Ischaemic contracture and myocardial perfusion in isolated rat heart

Left ventricular function after extended hypothermic preservation of the heart is dependent on functional coronary capillarity

BACKGROUND

A growing body of knowledge has led to the hypothesis that injury to the microcirculation during hypothermic myocardial preservation may result in decreased contractility of hearts upon reperfusion.

METHODS AND RESULTS

To test this hypothesis, we examined the relationship between no-reflow and left ventricular function after hypothermic cardiac preservation after reperfusion with solutions containing dilute whole blood (DWB) or washed red blood cells (K2RBC). Rat hearts were arrested with high-potassium cardioplegia, then flushed and stored for 6 hours in low-potassium cardioplegia at 4 degrees C. Hearts were reperfused at a constant flow rate (4 mL/min) with K2RBC for 60 minutes (group 1, n = 5) or DWB for 7 minutes followed by 53 minutes of K2RBC (group 2, n = 5). Left ventricular developed pressure (LVDP) was measured with an intraventricular balloon. Immediately after functional assessment, hearts were perfused with an india ink solution to mark flow, then glutaraldehyde. Morphometric techniques were used to determine the degree of capillary compression [delta d(c)], perfused capillary number per fiber area [QA(0)P], and perfused capillary surface area per fiber volume [Sv(c,f)P]. Capillaries were moderately compressed in both groups after reperfusion (group 1, 19 +/- 1%; group 2, 20 +/- 1%). QA(0)P and Sv(c,f)P were highly correlated with delta d(c) in hearts reperfused with K2RBC (r = .92 and r = .92; P < .01). Although statistically significant, the correlation was not as strong in DWB-reperfused hearts (r = .66 and r = .67; P < .05). LVDP was correlated to QA(0)P and Sv(c,f)P (r = .86 and r = .87, respectively) for groups 1 and 2.

CONCLUSIONS

The weaker correlation between capillary perfusion and capillary compression in DWB-reperfused hearts suggests that factors other than compression contribute to no-reflow after hypothermic preservation. Regardless of the composition of the reperfusate, recovery of left ventricular function after hypothermic ischemia is directly related to coronary capillary perfusion upon reperfusion.

The contraction state of myofibrils during global ischemia and after reperfusion following different forms of cardiac arrest. Correlation with metabolic parameters in the canine heart

This study was undertaken in order to obtain information on the mode of reaction of the contractile apparatus after different forms of cardiac arrest, global ischemia and reperfusion, as well as on possible correlations between the contraction state of myofibrils and biochemical parameters. During the survival time, before the level of 3 mumol/gww creatine phosphate (CP) is reached, the contraction state shows only minor changes. During the revival time in which ATP tissue concentrations decay to 4 mumol/gww, the contribution of ATP, lactate, anorganic phosphate (Pa) and acidosis to the degree of relaxation depends on the method of cardiac arrest. At defined biochemical values, the degree of relaxation is comparable after aortic cross clamping (ACC) and St. Thomas perfusion, but significantly different compared to HTK perfusion. Thus, during the revival time, the relaxation of sarcomeres depends predominantly on the composition of the solutions used for cardiac arrest. The re-entry of contraction below 3 mumol/gww ATP is correlated with the ATP concentration, independent of the form of cardiac arrest. Reperfusion after HTK or St. Thomas cardioplegia and reversible ischemia leads to the focal formation of contraction bands, which do not occur during ischemia. This contraction state is significantly more pronounced after reperfusion of St. Thomas arrested hearts. Thus, the contraction state of myofibrils is influenced not only by alterations in metabolite concentrations, but also by the composition of cardioplegic solutions and by the characteristic conditions (sufficient energy, oxygen and Calcium) during reperfusion.

Time course of regional flow distribution following reperfusion of the isolated ischaemic rat heart

1. The regional distribution of flow was studied at different times after the onset of reperfusion in isolated rat heart preparations. The hearts were submitted to 30 min of global ischaemia followed by 60 min of reperfusion. Microspheres labelled with various nuclides were added to the perfusate before ischaemia and 1, 5, 20, and 60 min after the onset of reperfusion. 2. One minute after the start of reperfusion, the flow to the left ventricular inner layer was restricted to 0.5 +/- 0.2 mL/min per g (2-3% of the pre-ischaemic flow). In this segment, the perfusion remained at the same low level during the entire reperfusion period studied. At the onset of reperfusion the flow to the outer layer of the left ventricle was 4.8 +/- 1.7 mL/min per g (37% of the pre-ischaemic flow), and 3.0 +/- 1.3 mL/min per g (27% of the pre-ischaemic value) to the free wall of the right ventricle. The flow was progressively reduced in the outer layer of the left ventricle in the course of reperfusion. After 60 min of reperfusion the flow to the left ventricular outer layer was 2.5 +/- 0.9 mL/min per g (19% of pre-ischaemic flow when compared with the onset of reperfusion [P less than 0.05]). 3. It is concluded that a 'no-reflow' condition develops very early during reperfusion and becomes more marked during this period.

The role of post-ischaemic reperfusion in the development of microvascular incompetence and ultrastructural damage in the myocardium

To determine the contribution of oxygenated reperfusion to the development of myocardial microvascular incompetence and ultrastructural damage following ischaemia, isolated buffer perfused rat hearts were subjected to either temporary (n = 15) or permanent (n = 15) ischaemia for 15, 30 or 45 minutes. The temporarily ischaemic hearts were reperfused for 5 min with oxygenated Krebs Henseleit buffer. All hearts were then fixed by perfusion fixation with nitrogen-bubbled glutaraldehyde. The transmural development of microvascular incompetence was determined quantitatively by scanning electron microscopy using nuclear track photographic emulsion as an intravascular marker of competent capillaries, and ultrastructural damage was examined by transmission electron microscopy. Thirty or more minutes of ischaemia where required to significantly reduce the mean density of competent capillaries in the subendocardial third of the left-ventricular wall. Such ischaemic myocardium contained relatively normal, open unobstructed vessels, indicating that the microvascular incompetence arising during ischaemia per se was not due to ultrastructural change in the capillaries. Subendocardial myocardium reperfused following 15 min ischaemia also showed little ultrastructural change, but did show a significant reduction in the density of competent capillaries. However, reperfusion of more severely ischaemic myocardium resulted in obvious ultrastructural damage as well as significant further reduction in capillary competence. These findings demonstrate that oxygenated reperfusion of ischaemic myocardium paradoxically results in the further development of microvascular incompetence and, in severely ischaemic myocardium, also to additional ultrastructural damage.

Ultrastructural changes in rat hearts following cold cardioplegic ischemia of differing duration and differing modes of reperfusion

Morphologic consequences of prolonged global hypothermic (15 degrees C), cardioplegic ischemia and two reperfusion techniques were studied in Langendorff-perfused rat hearts. A 'gentle' reperfusion technique, with gradual rise in perfusate temperature and pressure to physiologic levels over 30 min, was used for 12 hearts following 2-hour or 3 1/2-hour (6 in each group) ischemia. Abrupt reperfusion, with perfusate at 37 degrees C and 70 mmHg, was performed on 13 hearts (6 ischemic for 2 hours and 7 for 3 1/2 hours). Six nonischemic, perfused hearts served as controls. Randomly selected specimens from the left ventricle after 45-60 min reperfusion were prepared for transmission electron microscopy. Volume fractions of myocardial structural components were calculated from stereologic point-counting on the electron micrographs. Two-way analysis of variance revealed that interstitial edema developed with increasing ischemic time and was not influenced by reperfusion technique. The degree of endothelial damage was independent of ischemic time, but was lessened by 'gentle' reperfusion. Both mitochondrial injury and myocyte edema were less when perfusate temperature and pressure were slowly raised after 3 1/2-hour ischemia.

The transmural progression of the no-reflow phenomenon in globally ischemic hearts

A no-reflow phenomenon (NRP) develops in hearts subjected to global ischemia and prevents reperfusion of the subendocardial myocardium upon restoration of arterial supply. In the present study the transmural progression of the NRP across the left ventricular wall in globally ischemic rat hearts was quantitatively defined by using autoradiographic nuclear track emulsion (NTE) as an indicator of microvascular competence. Rat hearts were isolated and perfused for 10 min with oxygenated Krebs-Henseleit buffer, then were made completely globally ischemic for from 0 to 60 min and were maintained at 37 degrees C. They were then fixed by perfusion with glutaraldehyde after which NTE was injected into the coronary arteries. Transverse sections through the left ventricles were examined by scanning electron microscopy using back-scattered electron imaging and the vessels in a standard transmural contiguous series of photomicrographs were classified according to whether they did or did not permit the flow of NTE. Non-ischemic control myocardium showed a mean proportion of filled vessels of 99.4 +/- 0.5% SD, and those subjected to 15 min of ischemia showed only a slight overall reduction. After 30 min of ischemia 96 +/- 3% of vessels in the subepicardial third could be reperfused, but the proportion progressively diminished across the myocardium to total no-reflow near the endocardium. From 45-60 min of ischemia the totally non-reperfusible region remained confined to the subendocardial third but there was a significant reduction in the proportion of reperfusible vessels in the subepicardial third to 40% +/- 27%. Ischemia thus progressively reduces the capacity of myocardium to be reperfused.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased ischemia-reperfusion injury to the heart associated with short-term, diet-induced hypercholesterolemia in rabbits

The effects of increased dietary cholesterol content on coronary vascular hemodynamics and endothelial cell transport function were assessed in isolated rabbit hearts during 3.5 hours of reperfusion after 30 minutes of global, no-flow ischemia. In control hearts from rabbits fed normal chow, perfusion pressure, left ventricular end-diastolic pressure, maximum +dP/dt, and the rate of intravascular clearance of radiolabelled albumin remained constant during 5 hours of continuous perfusion, while the mean transit time of radiolabelled albumin increased 1.6 X baseline. In ischemic hearts from rabbits fed normal chow, perfusion pressure increased 59% during reperfusion while left ventricular end-diastolic pressure and maximum +dP/dt returned toward control levels. The rate of intravascular clearance of radiolabelled albumin decreased 36%, and the mean transit time of albumin increased approximately 3 X baseline. Ischemia-reperfusion injury to the cardiac vasculature and musculature was markedly increased in hearts of rabbits fed chow supplemented with 2% cholesterol for 2-3 weeks compared to rabbits fed the same diet for a longer duration (5-16 weeks) or rabbits fed normal chow. Prior to ischemia, permeation of the coronary vasculature by albumin was increased twofold in rabbits fed cholesterol for 2-3 weeks while myocyte contractile function was normal relative to chow-fed controls or the group fed cholesterol for 5-16 weeks. These effects of acute cholesterol feeding precede occlusive atherosclerotic coronary artery disease and occur at plasma cholesterol concentrations one third of those in rabbits fed cholesterol for the longer duration.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention of transcoronary macromolecular leakage after ischemia-reperfusion by the calcium entry blocker nisoldipine. Direct observations in isolated rat hearts

Coronary microvascular damage appears to play a role in reperfusion injury after myocardial ischemia. This study was designed to afford direct viewing of the effects of myocardial ischemia-reperfusion on the coronary microcirculation and to determine whether pretreatment with the calcium blocker nisoldipine would attenuate any microvascular damage during reperfusion. Four groups of isolated rat hearts were perfused with a solution that contained red cells and fluorescent albumin, but was essentially free of platelets and leukocytes. Group I served as a nonischemic control. Group II hearts were subjected to 30 minutes of no-flow ischemia followed by reperfusion. Group III hearts were pretreated with nisoldipine (1 microgram/min) for 5 minutes before ischemia, and group IV hearts were treated with nitroglycerin (93 micrograms/min) before and after ischemia to mimic the vasodilation caused by nisoldipine. Perfused coronary capillarity and transcoronary extravasation of plasma albumin were measured by direct visualization techniques before and after ischemia. For group I, there was no significant change in coronary resistance, perfused capillarity, or transcoronary extravasation with time. For both groups II and IV, ischemia-reperfusion caused no increase in coronary resistance, but a significant decrease in perfused capillarity and a marked increase in transcoronary extravasation of fluorescent albumin (P less than 0.05). The nisoldipine group (group III) demonstrated a similar decrease in perfused capillarity but no increase in protein extravasation during reperfusion. These results indicate that, in the heart, platelets and/or leukocytes are not absolutely necessary to induce either the no-reflow phenomenon or the permeability damage observed during reperfusion after ischemia. The protective effect of treatment with nisoldipine appeared to be independent of vasodilation. We speculate that this calcium blocker reduced endothelial uptake of calcium during reperfusion, preventing endothelial deformation and formation of interendothelial gaps.

Reversal of the no reflow phenomenon in globally ischemic rat hearts by ventricular dilation

Changes in the contracture and stiffness of ventricular walls during ischemia and their effect on vascular reperfusion were studied in isolated rat hearts. Global ischemia was induced by stopping the flow of oxygenated perfusate for 60 min. Contracture pressures generated against water-filled left ventricular balloons maintained at diastolic volume increased between 8 and 25 min of ischemia and declined thereafter. On the other hand, left ventricular wall stiffness, estimated from the pressure required to periodically inflate otherwise uninflated balloons to diastolic volume, increased rapidly between 20 and 30 min of ischemia, and more slowly thereafter. Inflation of balloons following 30 or more min of ischemia caused significant reductions in left ventricular wall stiffness. Similar brief inflation after 60 min of ischemia allowed aqueous sodium fluorescein to perfuse the subendocardial half of the left ventricular wall which was otherwise not accessible to reperfusion. This reversal of the no reflow phenomenon was accompanied by a reduction in myocardial wall stiffness, stretching of myocytes and increased patency of myocardial capillaries.

Hyaluronidase does not prevent deterioration of vascular functional integrity during reperfusion after no-flow ischemia in isolated rabbit hearts

Effects of hyaluronidase on myocardial water content and distribution, and on coronary vascular hemodynamics and endothelial cell transport function were assessed in isolated rabbit hearts during 3.5 hours of reperfusion after 30 minutes of global, no-flow ischemia. In nonischemic control hearts, perfusion pressure, left ventricular end-diastolic pressure, maximum +dP/dt, and intravascular clearance of radiolabeled albumin remained constant during 5 hours of continuous perfusion, while the mean-transit time and vascular into extravascular space clearance of radiolabeled albumin increased 1.5X and 2.5X baseline, respectively. During reperfusion after 30 minutes of no flow, perfusion pressure increased 53% and interstitial fluid volume increased 2-fold, while left ventricular end-diastolic pressure and maximum +dP/dt returned to control levels. The rate of intravascular clearance of radiolabeled albumin decreased 38%, and the mean-transit time and vascular-into-extravascular space clearance of albumin increased approximately 3X and 5X baseline, respectively. Hyaluronidase blocked the ischemia-reperfusion-induced increases in total water content and in interstitial fluid volume and reduced the increases in perfusion pressure and mean-transit time of radiolabeled albumin by 40% and 45%, respectively, but did not prevent the increase in albumin vascular-into-extravascular space clearance and the decrease in albumin clearance from the coronary vasculature. These findings indicate that hyaluronidase does not prevent ischemia-reperfusion-induced increases in albumin permeation of the coronary vasculature, and suggest that its protective effect on ischemic myocardium is mediated, instead, by reducing interstitial edema and vascular resistance.

Ischaemic contracture in isolated rat heart: reversible or irreversible myocardial injury?

An isolated rat heart preparation was reperfused at 37 degrees C for 10 min after 10, 20, 30 and 40 min of ischaemia. The left ventricular tension was measured by means of a balloon catheter filled with water and connected to a pressure recorder. The left ventricular resting tension began to increase at 9 +/- 1 min (mean +/- SEM) and was maximally developed (myocardial contracture) at 18 +/- 1 min of ischaemia. There was a striking and constant exacerbation of the resting tension during reperfusion after 30 and 40 min (but not after 10 or 20 min) of ischaemia with simultaneous acceleration of creatine phosphokinase (CK) release into the coronary effluent and with the loss of the recovery of contractile activity. Myocardial adenosine triphosphate (ATP)-content decreased during 20 min of ischaemia more in the endocardial (ENDO) (from 17.7 +/- 1.9 mumol/g to 0.7 +/- 0.1 mumol/g) than in epicardial (EPI) (from 15.5 +/- 0.9 mumol/g to 3.2 +/- 0.6 mumol/g) parts of myocardium. Reperfusion after 10 min of ischaemia resulted in a slight increase of myocardial ATP-content both in EPI (from 7.5 +/- 0.6 to 10.4 +/- 0.8 mumol/g, p less than 0.05) and ENDO (from 5.0 +/- 0.8 to 8.9 +/- 2.5 mumol/g, n.s.). Reperfusion after the completion of contracture (after 20 min) had no effect on myocardial ATP-content. The results indicate that there is a transmural ATP gradient in ischaemic isolated rat heart and that myocardial ATP net production during reperfusion (10 min) is prevented after the development of ischaemic contracture.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of calcium in coronary vasoconstriction due to prolonged hypoxia

In this study we employed a hypoxic rat heart model for investigating the mechanisms of coronary spasm. Perfusion of the isolated heart with hypoxic medium resulted in initial coronary dilatation followed by sustained constriction. Pretreatment of rats with 5 mg/kg reserpine did not alter the rate of the magnitude of constriction during 60 minutes of hypoxic perfusion. The hypoxia-induced vasoconstriction was not affected by the inclusion of either 1 or 10 micrograms/ml phentolamine in the perfusion buffer, but phenoxybenzamine (0.1 and 1 microgram/ml) significantly attenuated the degree of constriction. Since phenoxybenzamine and phentolamine were equally effective in preventing the phenylephrine-induced coronary constriction, it is unlikely that the effect of phenoxybenzamine was due to an alpha-receptor blocking property but instead may be accounted for by its calcium channel blocking action. Two calcium channel blockers, verapamil and D-600 (0.1 and 1 microgram/ml), were also effective. The rate of rise in coronary pressure was substantially reduced by decreasing the concentration of calcium and was increased by elevating the concentration of calcium in the perfusion medium, but the magnitude of hypoxia-induced constriction was not affected. These results are consistent with the suggestion that the coronary constriction seen during hypoxia does not involve adrenergic mechanisms but is dependent upon the availability of calcium.

Effect of verapamil on reperfusion damage and calcium paradox in isolated rat heart

The effect of verapamil on ischaemic contracture, reperfusion injury after global ischaemia, and calcium paradox was studied in isolated rat heart. The development of ischaemic contracture was measured by a balloon catheter inserted into the left ventricle, filled with water, and connected to a pressure recorder. Myocardial perfusability was tested by infusing sodium fluorescein solution into the cannulated aortic root. Creatine kinase (CK) activity and protein leakage from myocardium into the effluent were measured during reperfusion (30 min) after ischaemia (50 min) and during calcium repletion (30 min) after calcium depletion (5 min). Ischaemic contracture developed later in the verapamil-treated group (31.7 +/- 0.7 min) than in the control group (23.6 +/- 2.2 min). There was less CK (29.16 +/- 3.26 U/g) and protein leakage (4.81 +/- 0.41 mg/g) in the verapamil-treated group than in the control group (38.81 +/- 3.30 U/g and 6.14 +/- 0.49 mg/g, respectively) during reperfusion. Verapamil had no effect on the myocardial perfusability or on the strength of the ischaemic contracture. Verapamil did not reduce the CK or protein leakage in calcium paradox. It was concluded that verapamil has some protective effect against cell injury on myocardium during post-ischaemic reperfusion but none in calcium paradox.

Changes in vascular morphology associated with the no-reflow phenomenon in ischaemic myocardium

To investigate the pathogenesis of the reperfusion defect which develops in ischaemic myocardium, intravascular casts were prepared by injection of methyl methacrylate into the coronary arteries of isolated heparinised rat hearts. Using a scanning electron microscope, the vascular morphology following 60 min of global ischaemia at 37 degrees C was compared to that of non-ischaemic control hearts injected immediately after stopping perfusion with oxygenated Krebs-Henseleit buffer. Complete casts were obtained from control hearts and from all parts of ischaemic hearts except the subendocardial half of the left ventricular wall of ischaemic hearts where the blood vessels were not filled. At the border between the perfused subepicardial and unperfused left subendocardial regions, the resin which filled the radial penetrating arteries and their branches projected from the filled capillary plexus to an extent proportional to their diameter. Intravascular events such as erythrocyte plugging and thrombosis were excluded as causative factors by the use of a cell-free perfusate. Also, there was no morphological evidence that endothelial cell swelling or constriction of any particular population of vessels was involved. The observed pattern of vascular occlusion suggests that, during global ischaemia, blood vessels in the endocardial half of the left ventricular myocardium lose their ability to be reperfused because of extravascular compression.

A comparison of the effects of ischemia and of selective occlusion of capillaries, pre-capillaries and terminal arterioles on coronary reperfusion

To study the 'no-reflow' phenomenon is ischemic myocardium, the effects of ischemia and selective embolic blockade of capillaries, precapillaries and terminal arterioles were compared in isolated rat hearts. Hearts received oxygenated Krebs-Henseleit buffer for 10 min via an aortic cannula, and then coronary perfusion was stopped. The pattern and extent of reperfusion after 15-90 min of global ischemia and after the injection of 9, 15 or 55 mu diameter microspheres were determined from the distribution of injected 6.7% fluorescein isothiocyanate-dextran in frozen transverse sections of the ventricles. Following ischemia, progressively larger subendocardial regions surrounding the left ventricle could not be reperfused. In contrast, embolic occlusion of capillaries, precapillaries or terminal arterioles caused a transmural reduction in perfusion and a fine linear or herringbone pattern of fluorescence. Sixty min of ischemia followed by microsphere injection had no effect on the subendocardial zone of no-reflow but much reduced the intensity of fluorescence elsewhere. Thus thrombosis, erythrocyte plugging and occlusion of capillaries, precapillaries or terminal arterioles are unlikely to be primary causes of the reperfusion defect which develops in ischemic myocardium.

A method for the gross and microscopic investigation of vascular reperfusion in ischemic myocardium

Fluorescein-isothiocyanate dextran (FITC-dextran; MW approximately 70,000) was used in isolated rat hearts to compare normal vascular perfusion of ventricular myocardium with the pattern and extent of reperfusion following 60 minutes of global ischemia. Its gross distribution in frozen transverse sections through the ventricles was similar to that of sodium fluorescein. However, unlike 0.1% sodium fluorescein, 6.7% FITC-dextran has a viscosity similar to that of blood, and its much higher molecular weight prevents its diffusion beyond the ischemically injured vessels. Furthermore, staining by the alcoholic periodic acid-Schiff technique enabled tracer distribution to be confirmed microscopically and distinguished competent from incompetent vessels in paraffin embedded material.

The relationship between ischemic contracture and no-reflow phenomenon in isolated rat heart

The relationship between ischemic contracture and no-reflow phenomenon was studied in 59 isolated rat hearts during global ischemia. The contracture was measured by a water-filled balloon catheter placed in the left ventricular lumen. The time of onset of contracture was changed by preischemic infusion with buffer containing 0.5 mmoles/l iodoacetate (IAA) in order to get early contracture, and by hypothermia which delayed the development of contracture. The first signs of contracture were noticed in normothermia (37 degrees C) at 11 minutes, in hypothermia (26 degrees C) at 25 minutes, and in the IAA-infused group at 3 minutes. The completion of contracture occurred in these groups at 25, 90 and 11 minutes, respectively. The myocardial perfusability was tested at the pre- and postcontracture state by infusing 0.1% fluorescein in isotonic saline into the cannulated aortic root. The myocardial area perfused with fluorescein was quantified in colour photographs taken under ultraviolet light of frozen whole-heart sections. The myocardial perfusion - expressed ad percent of myocardial area - was 99% at the precontracture state in normothermia, 95% in hypothermia and 100% in the IAA-infused group. At the postcontracture state, the myocardial perfusion in these groups was 80, 56 and 18%, respectively. It was concluded that the no-reflow phenomenon in isolated rat heart is closely associated with the development of myocardial contracture during global ischemia.

The effect of an isovolumic left ventricle on the coronary vascular competence during reflow after global ischemia in the rat heart

During global ischemia in isolated rat hearts, the development of contracture, due to irreversible myofilament sliding, causes reduction of left ventricle luminal volume. Also, a considerable area of the myocardium cannot be reperfused after 1 hour's global ischemia. The purpose of this study was to reduce myofilament sliding by placing a fluid-filled isovolumic balloon in the left ventricular cavity of isolated rat hearts and assess the extent of reflow, after 60 minutes' ischemia, by perfusion of a 1% fluorescein tracer solution. Light and electron microscopy was used to determine the state of the vasculature and myofibrillar apparatus. In hearts without the left ventricular balloon (control) the ischemia produced a no-reflow zone comprising 45% of the myocardial wall. In contrast, if an isovolumic balloon was in place during the ischemic period, only 6% of the wall was involved. The volume of the capillary bed in the subendocardium of the control hearts was about 60% of that in th isovolumic hearts. In the isovolumic ("isometric") mode, ischemic contracture was associated with more severe myocardial cell injury than in the corresponding control ("isotonic") mode. Our results support the concept that intramyocardial pressure generated by ischemic contracture plays a major role in the production of the no-reflow phenomenon in globally ischemic rat hearts, and indicate that it is the series elastic component of cardiac muscle which imparts the stiffness necessary to prevent reopening of coronary vessels after a severe ischemic insult.

Effect of hypoxia on the microvascular function in isolated rat hearts

The effect of hypoxia on the microvascular function in continuously perfused isolated rat hearts was studied. The development of hypoxic contracture was determined by measuring the left ventricular pressure or the decrease of the left ventricular volume. The myocardial perfusability was tested by infusing 0.1 percent sodium fluorescein in isotonic saline into the cannulated aortic root at 0, 15, 30, 60 and 90 minutes of hypoxic glucose-free buffer perfusion. The percentage perfused with the fluorescent tracer in horizontal frozen myocardial sections was estimated by point counting from colour photographs taken under ultraviolet light. The coronary flow was estimated by measuring the flow of the perfusate through the vasculature under constant perfusion pressure. The hypoxic contracture was completed in 25 minutes in which time the coronary flow decreased gradually by 50 percent and thereafter the flow remained constant. The myocardium was fully perfused in controls and at 0 minutes of hypoxic glucose-free perfusion. There was an abrupt decrease in the perfusability between 15 and 30 minutes from 99 percent to 58 percent, respectively. At 60 and 90 minutes 65 percent of the myocardium were still perfused. It was concluded that the myocardial perfusability decreases when the left ventricular contracture develops during hypoxic perfusion.