The isolated neonatal rat-cardiomyocyte used in an in vitro model for 'ischemia'. I. A morphological study

Experimental models of cardiac physiology and pathology

Experimental models of cardiac disease play a key role in understanding the pathophysiology of the disease and developing new therapies. The features of the experimental models should reflect the clinical phenotype, which can have a wide spectrum of underlying mechanisms. We review characteristics of commonly used experimental models of cardiac physiology and pathophysiology in all translational steps including in vitro, small animal, and large animal models. Understanding their characteristics and relevance to clinical disease is the key for successful translation to effective therapies.

Combined hypoxia and sodium nitrite pretreatment for cardiomyocyte protection in vitro

Methods that increase cardiomyocyte survival upon exposure to ischemia, hypoxia and reoxygenation injuries are required to improve the efficacy of cardiac cell therapy and enhance the viability and function of engineered tissues. We investigated the effect of combined hypoxia/NaNO2 pretreatment on rat neonatal cardiomyocyte (CM), cardiac fibroblast, and human embryonic stem cell-derived CM (hESC-CM) survival upon exposure to hypoxia/reoxygenation (H/R) injury in vitro. Cells were pretreated with and without hypoxia and/or various concentrations of NaNO2 for 20 min, then incubated for 2 h under hypoxic conditions, followed by 2 h in normoxia. The control cells were maintained under normoxia for 4 h. Pretreatment with either hypoxia or NaNO2 significantly increased CM viability but had no effect on cardiac fibroblast viability. Combined hypoxia/NaNO2 pretreatment significantly increased CM viability but significantly decreased cardiac fibroblast viability. In rat neonatal CMs, cell death, as determined by lactate dehydrogenase (LDH) activity, was significantly reduced with hypoxia/NaNO2 pretreatment; and in hESC-CMs, hypoxia/NaNO2 pretreatment increased the BCL-2/BAX gene expression ratio, suggesting that hypoxia/NaNO2 pretreatment promotes cell viability by downregulating apoptosis. Additionally, we found a correlation between the prosurvival effect of hypoxia/NaNO2 pretreatment and the myoglobin content of the cells by comparing neonatal rat ventricular and atrial CMs, which express high and low myoglobin respectively. Functionally, hypoxia/NaNO2 pretreatment significantly improved the excitation threshold upon H/R injury to the level observed for uninjured cells, whereas pretreatment did not affect the maximum capture rate. Hence, hypoxia/NaNO2 pretreatment may serve as a strategy to increase CM survival in cardiac regenerative therapy applications and tissue engineering.

Coincident exposure of phosphatidylethanolamine and anionic phospholipids on the surface of irradiated cells

The major anionic phospholipid, phosphatidylserine (PS), and the neutral phospholipid, phosphatidylethanolamine (PE), are largely confined to the inner leaflet of the plasma membrane bilayer in mammalian cells under normal conditions. This asymmetry is lost when cells undergo apoptosis, become activated, or are exposed to irradiation, reactive oxygen species or certain drugs. It is not known whether exposure of anionic phospholipids (APLs) and PE occurs simultaneously or in the same region of the plasma membrane. Here we examined the coincidence of exposure of APLs and PE on the surface of bovine aortic endothelial cells and NS0 myeloma cells after irradiation. The cells were irradiated (5 Gy) and stained for APLs and PE using liposomes coated with either an Fab' fragment of a PS-binding antibody (bavituximab) or a PE-binding peptide (duramycin). Using live cell imaging and flow cytometry, we showed that irradiation leads to synchronous externalization of APLs and PE. The time course of appearance of APLs and PE on the cell surface was the same and the two phospholipid types remained colocalized over time. Distinct patches double positive for APLs and PE were visible. Larger areas of APLs and PE appeared to have detached from the cytoskeleton to form membrane blebs which protruded and drifted on the cell surface. We conclude that APLs and PE coincidently appear on the external leaflet of the plasma membrane of cells after irradiation. Probably, this is because PE and the major APL, PS, share common regulatory mechanisms of translocation.

Cardiomyocyte preparation, culture, and gene transfer

Neonatal rat ventricular myocytes (NRVMs) cultured in vitro have been used as a model system for easily recreating and studying several cardiac molecular conditions, such as hypertrophy, oxygen deprivation, and gene expression. However, low efficiency of gene transfer has often represented one of the major limitations of this technique. In this chapter we describe in detail how to isolate NRVMs from neonatal rat heart and the optimal conditions for their long-term culture. Different cardiomyocyte transfection methodologies, based on viral or viral/chemical delivery carriers, are also discussed.

Cell culture modifies Ca2+ signaling during excitation-contraction coupling in neonate cardiac myocytes

In heart, the excitation-contraction coupling (ECC) mechanism changes during development. Primary cell culture has been used to study Ca(2+) signaling in newborn (NB) rat heart. In this work, the effects of cell culture on the action potential (AP) and ECC Ca(2+) signaling during development were investigated. Specifically, AP, Ca(2+) currents (I(Ca)), and ryanodine receptor (RyR) properties (i.e. density, distribution, and contribution to Ca(2+) transients and Ca(2+) sparks) were defined in cultured myocytes (CM) from 0-day-old NB rat at different times in culture (1-4 days). Compared with acutely dissociated myocytes (ADM) from NB of equivalent ages (1-4 days), CM showed lower RyR density (50% at 1 day, 25% at 4 days), but larger RyR contribution to the Ca(2+) transient (25% at 1 day, 57% at 4 days). Additionally, Ca(2+) sparks were larger, longer, wider, and more frequent in CM than in ADM. RyR cellular distribution also showed different arrangement. While in CM, RyRs were located peripherally, in ADM of equivalent ages a sarcomeric arrangement was predominant. Finally, CM showed a two-fold increase in sarcolemmal Ca(2+) entry during the AP. These results indicated that primary culture is a feasible model to study Ca(2+) signaling in heart; however, it does not precisely reproduce what occurs in ECC during development.

The use of cis-parinaric acid to measure lipid peroxidation in cardiomyocytes during ischemia and reperfusion

cis-Parinaric acid (PnAc), a fluorescent, polyunsaturated fatty acid, was used to measure lipid peroxidation during simulated ischemia and reperfusion in cultured neonatal rat cardiomyocytes. PnAc was used both as free fatty acid, inserted in the membranes following cultivation of the cells, as well as constituent of the cellular complex lipids by metabolically integrating the fatty acid during growth. In the insertion experiments a pre-incubation with DL-aminocarnitine, an inhibitor of beta-oxidation, was necessary to prevent loss of fluorescent signal. Such a pre-incubation resulted in an enrichment of PnAc in the sarcolemma: In pre-treated cells 57 +/- 1.3% of total inserted PnAc is present in the sarcolemma compared to 27 +/- 5.7% in cells containing the integrated probe. Both methods to introduce PnAc into the cells were compared with respect to their sensitivity for an externally applied oxidative stress and thereafter lipid peroxidation during simulated ischemia and reperfusion was assayed. Going from normoxic to ischemic conditions lipid peroxidation did not increase and remained at a low level. When the ischemic cells were subsequently subjected to reperfusion (reintroduction of both oxygen and glucose), large scale lipid peroxidation was obvious. When, on the other hand, oxygen alone was reintroduced (reoxygenation) no increased lipid peroxidation was observed. These observations led to the conclusion that ischemia does not lead to an enhanced lipid peroxidation and that resumption of metabolic activity during reperfusion is necessary to induce lipid peroxidation.

Sarcolemmal phosphatidylethanolamine reorganization during simulated ischaemia and reperfusion: reversibility and ATP dependency

Exposure of cultured neonatal rat heart cells to simulated ischaemia results in a cessation of the spontaneous contractile activity and changes at both the level of sarcolemmal phospholipid topology and the ultrastructural level. Reperfusion at a timepoint before irreversible cell damage develops leads to a recovery of contractile activity. Furthermore, the shift in transbilayer distribution of sarcolemmal phosphatidylethanolamine in favour of the outer membrane leaflet, due to the ischaemic period, is reversed during subsequent reperfusion. Also the morphological changes (mitochondrial oedema, reorganization of the mitochondrial cristae and the formation of extrusions at the sarcolemma) are reversible. At the same time total intracellular ATP levels are restored to 80% of control. The role of cellular ATP content on sarcolemmal phospholipid topology was further studied by the use of the calcium antagonist verapamil (10 microM), which preserved cellular ATP content by inhibiting cell contractility before the onset of ischaemia. After 120 min of ischaemia, cell ATP content was still 63% of control in the presence of verapamil, versus 20% of control in untreated cells. Verapamil treatment also prevented the loss of the asymmetrical distribution of phosphatidylethanolamine and sarcolemmal disruption, the latter occurring during 120 min of ischaemia in untreated cells. It is proposed that maintenance of phospholipid asymmetry of the sarcolemma of the myocytes depends on the cellular ATP concentrations, indicating the involvement of an ATP dependent aminophospholipid translocase.

Reversible and irreversible damage in reoxygenated 'ischemic' ventricular myocytes in culture

The LDH release pattern from cardiomyocytes under 'ischemia-like' conditions shows two phases. In the initial slow phase, reoxygenation immediately stops further enzyme release. Accelerated LDH release, which occurs concomitantly with Iysosomal enzyme release, characterizes the second phase of 'ischemia.' Reoxygenation at this stage does not put a stop to further enzyme release. Reoxygenation during the first phase of 'ischemia' rapidly restored ATP level, while in the second phase, ATP levels remained low even after 6 h of reoxygenation. This study as well as previous data seem to suggest that irreversible cellular damage leading to cell death, occurs by synergistic action of many effectors, each of which does not necessarily cause irreversible damage.

Aggregation of myocardial sarcolemmal transmembrane proteins is not hindered by an interaction with the cytoskeleton. Possible implications for ischemia and reperfusion

Heart myocytes subjected to ischemia show a clustering of the sarcolemmal proteins. In the erythrocyte membrane, a system in which intramembranous particle (IMP) aggregation is extensively studied, it is found that an IMP aggregation can in principle only occur upon removal of the membrane skeleton of spectrin and actin by rather drastic experimental conditions. With regard to phospholipid composition and topology the sarcolemma and the erythrocyte membrane show large similarities and therefore it was proposed that a loss of the interaction of the IMPs and the cytoskeleton is also a prerequisite for the sarcolemmal IMP aggregation (Verkleij et al., 1990). Freezing myocardial tissue, both from adult and neonatal rat, from temperatures lower than 37 degrees C resulted in an aggregation of the sarcolemmal IMPs. The aggregation is proportional to the degree of lowering of the temperature at which the tissue is cryofixed. This in contrast to the erythrocyte membrane, where lowering the temperature only induces moderate IMP aggregation. The IMP aggregation in the sarcolemma is reversible upon a subsequent increase in incubation temperature. The results clearly demonstrate that the interaction between the sarcolemmal proteins does not hinder aggregation of the IMPs, as proposed previously, and suggest that loosening of this complex does not have to proceed the aggregation of the sarcolemmal intramembranous particles during ischemia.

Improved hypothermic preservation of rat hearts by furosemide

The effect of furosemide, a blocker of the Na+/K+/Cl- cotransporter, on hypothermic preservation of rat hearts was studied with use of the Langendorff perfusion system and electron microscopy. Furosemide significantly improved the mechanical recovery and the coronary flow of the hearts preserved for 8 hours in St. Thomas' Hospital cardioplegic solution at a temperature of 4 degrees C. Furosemide at the concentration of 100 mumol/L was found to have an optimal effect, whereas at high concentrations (1000 mumol/L) it was found to have toxic effects. In addition, furosemide reduces the time elapsed between the end of the preservation time and the resumption of myocardial contractions. Ultrastructural evaluations were done in which the presence of swollen mitochondria was chosen as a criterion of hypothermic ischemic damage to the myocardium. Morphometric analysis indicated that the mitochondrial volume of hearts stored for 8 hours in St. Thomas' Hospital cardioplegic solution increased by 72% as compared with the mitochondrial volume of hearts that were not exposed to the hypothermic ischemic conditions (control group). The addition of 100 mumol/L furosemide to the cardioplegic solution resulted in a significant reduction of mitochondrial swelling during the period of 8 hours' storage, which amounted only to 28% as compared with the figure for the control group. The reduction of mitochondrial swelling by furosemide and the improved mechanical and coronary flow recoveries are thought to be related to the blocking of the sarcolemmal Na+/K+/Cl- cotransporter and consequently the reduction of the Na+ influx during hypothermic ischemic storage.

Loss of asymmetric distribution of sarcolemmal phosphatidylethanolamine during simulated ischemia in the isolated neonatal rat cardiomyocyte

In the present study we have investigated the reorganization of the sarcolemmal phospholipids during the first 60 minutes of simulated ischemia ("ischemia") as induced by anoxia, volume restriction, and nutrient deprivation. Experiments were carried out on [3H]acetate-labeled neonatal rat cardiomyocytes and isolated (nonradiolabeled) sarcolemmal membranes obtained from the same culture system. After 60 minutes of "ischemia," cellular high-energy phosphate (ATP) levels had decreased to approximately 40% of the control values, but no significant phospholipid hydrolysis was detected. Labeling experiments using the nonpermeant (primary amine-containing phospholipid) probe trinitrobenzenesulfonic acid and nonlytic treatment with (different) exogenous phospholipases A2 were both indicative of a shifted transbilayer distribution of the hexagonalII phase-preferring and fusion-promoting sarcolemmal phosphatidylethanolamine in favor of the outer membrane leaflet. This specific change in sarcolemmal phospholipid asymmetry preceded the loss of integrity of the sarcolemma, monitored by the release of lactate dehydrogenase as well as by scanning electron microscopy. It is proposed that, in addition to the previously reported lateral phospholipid reorganization, uncontrolled transbilayer movement of the non-bilayer-preferring phosphatidylethanolamine from the inner to the outer leaflet of the sarcolemma is an additional factor in destabilizing the lipid bilayer, eventually leading to the irreversible membrane damage seen after a prolonged period of ischemia.