Preservation of myocyte contractile function after hypothermic, hyperkalemic cardioplegic arrest with 2, 3-butanedione monoxime

Effects of extended pharmacological disruption of zebrafish embryonic heart biomechanical environment on cardiac function, morphology, and gene expression

BACKGROUND

Biomechanical stimuli are known to be important to cardiac development, but the mechanisms are not fully understood. Here, we pharmacologically disrupted the biomechanical environment of wild-type zebrafish embryonic hearts for an extended duration and investigated the consequent effects on cardiac function, morphological development, and gene expression.

RESULTS

Myocardial contractility was significantly diminished or abolished in zebrafish embryonic hearts treated for 72 hours from 2 dpf with 2,3-butanedione monoxime (BDM). Image-based flow simulations showed that flow wall shear stresses were abolished or significantly reduced with high oscillatory shear indices. At 5 dpf, after removal of BDM, treated embryonic hearts were maldeveloped, having disrupted cardiac looping, smaller ventricles, and poor cardiac function (lower ejected flow, bulboventricular regurgitation, lower contractility, and slower heart rate). RNA sequencing of cardiomyocytes of treated hearts revealed 922 significantly up-regulated genes and 1,698 significantly down-regulated genes. RNA analysis and subsequent qPCR and histology validation suggested that biomechanical disruption led to an up-regulation of inflammatory and apoptotic genes and down-regulation of ECM remodeling and ECM-receptor interaction genes. Biomechanics disruption also prevented the formation of ventricular trabeculation along with notch1 and erbb4a down-regulation.

CONCLUSIONS

Extended disruption of biomechanical stimuli caused maldevelopment, and potential genes responsible for this are identified.

Methods in cardiomyocyte isolation, culture, and gene transfer

Since techniques for cardiomyocyte isolation were first developed 35 years ago, experiments on single myocytes have yielded great insight into their cellular and sub-cellular physiology. These studies have employed a broad range of techniques including electrophysiology, calcium imaging, cell mechanics, immunohistochemistry and protein biochemistry. More recently, techniques for cardiomyocyte culture have gained additional importance with the advent of gene transfer technology. While such studies require a high quality cardiomyocyte population, successful cell isolation and maintenance during culture remain challenging. In this review, we describe methods for the isolation of adult and neonatal ventricular myocytes from rat and mouse heart. This discussion outlines general principles for the beginner, but also provides detailed specific protocols and advice for common caveats. We additionally review methods for short-term myocyte culture, with particular attention given to the importance of substrate and media selection, and describe time-dependent alterations in myocyte physiology that should be anticipated. Gene transfer techniques for neonatal and adult cardiomyocytes are also reviewed, including methods for transfection (liposome, electroporation) and viral-based gene delivery.

Cellular and molecular therapeutic targets for treatment of contractile dysfunction after cardioplegic arrest

Transient left ventricular (LV) dysfunction can occur after hypothermic hyperkalemic cardioplegic arrest. This laboratory has developed an isolated LV myocyte system of simulated cardioplegic arrest and rewarming in order to examine cellular and molecular events that may contribute to the LV dysfunction after cardioplegic arrest. Contractile function was examined using high-speed video microscopy after reperfusion and rewarming. After cardioplegic arrest and reperfusion, indices of myocyte contractility were reduced by over 40% from normothermic control values. The capacity of the myocyte to respond to an inotropic stimulus was examined through beta-adrenergic receptor stimulation with isoproterenol. After cardioplegic arrest, the contractile response to isoproterenol was reduced by over 50% from normothermic values. The next series of studies focused upon preventing these changes in myocyte contractile processes after cardioplegic arrest. First, the cardioplegic solutions were augmented with adenosine or an ATP-sensitive potassium channel opener, aprikalim. Both adenosine and aprikalim augmentation significantly improved myocyte function compared with cardioplegia alone values. A potential intracellular mechanism for the protective effects of either adenosine or the ATP-sensitive potassium channel is the activation of protein kinase C (PKC). A brief period of PKC activation before cardioplegic arrest provided protective effects on myocyte contractility with subsequent reperfusion and rewarming. In another set of studies, the potential protective effects of the active form of thyroid hormone (T3) were examined. In myocytes pretreated with T3, myocyte contractile function and beta-adrenergic responsiveness were significantly improved after hypothermic cardioplegic arrest and rewarming. Thus, endogenous means of providing improved myocardial protection during prolonged cardioplegic arrest can be achieved through a brief period of PKC activation or pretreatment with T3. Future studies, which more carefully deduce the basis for these pretreatment effects, will likely yield novel methods by which to protect myocyte contractile processes during cardioplegic arrest.

Direct effects of oxygenated crystalloid or blood cardioplegia on isolated myocyte contractile function

The majority of myocardial protective techniques performed in the United States incorporate hypothermic, hyperkalemic blood or crystalloid cardioplegia. Oxygenated blood cardioplegia has not been compared with oxygenated crystalloid cardioplegia in an isolated myocyte model of hypothermic, hyperkalemic cardioplegic arrest in which direct measurements of contractile function and myocyte swelling can be made. Accordingly, isolated myocyte contractile function and myocyte profile surface area were examined after hypothermic arrest with oxygenated crystalloid or blood cardioplegia.

METHODS

Isolated left ventricular pig myocytes were randomly assigned to undergo cardioplegic arrest for 2 hours at 4 degrees C. Either oxygenated crystalloid or blood cardioplegia was used. After 2 hours, myocytes were reperfused with standard cell medium at 37 degrees C and contractile function was examined. A control group of myocytes was maintained in cell medium at 37 degrees C for 2 hours. Myocyte velocity of shortening (micrometers per second) was examined at baseline and after beta-adrenergic stimulation (isoproterenol, 25 nmol/L). Velocity of shortening declined equally from baseline control values (65 +/- 2 micron n/sec) in the groups subjected to oxygenated crystalloid cardioplegia and blood cardioplegia (37 +/- 2 micron n/sec and 42 +/- 1 micron n/sec, respectively; p < 0.05).

RESULTS

Although beta-adrenergic stimulation caused a significant increase in velocity of shortening in all myocyte groups, the increase was less pronounced in myocytes subjected to crystalloid cardioplegia (157 +/- 6 micron n/sec) and blood cardioplegia (159 +/- 6 micron n/sec) than in normothermic control myocytes (205 +/- microm/sec; p < 0.05). Myocyte profile surface area, an index of cell volume, was measured in all myocyte groups. Myocyte surface area increased equally after cardioplegic arrest and rewarming in both cardioplegia groups (crystalloid 4119 +/- 53 micron2; blood 3924 +/- 48 micron2); surface areas in both cardioplegia groups were significantly greater than in the normothermic control group (3158 +/- 39 micron2, p < 0.05).

CONCLUSION

Equivalent effects of oxygenated crystalloid and blood cardioplegia were observed with respect to myocyte contractile function, inotropic responsiveness, and intracellular volume regulatory processes.