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Am J Physiol Heart Circ Physiol 287: H2569-H2575, 2004. First published August 26, 2004; doi:10.1152/ajpheart.00526.2004
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Functional and transmural modulation of M cell behavior in canine ventricular wall

Norihiro Ueda,1 Douglas P. Zipes,1 and Jiashin Wu1,2

1Krannert Institute of Cardiology, Indiana University School of Medicine, and 2Department of Biomedical Engineering, Indiana University Purdue University Indianapolis, Indianapolis, Indiana 46202

Submitted 1 June 2004 ; accepted in final form 20 August 2004


   ABSTRACT
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 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
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Previous studies have demonstrated a discrete population of midmyocardial (M) cells in the ventricular myocardium having excessive action potential duration (APD) prolongation during long activation cycle lengths (CL) and under the influence of APD-prolonging agents. However, M cells have not been found in other studies. Existing explanations for the discrepancies appear inadequate. We hypothesized that instead of being a discrete group, M cell behavior is functional and conditionally expressed. We mapped APDs on the cut-exposed transmural surfaces of arterially perfused ventricular wedges from 26 dogs during Na+ current modification with anemone toxin II (ATX-II). Compared with the endocardium, APDs were not statistically different in the parallel layer having the longest mean APD (APDL) and were significantly shorter in the epicardium in the 26 wedges before ATX-II. ATX-II (≥5 nmol/l) prolonged APD heterogeneously (midmyocardium > endocardium > epicardium). The differences increased at longer CLs. ATX-II (20.0 nmol/l) shifted the APDL layer to 32 ± 6.2% (6 wedges, CL: 4,000 ms) of the transmural thickness from the (sub)endocardium (8.6 ± 7.2%, 26 wedges, ATX-II free). We detected the presence of M cell behavior (significantly longer APDs in the APDL layer than in the endocardium and epicardium,P ≤ 0.04, CL: 4,000 ms) in the 18 wedges having ≥5 nmol/l ATX-II but not (P > 0.36) in the other 18 wedges having ≤2.5 nmol/l ATX-II. Both the position of the APDL layer and presence of M cell-like behavior were modulated by ATX-II. The dynamic spatial modulation indicates that M cell behavior is functional and only becomes manifest under suitable conditions.

long QT syndrome; repolarization dispersion; optical mapping

TRANSMURAL DISPERSION of refractory periods in the canine ventricular wall has been known for more than 40 years (29). Recent in vitro experiments have demonstrated the existence of a distinctive class of midmyocardial (M) cells having exceedingly long action potential (AP) duration (APD) when activated at long cycle lengths (CLs), characteristically different and separated from the epicardial and endocardial myocytes (3, 5). M cells were also observed in vivo under the influence of QT-prolonging agents (11, 12, 33). However, M cells were less distinct or even not observed in other in vivo and in vitro studies without or before pharmacological prolongation of QT intervals (6, 7, 25, 26). Existing explanations do not completely resolve the above discrepancies. Previously, the strong intercellular coupling in cardiac tissue was suggested as the cause of the reduced dispersion of repolarization and absence of M cells in vivo (6). However, M cells were also observed in the experiments in which cells were well coupled, e.g., in the midmyocardial region of isolated ventricular wedges (microelectrode recordings) (22, 23, 38) and in intact hearts treated with QT-prolonging agents (plunge electrodes) (11, 12, 33). The inability to detect M cells in vivo (6, 7, 25, 26) was also explained on the basis of the positioning uncertainty in recording the epicardial, midmyocardial, and endocardial activations and low spatial resolution of the extracellular plunge electrodes (32). However, using high-resolution optical mapping, we (28, 34) and other investigators (2) observed only minor APD differences between the midmyocardium and endocardium in isolated wedges of ventricular wall, insufficient to identify a separate cell group, without, or before, pharmacological APD prolongation. Another explanation, suppression of M cell effects by barbiturates (4, 33), was also contradicted by the absence of M cells in other in vivo experiments that did not use barbiturates (25, 26).

Because M cells may play a role in generating the T and U waves in the ECG (5, 22, 38) and contribute to the dispersion of repolarization (3), which facilitates the occurrences of ventriclar tachycardia (VT) (11) and spontaneous arrhythmic death (19), it is necessary to have a coherent understanding of the M cells that reconciles the different observations. Because we (28) observed cells with M cell behavior in isolated canine ventricular wedges after anemone toxin II (ATX-II) treatment, but not before (28, 34), we hypothesized that cells exhibiting M cell behavior are not anatomically fixed; instead, they become manifest at different location under suitable conditions. Because of the regional differences in the densities of membrane ionic channels, uniform environmental changes could alter the rate dependency of APD and other electrophysiological properties heterogeneously and cause corresponding alterations in the dispersion of APDs and in the locations of the longest APDs inside the ventricular wall. Therefore, not only the occurrence but also the locations of cells with M cell behavior could be manipulated experimentally if the hypothesis is true. To test the hypothesis, we statistically evaluated the presence of M cell behavior and locations of the longest APDs in the canine left ventricular free wall during pharmacological modifications of the late Na+ currents.


   METHODS
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 METHODS
RESULTS
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The investigation conforms to the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health. We isolated wedges of canine ventricular wall as previously described (28, 34, 35, 36). In brief, we harvested hearts from 26 adult male mongrel dogs (25–30 kg) after pentobarbital sodium (30 mg/kg body wt) anesthesia, and we immediately perfused the hearts through the aorta with a cardioplegic solution (Tyrode solution, with 15 mmol/l KCl, 4°C, see below). The cardioplegic perfusion washed out the blood and pentobarbital sodium and protected the hearts during the subsequent period of wedge isolation. We then isolated a transmural wedge from the left ventricular free wall of each cardioplegic solution-perfused heart. Each wedge (20–30 mm long by 4–7 mm wide on the epicardium and 12–20 mm transmural) contained a section of coronary artery (diameter: ≥1 mm) along its length. Two plastic cannulas, one for perfusion and the other for arterial pressure monitoring, were inserted into the two openings of the artery. Major arterial leaks in the wedges were ligated with silk sutures. Unperfused tissue was trimmed from the wedges. The isolated tissues were mounted in a warmed chamber with the cut-exposed transmural observation surface up, perfused with 37°C Tyrode solution (in mmol/l: 128.0 NaCl, 4.69 KCl, 22.0 NaHCO3, 0.65 NaH2PO4, 0.50 MgCl2, 11.1 dextrose, and 2.0 CaCl2, and gassed with 95% O2-5% CO2) at an arterial pressure of 40–50 mmHg and immersed in the perfusion efflux.

The wedges were preconditioned with 5-min ischemia after >20 min of Tyrode perfusion, as we have done routinely (28, 34, 35, 36), to reduce the interwedge variations of ischemic insult during heart removal and wedge isolation. Similar short periods of ischemia (~4 min between the heart isolation and wedge perfusion) also occurred during the preparation of canine ventricular wedges by other investigators (22, 23, 38). Each wedge was stained with 4-[{beta}-[2-(di-n-butylamino)-6-naphthyl]vinyl]pyridinium (di-4-ANEPPS, ~4 µmol/l in perfusate, Molecular Probes; Eugene, OR), a membrane potential-sensitive fluorescent dye having no known electrophysiological effects and used widely in optical mapping studies, after >100 min of tissue recovery and equilibration during continued Tyrode perfusion. Wedges were allowed additional recovery time if their APs, arterial pressure, or contraction strengths were changed during an observation period of 10 min. We evaluated the healthiness of the wedges according to our published criteria (28, 34, 35, 36). These procedures produced stable wedges that had transmural dispersions of APD and conduction velocity similar to in vivo observations (7, 18).

We immobilized the wedges that were verified healthy with cytochalasin D (20–30 µmol/l, Sigma Chemical; St. Louis, MO), which binds to actin with high affinity, thus depolymers the rapidly recycling actin filaments in the cytoskeleton, and interrupts the transmission of contraction. Cytochalasin D also reduces the Ca2+ sensitivity of myofilaments in cardiac myocytes (10). We (8) verified previously that canine ventricular APs were not affected by cytochalasin D at up to 80 µmol/l using microelectrodes in isolated canine ventricular trabeculae and at up to 40 µmol/l using optical mapping in isolated canine ventricular wedges (34). Although cytochalasin D does not alter APs in canine ventricular muscle or in isolated rat ventricular myocytes (5–50 µmol/l) (40), it may affect APs in other animal species [e.g., mouse (14)] due to the species dependency of repolarization currents (42).

The wedges were paced (2 ms duration, 2x diastolic current threshold, with a bipolar electrode) from the endocardium at a CL of 1,000 ms or as otherwise specified. Two silver electrodes were inserted, one into the epicardium and the other into the endocardium, to record the transmural ECG. An optical mapping system (34) with a 256-element (16 x 16) photodiode camera (C4675, Hamamatsu, Japan) collected the fluorescence from an area of 19.5 x 19.5 mm2 on the cut-exposed transmural surface of the wedge (with minor contributions from subsurface cells) and converted it into 256 channels of electrical signals. Each channel of electrical signal (fluorescence AP) corresponded to a surface area of 1.1 x 1.1 mm2. A custom data- acquisition system recorded the APs, ECG, and arterial perfusion pressure during increasing pacing CLs of 500, 1,000, 2,000, and 4,000 ms (sequentially in 3-s segments at 1,000 samples·channels–1·s–1), after the wedges were fully immobilized (baseline recording). We then added ATX-II (Calbiochem-Novabiochem; San Diego, CA), an agent that delays the inactivation of late sodium current (type 3 long QT syndrome) (9, 22, 24, 38), to the perfusate at one of the following concentrations: 1.25, 2.5, 5.0, 10.0, and 20.0 nmol/l (six wedges at each concentration) and repeated the above recording sequences >20 min later. We carried out a second protocol with a higher concentration (≥5.0 nmol/l) of ATX-II in 10 wedges in which an initial protocol of lower ATX-II concentration (≤2.5 nmol/l) had only minor effects on APD and thus did not affect the quality of data in the second protocol. The two sequential ATX-II concentrations also provided same tissue comparison of the dosage effects of ATX-II and reduced the total number of animals. Six additional wedges with continued ATX-II-free perfusion were used to evaluate the tissue stability and served as the control group.

We measured APD from the interval between the maximum rate of depolarization and peak of the second-order derivative of AP with visual inspections and manual corrections, as we did previously (28, 34, 35, 36). Epicardial, endocardial, and midmyocardial APDs were measured at the recording sites along the first epicardial row, first endocardial row, and a parallel row having the longest mean APD (APDL layer). The optical recording and statistical representations of the endocardial, epicardial, and midmyocardial APs could cause averaging/filtering effects that could reduce the dispersion of APDs. However, these effects should not affect the locations of the APDL layers and conclusions of this study. We detected the presence of cells exhibiting M cell behavior as having statistically significant longer APDs (P < 0.05) in the APDL layer than in the endocardial and epicardial rows of recording sites in each wedge. This statistics-based method of confirming the existence of M cell-type behavior could be applied to all tissues uniformly without the need of identifying individual M cells as was done previously with microelectrode recordings (22, 23, 38). ANOVA and Fisher's protected least-significant difference test were used for statistical analysis. Differences were considered significant if P < 0.05.


   RESULTS
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All wedges were verified healthy and were fully stabilized after ~100 min recovery perfusion, with insignificant interwedge differences (P > 0.05) in the baseline APDs, similar to our previously used wedges (28, 34, 35, 36). Compared with the endocardium, APDs were similar (P > 0.05) in the APDL layer and shorter (P < 0.05) in the epicardium during baseline recordings (Fig. 1A). Therefore, M cell-like behavior was not observed under the baseline condition. The baseline transmural dispersion of APDs (Fig. 1B) was similar to the previous observations in isolated ventricular wedges (28, 34) and in vivo (6, 7, 25, 26) and differed from the observations in isolated cells (3, 4). We evaluated the stability of the preparations in six control wedges with continuous ATX-II-free perfusion and observed no statistical differences between the APDs recorded initially (baseline) and again 91 ± 10 min later.



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  		Fig. 1. Baseline action potential durations (APDs) [after ~100 min of recovery perfusion but before anemone toxin II (ATX-II), A] and their transmural dispersions (B) in all 26 wedges. Wedges were paced at the endocardium. APDs were measured from the epicardial and endocardial rows of recording sites and from a parallel row having the longest mean APD (APDL layer). APDs were shorter in the epicardium than in the endocardium and APDL layer (P < 0.05) but statistically indifferent between the APDL layer and endocardium at all pacing cycle lengths (CLs) (paired comparisons in each of the 26 wedges, A). Dispersions of APD (B) are calculated from the same-wedge differences in the mean APDs between the APDL layer and epicardium in the 26 wedges. Error bars indicate means ± SD.

 
Although lower concentrations of ATX-II (≤2.5 nmol/l, >20 min) had insignificant effects, higher concentrations (≥5 nmol/l) prolonged APDs significantly and heterogeneously (midmyocardium > endocardium > epicardium) and with more prolongation at longer CLs (Fig. 2). ATX-II altered the dependency of APD on the pacing CL heterogeneously with greater prolongation in the APDL layer and endocardium than in the epicardium (Fig. 3). The shortest mean APDs always occurred in the epicardium, whereas the longest mean APDs (APDL layer) were observed 0–5 mm (0%-31% of the transmural thickness) from the endocardium with lower ATX-II concentrations (≤2.5 nmol/l) and 2–8 mm (15–40% transmural thickness) from the endocardium with higher ATX-II concentrations (≥5 nmol/l, Fig. 4A). For example, the APDL layer was 32 ± 6.2% of the transmural thickness from the endocardium in the 6 wedges treated with 20.0 nmol/l ATX-II in contrast to 8.6 ± 7.2% in all 26 wedges before or without ATX-II at the pacing CL of 4,000 ms. The ATX-II shifting of the APDL layer deeper into the ventricular wall was also demonstrated clearly in the 10 wedges having two sequential concentrations of ATX-II (Fig. 4B). The presence of M cell behavior (longer APDs in the APDL layer than in the endocardium, P ≤ 0.04, CL: 4,000 ms) was statistically confirmed in the 18 wedges having ≥5 nmol/l ATX-II but not in the other 18 wedges having ≤2.5 nmol/l ATX-II (P > 0.36, CL: 4,000 ms). Figure 5 shows a typical example of the midmyocardial preferential prolongation of APD and shifting of the region having the longest APDs by increasing concentrations of ATX-II. Therefore, both the transmural position of the APDL layer as well as the presence of M cell behavior were modulated by ATX-II.



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  		Fig. 2. Effects of ATX-II on APDs. Wedges were paced at the endocardium at the CL of 500 m (A), 1,000 ms (B), 2,000 ms (C), and 4,000 ms (D). APDs were measured from all recording sites in the epicardial, APDL layer, and endocardial rows of recording sites. The epicardial APDs were always shorter than the APDs in the APDL layer and in the endocardium. *M cell behavior was statistically confirmed only in the wedges treated with high concentrations (≥5 nmol/l) of ATX-II. Each data point contains six wedges.

 


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  		Fig. 3. Effects of ATX-II (in nmol/l) on the cycle length dependency of APD in the epicardium (A), parallel layer having the longest APD (APDL) layer (B), and endocardium (C). The effects of ATX-II are concentration and transmural position dependent. *Statistical confirmation of M cell behavior. These results are the alternative expressions of the same measurements shown in Fig. 2. The differences between the APDL and endocardial layers are demonstrated in Fig. 2. Each data point contains the mean and SD from 6 wedges.

 


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  		Fig. 4. The ATX-II and CL dependencies of the transmural position of APDL layer (percent from the endocardium to epicardium). A: shifting of APDL layer from the endocardium into midmyocardium with higher concentrations of ATX-II (0 nmol/l: 26 wedges, nonzero concentrations: 6 wedges each) at pacing CL of 4,000 ms. *Statistical confirmation (P < 0.05) of M cells. #Significant (P < 0.05) changes in the APDL layer position from the corresponding control positions (0 nmol/l ATX-II). B: paired comparisons in the same 10 wedges having two sequential exposures of ATX-II (Control: 0 nmol/l, low dose: 1.25 or 2.5 nmol/l, high dose: ≥5 nmol/l). *Satistical confirmation (P < 0.05) of M cells. #Significant (P < 0.05) changes in the APDL layer position from the corresponding control positions (0 nmol/l ATX-II). &Significant (P < 0.05) changes in the APDL layer position from the corresponding positions at pacing CL of 500 ms. Note how the locations of the longest APD shifted, in the same wedges, with exposure to the higher dose.

 


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  		Fig. 5. An example showing that the longest APDs occurred at deeper positions in ventricular wall with higher concentrations of ATX-II (control, 1.25 and 20 nmol/l). Isochronal maps show the transmural distributions of APDs with the diagonal hatched lines and solid black highlighting the regions of longer and longest APDs, respectively. The thick horizontal dash lines indicate the APDL layer having the longest mean APDs. *Selected columns of APs from each map are displayed at lower right indicating the longest three APDs. All numbers are time in milliseconds. Transmural ECGs are shown under the APs. The wedge was stimulated at the endocardium at the CL of 1,000 ms. The mapping area is 19.5 x 17.0 mm2. The longest APDs are mostly in the (sub)endocardium during control and appeared as islands in the midmyocardium after 20 nmol/l ATX-II.

 

   DISCUSSION
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 ABSTRACT
 METHODS
 RESULTS
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GRANTS
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Relationship to previous studies. Ventricular myocytes are heterogeneous (3, 7, 21) due to regional variations in the densities of membrane ionic currents. Compared with the endocardial myocytes, epicardial myocytes have a larger transient outward current (Ito), lower peak Na+ current (INa), larger slow component of delayed rectifier K+ current (IKs), larger Na+/Ca2+ exchange current (INaCa), and APs with a larger spike and dome morphology and more pronounced rate dependency. M cells have the lowest density of IKs in the canine ventricle (17) contributing to the dispersion of APDs (3, 4, 31) and its rate dependence (31). Transmural dispersion of APDs can be amplified by blockade of the rapid component of delayed rectifier K+ current (IKr) or by slowing the inactivation of late INa and can be minimized by IKs blockade (30). The heterogeneic ventricular ionic currents provide the basis for functional modulations of the presence of M cell behavior and position of APDL layers.

The experimental procedures and artificial environment can modulate the electrophysiological behavior of isolated cardiac cells and tissue. Many cardiac ionic channels are in rapid cycles of synthesis, intracellular trafficking, assembly into channels, and degradation. Rapid turnover of Kv channel proteins [e.g., ~4 h half-life for the Kv1.5 proteins, which form one of the delayed rectifier K+ channels (IK) (27)] can regulate the voltage-gated K+ channels and tissue excitability (15). The IK can be regulated even during the short period of cell isolation. Canine atrial IK was observed in only 4% of the cells isolated by exposing chunks of tissue to collagenase but in 99% of the cells isolated by arterial perfusion of collagenase (41). Rapid pacing for 30–60 min caused persistent remodeling of refractory period (13) and APD (16), the K+ channel mRNA levels (37), and the membrane Ito and L-type calcium (ICaL) currents (20) in cardiac muscle. Therefore, the procedures of cell/tissue isolation and experiments could cause sufficient modifications that affect the electrophysiological property of cardiac cells.

The relatively minor M cell effects in the ATX-II-free wedges as we observed in this study are consistent with our previous observations (28, 34, 35, 36) as well as in intact ventricles (7, 18). Similar transmural profiles of APDs with the longest APD near the endocardium were also reported previously (1, 28, 34, 38). The dose-dependent preferential midmyocardial APD prolongation of ATX-II (Fig. 2) is also consistent with the previously reported effects of 20 nmol/l ATX-II (22, 28, 38).

Definition of M cells and confirmation of M cell behavior. The original definition of M cells, a distinctive class of midmyocardial cells having exceedingly long APD when activated at long CLs characteristically different and separated from the epicardial and endocardial myocytes (3, 4), was based on observations with microelectrode recordings at selected sites. Although M cells were defined, the criteria for separating the M cells from other cell types have been less clear. Cells and tissue slices isolated from the M region displayed APDs that were 170 ms (4) and >100 ms (3) longer than the epicardium or endocardium, respectively, at pacing CLs of 2,000 ms or greater. However, the differences in the APDs between the M cells and endocardium were much smaller in intact ventricular wall. Yan et al. (39) reported that APDs of the M cells and endocardium were 281 ± 25 and 266 ± 21 ms, respectively, at pacing CL of 2,000 ms in arterially perfused wedges isolated from canine ventricular wall. The difference between the above mean APDs (15 ms) was actually smaller than their standard deviations (25 and 21 ms). Such a relatively small difference in APD may not be easily detectable, especially when we compared APDs between all mapping sites in the APDL layer and in the endocardium, instead of between small numbers of microelectrode recordings at selected sites. Alternatively, M cells were also defined as nonepicardial and nonendocardial cells with the longest (10th percentile) APDs (2). This definition cannot be used to evaluate the presence of M cells, because the top 10% APDs can always be determined even when there are no statistical differences.

We used a statistical criterion to evaluate the presence of cells with M cell behavior in the ventricular wall. Because the occurrence of M cells depended on the depth inside the ventricular wall (mostly in the midmyocardium) (3, 4), a parallel layer with the longest mean APD (APDL layer) was most statistically sensitive compared with the endocardium and could be applied uniformly to all tissue. This statistical method avoided the difficulty of identifying individual M cells based on the property of single cells. However, this statistical method could not determine all the regions of apparent M cells, even when the presence of M cells was statistically confirmed, because not all sites in the APDL layer had long APDs, and long APDs also occurred in other regions in addition to the APDL layer (e.g., Fig. 5).

New observations. We demonstrated in this study that both the presence of M cell behavior and position of the APDL layer were modulated by ATX-II. M cell behavior was confirmed in wedges treated with ≥5 nmol/l but not with lower concentrations of ATX-II. ATX-II shifted the APDL layer deeper into the midmyocardium from the (sub)endocardium.

Because the combined effects of all membrane ionic currents determine APD, heterogeneities in the densities of membrane ionic channels cause regional variations in the APD dependency on the environment (i.e., ATX-II and pacing CL). In other words, uniform changes in the environment (i.e., ATX-II) can alter the CL dependency of APD heterogeneously and cause the longest APDs to appear at different locations. The ATX-II dependencies of the APDL layer location (Figs. 4 and 5) and of the occurrence of M cell behavior (Fig. 2) suggest that the M cell-like behavior is a functional state of the heterogeneous ventricular myocytes instead of a manifestation by an anatomically separated distinctive cell type having its own electrophysiological characteristics and pharmacological profiles. Different (heterogeneous) cells can enter this functional state under different conditions. If M cells were a distinctive group of cells, then external modulators (e.g., ATX-II, CL, etc.) should only suppress or enhance their functional expressions, not shift their locations, or one would have to postulate that there were different populations of M cells that responded differently to the different concentrations of ATX-II.

Similar ATX-II-promoted M cell behavior was also reported in our previous study in ventricular wedges (28), in which we observed M cell behavior with the APDL layer in the midmyocardium (31% ± 4% transmurally from the endocardium, n = 8, CL: 4,000 ms, control group) after 20 nmol/l ATX-II but no statistical evidence of M cell behavior before ATX-II. The current study demonstrated the progressive effects of increasing concentrations of ATX-II and unveiled the functional nature of M cell behavior as phenotypical expressions of the ventricular heterogeneity under suitable conditions. The functional nature of M cell behavior as we demonstrated in this study also supports previous in vivo observations that the dispersion of repolarization could be amplified by anthopleurin-A (11) or by D-sotalol (33) because both agents modify membrane ionic currents.

Unifying hypothesis. The results of this study suggest a single mechanism for the seemingly contradicting observations on the transmural dispersion of repolarization, which now can be explained by the different dynamic and spatial responses of cellular heterogeneity to the experimental conditions. The extensive procedures, artificial environment, and experimental manipulations in the in vitro studies with isolated cells and tissue, as well as QT-prolonging agents, could modify the densities and function of membrane ionic currents and cause some cells to enter the state of M cell behavior. In contrast, in vivo studies without pharmacologically induced QT prolongation produced much less disturbance of the ventricular cells, thus, demonstrated transmural profiles of repolarization closer to their physiological states with much less expression of M cell behavior.

Limitations. We immobilized the wedges with 20–30 µmol/l cytochalasin D. Tissue immobilization disables stretch-activated channels and may affect other ionic channels that interact with the cytoskeleton.


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This research was supported by American Heart Association Midwest Affiliation Awards 0256112Z and 0455517Z, by a grant from Alcoholic Beverage Medical Research Foundation, and by the Herman C. Krannert Fund, Indianapolis, IN.


   FOOTNOTES
 

Address for reprint requests and other correspondence: J. Wu, Krannert Institute of Cardiology, Indiana Univ. School of Medicine, 1800 N. Capitol Ave., Indianapolis, IN 46202 (E-mail: jiaswu{at}iupui.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


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