Measurement of Cardiac Mechanical Function in Isolated Ventricular Myocytes from Rats and Mice by Computerized Video-Based Imaging.

Isolated adult cardiac ventricular myocytes have been a useful model for cardiovascular research for more than 20 years. With the recent advances in cellular physiology and transgenic techniques, direct measurement of isolated ventricular myocyte mechanics is becoming an increasingly important technique in cardiac physiology that provides fundamental information on excitation-contraction coupling of the heart, either in drug intervention or pathological states. The goal of this article is to describe the isolation of ventricular myocytes from both rats and mice, and the use of real-time beat-to-beat simultaneous recording of both myocyte contraction and intracellular Ca(2+) transient.

Introduction

The essential contractile unit of the heart is the ventricular myocyte. Measurement of ventricular contractile function provides the most direct information on cardiac contractile function either with drug intervention or under certain pathological conditions. Over the past two decades, a plethora of new techniques have been presented to the physiologist for studies of cardiac contractile function. Several techniques have been developed including video-based edge-detection systems, photodiode arrays, and spatial video-based edge imaging (1,2). The most commonly used approach is the edge-detection method, which measures changes in myocyte length (isotonic contraction).

The application of isolated myocytes provides unique information on cardiac physiology. The presence of heterogeneous cell types and nerve terminals often makes the results obtained from multicellular preparations such as papillary muscles difficult to interpret. Mechanical function of the myocardium may be affected by non-myocyte factors such as the coronary vasculature and/or interstitial connective tissue. For example, alterations in contractile performance under ethanol exposure may simply be due to enhanced interstitial fibrosis but not reduced function of individual myocytes. Therefore, it is imperative that there be a tool for physiologists to study the myocyte specifically, regardless of the function of its surrounding tissues. This is accomplished with video-based edge-detection of the isolated ventricular myocyte.

Materials and Methods

This section describes the typical methods used to ultimately obtain individual ventricular myocytes. Some of these methods are specific to the type of animal used, so we have noted in the text if the method is specific to either rat or mouse.

Preparation of the animals

Animals are usually injected with heparin (1000U/kg, i.p.) 20 mins. prior to the experimental protocol. To anesthetize the animal, ketamine/xylazine has been the drug of choice in both rats and mice (0.1ml/100g, i.p.) since it has little direct cardiac toxicity. Once the animal is anesthetized, a surgical incision is made posterior to the xiphoid process. Cut superiorly until reaching the xiphoid process and make another incision running down the chest cavity. Continue cutting through the sternum until the diaphragm is evident. Cut down along the diaphragm to avoid damaging the heart. The rib cage may be dissected and removed. To remove the heart, pick the organ up between the index finger and thumb and cut the aorta below. Place the heart in a weigh bath with some of the Ca2+-containing buffer and weigh the organ. While the heart is in the buffer, carefully remove all remnant tissue around the organ, specifically exposing the aorta. Make sure that there is enough aortic tissue to allow adequate perfusion superiorly to the carotid bifurcation. Holding the aorta with two forceps, lift the heart from the buffer and place on the perfusion cannula, which should have buffer running through at a slow rate. Clamp the aorta to the perfusion needle and tie it (below the branches) with a piece of surgical thread. Increase the buffer flow rate until the drops fall between 5 and 10 times per minute. If the procedure is performed successfully, the carotid arteries will fill with buffer and become clear, the heart will begin to beat rhythmically and the drops will become clear.

Isolation Procedure of Rat Ventricular Myocytes

Once the heart is successfully hung, it is perfused (at 37°C) with Krebs-Henseleit bicarbonate (KHB) buffer containing (in mM): 118 NaCl, 4.7 KCl, 1.25 CaCl2, 1.2 MgSO4, 1.2 KH2PO4, 25 NaHCO3, 10 N-[2-hydro-ethyl]-piperazine-N'-[2-ethanesulfonic acid] (HEPES) and 11.1 glucose, equilibrated with 5%CO2-95% O2. Hearts are subsequently perfused with a nominally Ca2+-free KHB buffer for 2-3 min until spontaneous contractions cease, followed by a 15-20 min perfusion (depending on the physiologic condition of the animal) with Ca2+-free KHB containing 223 U/ml collagenase (Worthington Biochemical Corp., Freehold, NJ) and 0.1 mg/ml hyaluronidase (Sigma Chemical, St. Louis, MO). After perfusion, ventricles are removed and minced, under sterile conditions, and incubated with the Ca2+-free KHB with collagenase solution for 3-5 min. The cells are further digested with 0.02 mg/ml trypsin (Sigma) before being filtered through a nylon mesh (300 μm) and subsequently separated from the collagenase-trypsin solution by centrifugation (60 x g for 30 sec). Myocytes are resuspended in a sterile-filtered, Ca2+-free Tyrode's buffer containing (in mM): 131 NaCl, 4 KCl, 1 MgCl2, 10 HEPES, and 10 glucose, supplemented with 2% bovine serum albumin, with a pH of 7.4 at 37oC. Cells are initially washed with Ca2+-free Tyrode's buffer to remove residual enzyme and extracellular Ca2+ is slowly added back up to 1.25 mM. Myocytes with obvious sarcolemmal blebs or spontaneous contractions are not used. Only rod-shaped myocytes with clear edges are selected for recording of mechanical properties or intracellular Ca2+ transients as previously described (3).

Isolation Procedure for Mouse Ventricular Myocytes

A similar hanging procedure is utilized in the mouse, however a light magnifier is helpful in isolating the aorta. Hearts are rapidly removed from anesthetized mice via cardiectomy and immediately mounted on a temperature controlled (37°C) perfusion system. After perfusion with modified Tyrode solution (Ca2+ free) for 2 min, the heart is digested for 15-20 min with 0.9 mg/ml collagenase D (Boehringer Mannheim Biochemicals) in modified Tyrode solution. The modified Tyrode solution (pH 7.4) contained the following (in mM): NaCl 135, KCl 4.0, MgCl2 1.0, HEPES 10, NaH2PO4 0.33, glucose 10, butanedione 10, and the solution was equilibrated with 5% CO2-95% O2. The digested heart is removed from the cannula, and the left ventricle is cut into small pieces in the modified Tyrode solution. These pieces are gently agitated and the pellet of cells is resuspended in modified Tyrode solution and allowed to settle for another 20 min at room temperature during which time extracellular Ca2+ is added incrementally back up to 1.20 mM. Isolated myocytes are always used for experiments within 8 h after isolation, however we have recently had some success with culturing mouse myocytes for up to 48 hours.

IonOptixTM video-based edge-detection cartoon. As the above panel illustrates, cell shortening and intracellular Ca2+ transients can be monitored simultaneously within the same cell. Figure provided courtesy of Dr. Doug Tillotson, IonOptixTM.

Cell Shortening/Relengthening

Mechanical properties of ventricular myocytes are assessed using a video-based edge-detection system (IonOptix Corporation, Milton, MA) (3). Cells are placed in a Warner chamber mounted on the stage of an inverted microscope (Olympus, X-70) and superfused (~1 ml/min at 25°C) with a buffer containing (in mM): 131 NaCl, 4 KCl, 1 CaCl2, 1 MgCl2, 10 glucose, 10 HEPES, at pH 7.4. The cells are field stimulated with a suprathreshold (50%) voltage and at a frequency of 0.5 Hz, 3 msec duration, using a pair of platinum wires placed on opposite sides of the chamber connected to a FHC stimulator (Brunswick, NE). The polarity of the stimulatory electrodes is reversed frequently to avoid possible build up of electrolyte by-products. The myocyte being studied is displayed on the computer monitor using an IonOptix MyoCam camera, which rapidly scans the image area every 8.3 msec such that the amplitude and velocity of shortening/relengthening is recorded with good fidelity. The soft-edge software (IonOptix) is used to capture changes in cell length during shortening and relengthening.

Intracellular Fluorescence Measurement

A separate cohort of myocytes is loaded with fura-2/AM (0.5 μM) for 15 min and fluorescence measurements are recorded with a dual-excitation fluorescence photomultiplier system (Ionoptix) as previously described (3). Myocytes are placed in a chamber on the stage of an Olympus X-70 inverted microscope and imaged through a Fluor 40x oil objective. Cells are exposed to light emitted by a 75W lamp and passed through either a 360 or a 380 nm filter (bandwidths are ± 15 nm), while being stimulated to contract at 0.5 Hz. Fluorescence emissions are detected between 480-520 nm by a photomultiplier tube after first illuminating cells at 360 nm for 0.5 sec then at 380 nm for the duration of the recording protocol (333 Hz sampling rate). The 360 nm excitation scan is repeated at the end of the protocol and qualitative changes in intracellular Ca2+ concentration ([Ca2+]i) are inferred from the ratio of the fluorescence intensity at the two wavelengths.

Results and Discussion

This section describes several problems that may be encountered during the isolation of single ventricular myocytes.

Myocyte Isolation and Yield:

The nature and quality of the myocyte isolation procedure may be the single most important factor in the success of mechanical assessment of the myocytes, and also for reliable comparison between the experimental and control groups. Although the cell culture medium may provide a favorable environment for the cells to recover from enzymatic trauma, the yield, survival and viability of the myocytes are largely dependent on the quality of isolation. Caution has to be taken when working with the mice myocytes in such variables as temperature, type of enzyme, and difficulty of culturing. A good isolation will yield approximately 70-80% viable cells or 4-5 million rod-shaped myocytes in both rats and mice.

Inadequate Heparinization:

Inadequate heparinization due to injection of heparin (i.p.) into the gut, gall bladder or other organs in the abdominal cavity may result in inadequate heparinization and blood clotting in the coronary arteries. This will significantly affect the outcome of enzymatic digestion and yield of myocytes. If after perfusion there are blood clots in the ventricles of the heart, increase the amount of heparin given and take particular caution to inject medial to the hindlimb in further studies. This will secure that the heparin is given intraperitoneally.

Air bubble accumulation

Air bubbles may build up and be trapped over time in the heart, causing a physical occlusion of the perfusion. Therefore, the perfusion system should be clear of any air bubbles at the start of the perfusion. Should any air bubbles develop during the perfusion, an air bubble trap device (usually a valve in the perfusion route or a 0.45-μm pore size cellulose acetate filter) may be inserted into the perfusion path. Make sure to replace the filter frequently as it may become plugged.

Cleaning of the perfusion system

It is essential to wash the entire perfusion system with 70% ethanol and distilled water before and after each isolation. Enzyme may stick to the glassware tubing due to inappropriate washing after perfusion or bacteria may grow within the system, contaminating the perfusion system.

Inadequate cell yield

The number one problem in myocyte isolation is inadequate cell yield. If the animal under study is older than average or substantially overweight (such as in an obesity study), the heart may need to be digested for a longer period of time. Also, the perfusion pressure can be increased to increase the digestion pressure within the heart. On the other hand, younger animals will require less perfusion pressure and shorter digestion times. All of this comes with practice in determining adequate perfusion and digestion.

Inability to stimulate myocyte

A particular problem normally encountered in myocyte studies is the inability to get the cells to beat. Make sure to clean the platinum wires routinely to remove any electrolyte by-product buildup. It is also helpful to allow the cells to recover for a short period of time (~15 mins.) once in the Warner chamber. It is essential that CaCl2 be added to the HEPES buffer in order to facilitate contraction.

Inability to obtain an adequate fluorescence signal

Fura-2/AM is cleaved into AM and fura-2, which is able to penetrate the cell membrane and bind to intracellular Ca2+. If the fura-2-loaded cell is excited, it will fluoresce during contraction and the signal will be displayed on the computer monitor. Make sure that the shutter is open, the emission filter is incorporated and the room is as dark as possible. We have found that the addition of 1 μl of fura-2/AM stock to the Warner chamber will provide a beautiful signal after 15 mins. loading and wash.

Applications

In this section, the authors present several different applications that have been used to provide models of compromised heart function in which myocytes can be studied. This is by no means an exhaustive list, simply several models that have benefited from video-based edge-detection.

(1). Drug intervention: Drugs are commonly used in an attempt to ameliorate the devastating effects of a particular disorder. We have used a model of fetal alcohol syndrome whereby dams are administered alcohol in liquidiet form during gestation, in concert with magnesium supplementation (4) (alcohol is known to contribute to hypomagnesemia). Isolation of ventricular myocytes from this model has allowed us to examine the effect of magnesium supplementation on the heart, specifically the functional unit or myocyte. We have also utilized video-based edge-detection in the study of leptin and cardiac contractile function. We found that leptin elicits a dose dependent decrease in myocyte shortening and intracellular Ca2+ transients that is attenuated by pretreatment with L-NAME, a NOS inhibitor (5).

(2). Disease models: Overt cardiac mechanical abnormalities are generally observed in diabetes of both chemical and genetic origin, with the severity of dysfunctions increasing with time (6-8). Characteristics of abnormal function include prolonged duration and reduced rate of ventricular contraction and relaxation, as measured in isolated ventricular myocytes (6,8). Diabetes is commonly studied by our laboratory and others and the heart is of particular interest due to the development of diabetic cardiomyopathy. Cardiomyopathy is a heart disorder that presents itself in the diabetic patient, regardless of arterial disease. Isolated myocytes from this model of diabetic cardiomyopathy have allowed the researcher to probe the effects of the disease on the heart. We have also used models of hypertension (9) and obesity (10) to examine cardiac specifics effects.

(3). Transgene: Transgenic animals are particularly important in the study of disease and its progression. We have examined the effect of metallothionein overexpression on the development of diabetic cardiomyopathy in the mouse (11). Myocytes isolated from this animal model has allowed us to probe the effect of transgenic alteration on the heart.

Summary

Isolation of ventricular myocytes is a powerful tool that allows the researcher the freedom to examine myogenic, cardiac specific effects, with all other tissues removed. The development of video-based edge-detection was an extremely important and useful tool that has expanded the level of understanding of cardiac specific effects of numerous disease states. We have included several helpful hints and techniques that will aid in proper isolation and maintenance of ventricular myocytes, however this technique is extremely touchy and requires patience beyond that of the beginning researcher. However, mastery of the technique will allow the researcher power beyond what was initially available to study cardiac specific problems under disease.