Prevention of cellular edema directly caused by hypothermic cardioplegia: studies in isolated human and rabbit atrial myocytes

Resolution of hyposmotic stress in isolated mouse ventricular myocytes causes sealing of t-tubules

It has recently been shown that various stress-inducing manipulations in isolated ventricular myocytes may lead to significant remodelling of t-tubules. Osmotic stress is one of the most common complications in various experimental and clinical settings. This study was therefore designed to determine the effects of a physiologically relevant type of osmotic stress, hyposmotic challenge, to the integrity of the t-tubular system in mouse ventricular myocytes using the following two approaches: (1) electrophysiological measurements of membrane capacitance and inward rectifier (IK1) tail currents originating from K(+) accumulation in t-tubules; and (2) confocal microscopy of fluorescent dextrans trapped in sealed t-tubules. Importantly, we found that removal of '0.6 Na' (60% NaCl) hyposmotic solution, but not its application to myocytes, led to a ∼27% reduction in membrane capacitance, a ∼2.5-fold reduction in the amplitude of the IK1 tail current and a ∼2-fold reduction in the so-called IK1 'inactivation' (due to depletion of t-tubular K(+)) at negative membrane potentials; all these data were consistent with significant detubulation. Confocal imaging experiments also demonstrated that extracellularly applied dextrans become trapped in sealed t-tubules only upon removal of hyposmotic solutions, i.e. during the shrinking phase, but not during the initial swelling period. In light of these data, relevant previous studies, including those on excitation-contraction coupling phenomena during hyposmotic stress, may need to be reinterpreted, and the experimental design of future experiments should take into account the novel findings.

Diazoxide maintains human myocyte volume homeostasis during stress

Background

Exposure to hypothermic hyperkalemic cardioplegia, hyposmotic stress, or metabolic inhibition results in significant animal myocyte swelling (6% to10%) and subsequent reduced contractility (10% to 20%). Both are eliminated by the adenosine triphosphate-sensitive potassium channel opener diazoxide (DZX). The relationship between swelling and reduced contractility suggests that the structural change may represent one mechanism of postoperative myocardial stunning. This study evaluated human myocyte volume during stress to investigate if similar phenomena exist in human myocytes.

Methods and Results

Human atrial myocytes isolated from tissue obtained during cardiac surgery were perfused with Tyrode's physiological solution (20 minutes, 37°C), test solution (20 minutes), and Tyrode's (37°C, 20 minutes). Test solutions (n=6 to 12 myocytes each) included Tyrode's (37°C or 9°C), Tyrode's+DZX (9°C), hyperkalemic cardioplegia (9°C)±DZX, cardioplegia+DZX+HMR 1098 (sarcolemmal adenosine triphosphate-sensitive potassium channel inhibitor, 9°C), cardioplegia+DZX+5-hydroxydeconoate (mitochondrial adenosine triphosphate-sensitive potassium channel inhibitor, 9°C), mild hyposmotic solution±DZX, metabolic inhibition±DZX, and metabolic inhibition+DZX+5-hydroxydeconoate. Myocyte volume was recorded every 5 minutes. Exposure to hypothermic hyperkalemic cardioplegia, hyposmotic stress, or metabolic inhibition resulted in significant human myocyte swelling (8%, 7%, and 6%, respectively; all P<0.05 vs control). In all groups, the swelling was eliminated or lessened by DZX. The addition of channel inhibitors did not significantly alter results.

Conclusions

DZX maintains human myocyte volume homeostasis during stress via an unknown mechanism. DZX may prove to be clinically useful following the elucidation of its specific mechanism of action. (J Am Heart Assoc. 2012;1:jah3-e000778 doi: 10.1161/JAHA.112.000778.)

Myocardial microvascular permeability, interstitial oedema, and compromised cardiac function

The heart, perhaps more than any other organ, is exquisitely sensitive to increases in microvascular permeability and the accumulation of myocardial interstitial oedema fluid. Whereas some organs can cope with profound increases in the interstitial fluid volume or oedema formation without a compromise in function, heart function is significantly compromised with only a few percent increase in the interstitial fluid volume. This would be of little consequence if myocardial oedema were an uncommon pathology. On the contrary, myocardial oedema forms in response to many disease states as well as clinical interventions such as cardiopulmonary bypass and cardioplegic arrest common to many cardiothoracic surgical procedures. The heart's inability to function effectively in the presence of myocardial oedema is further confounded by the perplexing fact that the resolution of myocardial oedema does not restore normal cardiac function. We will attempt to provide some insight as to how microvascular permeability and myocardial oedema formation compromise cardiac function and discuss the acute changes that might take place in the myocardium to perpetuate compromised cardiac function following oedema resolution. We will also discuss compensatory changes in the interstitial matrix of the heart in response to chronic myocardial oedema and the role they play to optimize myocardial function during chronic oedemagenic disease.

Towards modeling of cardiac micro-structure with catheter-based confocal microscopy: a novel approach for dye delivery and tissue characterization

This work presents a methodology for modeling of cardiac tissue micro-structure. The approach is based on catheter-based confocal imaging systems, which are emerging as tools for diagnosis in various clinical disciplines. A limitation of these systems is that a fluorescent marker must be available in sufficient concentration in the imaged region. We introduce a novel method for the local delivery of fluorescent markers to cardiac tissue based on a hydro-gel carrier brought into contact with the tissue surface. The method was tested with living rabbit cardiac tissue and applied to acquire three-dimensional image stacks with a standard inverted confocal microscope and two-dimensional images with a catheter-based confocal microscope. We processed these image stacks to obtain spatial models and quantitative data on tissue microstructure. Volumes of atrial and ventricular myocytes were 4901 +/- 1713 and 10 299 +/-3598 mum (3) (mean+/-sd), respectively. Atrial and ventricular myocyte volume fractions were 72.4 +/-4.7% and 79.7 +/- 2.9% (mean +/-sd), respectively. Atrial and ventricular myocyte density was 165 571 +/- 55 836 and 86 957 +/- 32 280 cells/mm (3) (mean+/-sd), respectively. These statistical data and spatial descriptions of tissue microstructure provide important input for modeling studies of cardiac tissue function. We propose that the described methodology can also be used to characterize diseased tissue and allows for personalized modeling of cardiac tissue.

Hyperkalemic cardioplegia-induced myocyte swelling and contractile dysfunction: prevention by diazoxide

BACKGROUND

Hyperkalemic cardioplegia (9 degrees C) results in significant myocyte swelling and reduced contractility, representing a possible mechanism of myocardial stunning. Adenosine triphosphate-sensitive potassium channel (KATP) openers have been shown to ameliorate stunning. This study evaluated the hypothesis that a KATP opener would prevent hyperkalemic cardioplegia-induced myocyte swelling and reduced contractility.

METHODS

Isolated rabbit myocytes were perfused with 37 degrees C Tyrode's solution for 20 minutes, followed by test solution (9 degrees C or 37 degrees C) including control Tyrode's, Tyrode's + 100 micromol/L diazoxide (KATP opener), St. Thomas's solution; or 9 degrees C St. Thomas's + 100 micromol/L diazoxide or St. Thomas's + 100 micromol/L diazoxide + 20 micromol/L HMR1098 or 50 micromol/L 5-hydroxydeconoate (KATP blockers) for 20 minutes (n = 8 per group). Myocytes were then reexposed to 37 degrees C Tyrode's solution for 20 minutes. Volume and contractility were measured by videomicroscopy and video-based edge detection, respectively.

RESULTS

St. Thomas's solution (9 degrees C) caused significant myocyte swelling and associated reduced contractility (p < 0.05). The addition of diazoxide abolished myocyte swelling (p < 0.0001), and eliminated the associated reduced contractility (p < 0.05). Findings were unchanged by the addition of HMR 1098 and 5-hydroxydeconoate.

CONCLUSIONS

Diazoxide prevented myocyte swelling and reduced contractility secondary to hyperkalemic cardioplegia, and this was unchanged by the addition of either KATP channel blocker. Prevention of myocyte swelling was associated with improved contractility, consistent with the hypothesis that myocyte swelling may be a mechanism of myocardial stunning. Diazoxide may play a role in myocyte volume homeostasis by means of a mechanism separate from opening the KATP channel.

Role of the sarcolemmal adenosine triphosphate-sensitive potassium channel in hyperkalemic cardioplegia-induced myocyte swelling and reduced contractility

BACKGROUND

Hyperkalemic cardioplegia (Plegisol) has been shown to result in myocyte swelling and reduced contractility. We have demonstrated the elimination of these detrimental effects by the addition of an adenosine triphosphate-sensitive K+ (KATP) channel opener. To examine whether the mitochondrial or sarcolemmal KATP channel might be involved, volume and contractility in isolated myocytes from wild-type mice and mice lacking the sarcolemmal KATP channel (Kir6.2-/-) were evaluated.

METHODS

Myocytes were perfused for 20 minutes each with control 37 degrees C Tyrode's solution, test solution, and then control solution. Test solutions were (n = 10 per group) either 9 degrees C Plegisol or 9 degrees C Plegisol with 100 micromol/L of diazoxide, a putative mitochondrial-specific KATP channel opener. Cell volume and contractility were measured by digital video microscopy at baseline and during the test solution and reexposure periods.

RESULTS

Myocytes from wild-type mice, perfused with 9 degrees C Plegisol, demonstrated significant cell swelling (11.2% +/- 0.4%; p < 0.01) and diminished contractility (32.5% +/- 9.6% reduction in percent shortening, 47.2% +/- 10.1% reduction in peak velocity of shortening, and 52.0% +/- 8.8% reduction in peak velocity of relengthening; p < 0.05) versus baseline. Cell swelling and diminished contractility were significantly reduced by the addition of diazoxide. In Kir6.2-/- myocytes, Plegisol caused a greatly reduced level of cell swelling (3.2% +/- 0.1%; p < 0.01), and this was unaffected by diazoxide. Contractility was unchanged in Kir6.2-/- myocytes after Plegisol.

CONCLUSIONS

The sarcolemmal KATP channel appears necessary for exaggerated cell swelling and reduced contractility to occur after hyperkalemic cardioplegia in mouse myocytes.

Myocyte volume and function in response to osmotic stress: observations in the presence of an adenosine triphosphate-sensitive potassium channel opener

BACKGROUND

Hypothermic hyperkalemic cardioplegia results in significant myocyte swelling and impaired contractility. These detrimental effects may be eliminated by the addition of an adenosine triphosphate-sensitive potassium (KATP) channel opener. This study evaluated the hypothesis that a KATP channel opener (diazoxide) would benefit volume homeostasis by limiting volume and subsequent contractility changes during osmotic stress.

METHODS AND RESULTS

Isolated rabbit ventricular myocyte volume and contractility were evaluated using video microscopy and field stimulation after exposure to osmotic stress at 37 degrees C. Myocytes were exposed to Tyrode's physiological solution for 20 minutes and test solution for 20 minutes, and then reexposed to Tyrode's for 20 minutes. Test solutions included control Tyrode's (1T) and osmotically altered Tyrode's (2.6T, 0.9T, and 0.6T) solutions with or without the KATP channel opener diazoxide. Severe osmotic stress (2.6T and 0.6T) resulted in significant cell shrinkage and swelling, respectively. This was unchanged by the addition of diazoxide. Mild hyposmotic stress (0.9T) resulted in significant cell swelling that was eliminated by the addition of diazoxide. Cell swelling was associated with reduced contractility.

CONCLUSIONS

Cell swelling, but not shrinkage, was detrimental to myocyte contractility. Diazoxide eliminated volume change due to mild hyposmotic stress, similar to that previously noted with hyperkalemic cardioplegia, but did not alter volume change secondary to severe osmotic stress.

Different preservation of myocardial capillary endothelial cells and cardiomyocytes during and after cardioplegic ischemia (25 degrees C) of canine hearts

Complete resumption of cardiac function after cardioplegic arrest presupposes a well-preserved myocardial ultrastructure during and after ischemia. Therefore, we determined ischemia-induced ultrastructural alterations in the myocardium during and after reversible cardioplegic ischemia using stereological methods. Cardiac arrest was induced with St. Thomas' Hospital- or Custodiol (HTK) solution. Reperfusion with Tyrode's solution followed after reversible cardioplegic ischemia in situ. Samples were taken 1) from beating hearts, 2) from cardioplegically arrested hearts immediately after the end of coronary perfusion, 3) from ischemic hearts incubated in the cardioplegic solution at 25 degrees C, and 4) from reperfused beating hearts after ischemia in situ at 22 degrees C. Cellular swelling was determined as the barrier thickness of capillary endothelium and as the sum of cardiomyocyte volume fractions of free sarcoplasm and mitochondria. In St. Thomas'-arrested hearts, intraischemic volume increase was significantly more pronounced in endothelial cells than in cardiomyocytes. Reperfusion at the intraischemic practical limit of resuscitability (ATP levels of 4 micromol/gww) significantly reduced intraischemic swelling of cardiomyocytes, but not of capillary endothelial cells. Mitochondrial damage was more pronounced in capillary endothelial cells during ischemia and after reperfusion. Thus, after reversible cardioplegic arrest, structural recovery of cardiomyocytes is better than that of capillary endothelial cells. An incomplete structural protection of capillary endothelial cells may predominantly contribute to postischemic dysfunction in the reperfused heart.

Cardiac swelling associated with linear radiofrequency ablation in the atrium

OBJECTIVES

To characterize myocardial swelling in response to application of endocardial radio-frequency ablation lesions.

BACKGROUND

In individual patients, we have observed that ablation in the posterior right atrium was associated with echocardiographic evidence of atrial and contiguous right pulmonary vein wall swelling.

METHODS

1. Human Subjects: "linear" ablation was performed in the posterior right atrium in 10 subjects; a portion of the ablation lesion was contiguous to the right pulmonary vein; this area was defined as the "contiguity zone". In the contiguity zone, right atrial wall thickness and pulmonary vein lumen diameter were measured utilizing intracardiac echocardiography. Measurements were made just prior to (baseline) and immediately after ablation.2. Porcine Subjects: linear ablation was performed in the posterior right atrium of 14 pigs. In the contiguity zone, atrial wall thickness, interstitial space thickness, right pulmonary vein wall thickness and lumen diameter were measured using intracardiac echocardiography. Measurements were made at baseline, immediately after ablation, and at 1, 4, 8 or 12 weeks after ablation (followup). Post-mortem pathologic evaluation of the contiguity zone was performed.

RESULTS

1. Human Subjects: Immediately after ablation, relative to baseline right atrial wall thickness was significantly increased (9.4+/-3.1mm versus 5.4+/-1.5 mm) and right pulmonary vein lumen diameter was significantly decreased (6.2+/-2.9 mm versus 8.1+/-2.9 mm).2. Porcine Subjects: Immediately after ablation, right atrial wall thickness (4.1+/-1.2 mm), interstitial space thickness (1.9+/-1.1mm), and right pulmonary vein wall thickness (1.2+/-0.4 mm) were each significantly increased relative to baseline (1.0+/-0.3 mm, 0+/-0 mm, and 0.7+/-0.2 mm, respectively) and pulmonary vein lumen diameter was significantly decreased (5.0+/-1.4 mm versus 6.9+/-2.2 mm). Similar findings were made at the 1 week followup interval. At 4, 8 and 12 week followup intervals, thicknesses and lumen diameter were not significantly different from baseline. At post-mortem examination, direct measurements of wall thickness were significantly correlated with echocardiographic measurements. Histologic analysis demonstrated edema to be the cause of the early wall thickness and lumen diameter changes. Ablation lesions were transmural in the right atria of all animals; in some animals, lesion formation was also observed in the pulmonary vein wall.

CONCLUSIONS

Cardiac edema resulting from right atrial linear ablation results in swelling of atrial and contiguous right pulmonary vein walls, as well as the interposed extracardiac interstitial space. These changes are associated with a decrease in pulmonary vein lumen diameter. Swelling evolves rapidly and resolves within 4 weeks.

Mechanisms responsible for cell volume regulation during hyperkalemic cardioplegic arrest

BACKGROUND

Cardioplegia has been shown to induce significant cell swelling. This study tested the hypothesis that (1) the [K+][Cl-] product of the cardioplegia solution is the main determinant of myocyte swelling, and (2) reperfusion myocyte shrinkage results from a rectifying Cl- conductance.

METHODS

Rabbit ventricular myocytes were superfused with 37 degrees C Krebs-Henseleit solution for 10 minutes. Then cells underwent 20 minutes of superfusion with standard St. Thomas' solution ([K+][Cl-] product = 2566 mmol/L2) and two solutions with lower [K+][Cl-] product (1500 and 700 mmol/L2) at 9 degrees C. Cells were then resuperfused with 37 degrees C Krebs-Henseleit solution for 30 minutes. Cell volume was measured by videomicroscopy.

RESULTS

Cells superfused with St. Thomas' having [K+][Cl-] products of 2,566, 1,500, and 700 mmol/L2 swelled by 9.18%+/-3.57%, 5.51%+/-1.08%, and 1.49%+/-1.56%, respectively. Reexposure to Krebs-Henseleit solution caused these cells to shrink by 5.79%+/-1.41%, 8.72% +/-3.68%, and 13.46%+/-5.60%, respectively. This shrinkage was blocked by Cl- channel blockers given at the onset of superfusion.

CONCLUSIONS

Lowering the [K+][Cl-] product of St. Thomas' solution attenuated myocyte edema. Myocyte shrinkage during reexposure to Krebs-Henseleit solution resulted from the volume-activated Cl- channel.

Cardioplegia-induced cell swelling: prevention by normothermic infusion

BACKGROUND

Previous work has shown significant swelling of isolated rabbit myocytes exposed to cold hyperkalemic cardioplegia; however, the effect of warm hyperkalemic cardioplegia on myocyte volume is unknown. This study examined the effect of warm hyperkalemic cardioplegia (St. Thomas' solution) on myocyte volume.

METHODS

Myocytes were enzymatically isolated and placed on an inverted video microscope. Tyrode's solution (37 degrees C) was infused for 10 minutes to establish baseline cell volumes. Subsequently, either the control Tyrode's or St. Thomas' was infused either at 37 degrees C and 9 degrees C respectively (n = 5 for all groups) for 20 minutes, followed by a 30-minute reperfusion with 37 degrees C Tyrode's. Cell volume was determined from cell images captured every 5 minutes.

RESULTS

Myocyte swelling occurred rapidly on exposure to cold St. Thomas' solution to a maximum of 9.8 +/- 2.1% (p < 0.001). In contrast, myocytes exposed to warm cardioplegia did not show any volume changes during exposure to cardioplegia. However, upon reexposure to Tyrode's, these cells showed shrinkage below their baseline volume (p < 0.001).

CONCLUSIONS

The cell swelling associated with hypothermic cardioplegia is prevented by normothermic infusion.

Rabbit ventricular myocyte volume changes as a direct result of crystalloid cardioplegia in congestive heart failure induced by aortic regurgitation

OBJECTIVES

We hypothesized that the cell volume of ventricular myocytes isolated from hearts in volume-overload congestive failure would respond differently to hypothermic cardioplegia than would sham-operated cohorts.

METHODS

Adult rabbits underwent either valvotomy and aortic regurgitation-induced heart failure or sham surgery. Congestive failure was confirmed clinically and by means of echocardiography. Cell volumes of isolated myocytes were measured by digital video microscopy. After equilibration in 37 degrees C physiologic solution, cells were suprafused with 9 degrees C standard or low-Cl(-) St Thomas' Hospital solution followed by reperfusion in 37 degrees C physiologic solution.

RESULTS

Exposure to cold St Thomas' Hospital solution for 20 minutes caused sham myocytes to swell by 8% (n = 9); cell volumes fully recovered on normothermic reperfusion. In contrast, congestive failure myocytes (n = 9) maintained their cell volume in cold St Thomas' Hospital solution and during reperfusion. Lowering the [K(+)][Cl(-)] product of St Thomas' Hospital solution by partially replacing Cl(-) with an impermeant anion prevented cellular edema in the sham group (n = 8) but caused a 4% swelling in failure myocytes (n = 10) on reperfusion. Osmotically shrinking the failure cells (n = 9) converted their behavior to that of sham cells.

CONCLUSIONS

In the absence of ischemia, congestive failure myocytes are less sensitive to cardioplegia-induced edema than sham cells. Low-Cl(-) cardioplegia, which prevents edema and protects the normal heart, induced swelling and may be detrimental in myopathic hearts. Differences in volume regulation in failure and sham myocytes may be due to activation of volume-sensitive channels that are turned off by osmotic shrinkage.

Age-related effects of St Thomas' Hospital cardioplegic solution on isolated cardiomyocyte cell volume

OBJECTIVES

We tested the hypothesis that neonatal cells are more sensitive to cardioplegia-induced cell swelling than more mature cells and spontaneous swelling in the absence of ischemia can be prevented by cardioplegia with a physiologic KCl product.

METHODS

Cell volumes of isolated ventricular myocytes from neonatal (3-5 days), intermediate (10-13 days), and adult (>6 weeks) rabbits were measured by digital video microscopy. After equilibration in 37 degrees C physiologic solution, cells were suprafused with 37 degrees C or 9 degrees C St Thomas' Hospital solution (standard or low Cl(-)) or 9 degrees C physiologic solution followed by reperfusion with 37 degrees C physiologic solution.

RESULTS

Neonatal cells swelled 16.2% +/- 1.8% (P <.01) in 37 degrees C St Thomas' Hospital solution and recovered during reperfusion, whereas more mature cells maintained constant volume. In contrast, 9 degrees C St Thomas' Hospital solution caused significant age-dependent swelling (neonatal, 16.8% +/- 1.5%; intermediate, 8.6% +/- 2.1%; adult, 5.6% +/- 1.1%). In contrast to more mature cells, neonatal cells remained significantly edematous throughout reperfusion (8.1% +/- 1.5%). Swelling was not due to hypothermia because 9 degrees C physiologic solution did not affect volume. Lowering the KCl product of St Thomas' Hospital solution by partially replacing Cl(-) with an impermeant anion prevented cellular edema in all groups.

CONCLUSION

In the absence of ischemia, neonatal cells were more sensitive to cardioplegia-induced cellular edema than more mature cells, and edema observed in all groups was avoided by decreasing the KCl product of St Thomas' Hospital solution to the physiologic range. Differences in cell volume regulation may explain the sensitivity of neonatal hearts to hyperkalemic cardioplegic arrest and suggest novel approaches to improving myocardial protection.

Prevention of cell swelling with low chloride St. Thomas' Hospital solution improves postischemic myocardial recovery

OBJECTIVE

In isolated myocytes cardioplegia-induced cell swelling can be prevented by lowering the KCl product by replacing Cl- with an impermeant ion. This study tested the hypothesis that Cl- substitution in St. Thomas' Hospital cardioplegic solution would result in superior myocardial protection in the intact, blood-perfused heart.

METHODS

Using a parabiotic, isolated rabbit heart Langendorff model, hearts were exposed to 1 hour of hypothermic (10 degrees to 12 degrees C), global ischemia followed by 30 minutes of reperfusion. Isosmotic cardioplegia was administered as a single 50 ml bolus of either standard St. Thomas' Hospital solution ([K+]o x [Cl-]o = 2566.4 (mmol/L)2) or low Cl- St. Thomas' Hospital solution ([K+]o x [CI-]o = 700 (mmol/L)2). Chloride was replaced by a large, impermeant ion, methanesulfonate. Postreperfusion systolic function and atrioventricular conduction times were measured before ischemia and after reperfusion.

RESULTS

Hearts receiving low Cl- St. Thomas' Hospital cardioplegia demonstrated significantly better postischemic functional recovery (74% +/- 3%) compared with those treated with standard high Cl- St. Thomas' Hospital solution (55% +/- 4%, p = 0.003). In addition, atrioventricular conduction times remained normal in the low Cl- group but were significantly prolonged in the St. Thomas' Hospital group.

CONCLUSIONS

Lowering the KCl product of St. Thomas' Hospital solution makes it isotonic with plasma and prevents cellular edema. This ameliorates the detrimental functional and electrophysiologic sequelae of hypothermic, hyperkalemic cardioplegia.