protects the heart.

Pretreatment with endothelin-1 mimics ischemic preconditioning against infarction in isolated rabbit heart

We have proposed that ischemic preconditioning in rabbit hearts is initiated by adenosine receptor stimulation resulting in activation of protein kinase C. If this theory is correct then any agonist which can activate PKC should also put the heart into a preconditioned state. This study sought to determine whether endothelin-1 (ET-1), which is known to activate protein kinase C can also mimic ischemic preconditioning. Isolated rabbit hearts experienced 30 min of regional ischemia followed by 120 min of reperfusion. Infarct size was measured with triphenyltetrazolium chloride. In control hearts infarction was 30.3 +/- 2.5% of the risk zone. Preconditioning with 5 min global ischemia and 10 min reperfusion reduced infarct size to 5.6 +/- 0.7% (P < 0.01). Perfusion with either 10 PM ET-1 at constant coronary artery flow for 5 min in lieu of ischemia or 50 PM ET-1 with 10 nM nicardipine to block the former's coronary constructive effect was quite protective and equipotent with preconditioning. Infarction averaged 7.2 +/- 0.8% and 5.8 +/- 1.7% of the risk zone, respectively. This protection could be blocked by PD 156 707 (10 microM), a highly specific endothelin receptor antagonist. Chelerythrine (5 microM), a PKC inhibitor, also aborted protection (22.0 +/- 1.7% infarction). However, 8-(p-sulfophenyl)theophylline (100 microM), an adenosine receptor blocker, given during ET-1 administration did not block ET-1's protective effect indicating that adenosine was not involved in the effect. PD 156707 failed to block the protection from ischemic preconditioning (12.6 +/- 2.3% infarction) revealing that endothelin is not an important physiological mediator of ischemic preconditioning. We conclude that ET-1 can mimic ischemic preconditioning in isolated rabbit hearts as would be predicted since its receptors are PKC-coupled, but that endogenous endothelin contributes little to ischemic preconditioning.

Endogenous myocardial norepinephrine is not essential for ischemic preconditioning in rabbit heart

To determine whether endogenous cardiac catecholamines mediate ischemic preconditioning (PC) in the rabbit heart, myocardial catecholamines were depleted by reserpine (5 mg/kg, 18-24 h pre-PC) or surgical sympathectomy (2 wk pre-PC). In vivo hearts were subjected to 30 min of regional ischemia and 3 h of reperfusion. PC involved either one or four cycles of 5-min ischemia and 10-min reperfusion before the 30-min ischemic period. Right ventricular norepinephrine content (pmol/mg protein), 51.4 +/- 11.1 in untreated rabbits, was reduced to 0.6 +/- 0.2 and 1.8 +/- 0.5 by surgical sympathectomy and reserpine, respectively. Infarct size (IS) was measured by tetrazolium and expressed as percentage of the risk zone. In untreated animals exposed solely to 30 min of regional ischemia IS was 35.5 +/- 1.6% and was unchanged by reserpine (43.3 +/- 5.4%) or surgical sympathectomy (33.4 +/- 3.5%). compared with infarction in the respective non-PC controls, IS in untreated (7.4 +/- 1.5%, P < 0.0001) and surgically sympathectomized (11.2 +/- 1.5%, P < 0.0001) animals was significantly diminished by a single cycle of PC, but the latter exerted less protection in reserpinized animals (27.6 +/- 3.5%, P < 0.0025). Four cycles of PC, however, reduced IS to 10.3 +/- 1.2% in reserpinized animals. Therefore, despite comparable depression of myocardial norepinephrine content, surgical and chemical sympathectomy had different effects on the level of protection afforded by ischemic PC. These data demonstrate that endogenous myocardial catecholamines are not essential for protection from PC in the rabbit.

Myocardial preconditioning promises to be a novel approach to the treatment of ischemic heart disease

In the phenomenon termed "ischemic preconditioning," a brief period of ischemia prior to a more prolonged one improves myocardial function (after reperfusion) and diminishes infarction. This phenomenon has been described extensively in experimental animals and now in humans. It is triggered by several agents released by ischemic cells and can be reproduced by infusion of agonists coupled to protein kinase C (PKC), e.g. adenosine, angiotensin, phenylephrine, bradykinin, and endothelin. The intracellular signaling pathway involves a phospholipase, either C or D, which metabolizes membrane phospholipids to produce diacylglycerol, a necessary endogenous cofactor for PKC activation. Which protein(s) is phosphorylated by PKC is not yet known, nor is the identity of the end-effector that actually mediates protection of the ischemic cell. Identification of the end-effector may make it possible in the routine treatment of patients with ischemic heart disease to precondition and thereby salvage ischemic myocardium and improve survival.

Relation of duration of morbid obesity to left ventricular mass, systolic function, and diastolic filling, and effect of weight loss

Longer duration of morbid obesity is associated with higher LV mass, poorer LV systolic function, and greater impairment of LV diastolic filling. Weight loss-induced decreases in LV mass and improvements in LV systolic function and diastolic filling are due in part to favorable alterations in LV loading conditions.

Influence of left ventricular mass on left ventricular diastolic filling in normotensive morbid obesity

To identify factors influencing left ventricular (LV) diastolic filling in patients with morbid obesity, we performed transthoracic and Doppler echocardiography on 50 subjects whose actual body weight was > or = twice their ideal body weight and on 50 normal lean control subjects. The transmitral Doppler E/A ratio and E wave deceleration half-time were used to assess LV diastolic filling. Significant negative correlations were seen between the E/A ratio and the LV internal dimension in diastole (r = 0.819, p = 0.0001), systolic blood pressure (r = 0.751, p = 0.0001), LV end-systolic wall stress (r = 0.782, p = 0.0001), and LV mass/height index (r = 0.901, p = 0.0001). Significant positive correlations were seen between the E wave deceleration half-time and the LV internal dimension in diastole (r = 0.743, p = 0.0001), systolic blood pressure (r = 0.789, p = 0.0001), LV end-systolic wall stress (r = 0.828, p = 0.0001), and LV mass/height index (r = 0.831, p = 0.0001). No correlation was seen between diastolic blood pressure and either index of LV diastolic filling. Thus increasing LV mass is associated with progressive impairment of LV diastolic filling in morbidly obese individuals. The aforementioned alterations in LV loading conditions may contribute to impairment of LV diastolic filling directly or by increasing LV mass.

Effects of anesthesia and K+ ATP channel blockade on interstitial adenosine accumulation in ischemic rabbit myocardium

Glibenclamide, a K+ ATP channel antagonist, blocks the anti-infarct effect of ischemic preconditioning in rabbits, but only when the latter are anesthetized with ketamine-xylazine. Furthermore, the protection triggered by pinacidil, a K+ ATP channel opener, can be aborted by treatment with the adenosine antagonist 8-(P-sulfophenyl)theophylline. This study tests whether either the anesthetic regimen or glibenclamide affects infarct size by modulating interstitial adenosine levels. Interstitial adenosine and total purine concentrations were assessed in open-chest rabbits by the microdialysis technique. Dialysis fibers were inserted into myocardium served by a coronary artery branch surrounded by a snare. All animals sustained a 30-min coronary occlusion and then 120-min reperfusion. Rabbits were anesthetized with either sodium pentobarbital or a ketamine-xylazine mixture. Half of the latter animals also received glibenclamide. The control levels of adenosine in the dialysate were comparable in the three groups, as were those of total purines, and the infusion of glibenclamide caused no change. Ischemia led to 10- to 20-fold increases in interstitial adenosine and 10- to 40-fold rises in total purine concentrations. These increases were equivalent in all groups. Further-more, infarct size as a percentage of the myocardium at risk was also comparable in the three groups. Neither the anesthetic agent nor glibenclamide appears to modulate interstitial adenosine release from ischemic tissue.