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. 2001 Sep 1;535(Pt 2):611–617. doi: 10.1111/j.1469-7793.2001.00611.x

Cardiac performance in inbred rat genetic models of low and high running capacity

J Chen 1, G M Feller 1, J C Barbato 1, S Periyasamy 1, Z-J Xie 1, L G Koch 1, J I Shapiro 1, S L Britton 1
PMCID: PMC2278800  PMID: 11533149

Abstract

  1. Previous work demonstrating that DA inbred rats are superior to COP inbred rats in aerobic treadmill running capacity has indicated their utility as genetic models to explore this trait. We tested the general hypothesis that intermediate phenotypes of cardiac function and calcium metabolism are responsible for the difference in capacity between these strains.

  2. Logical cardiac trait differences were estimated at a tissue (isolated papillary muscle), cellular (isolated left ventricular cells), and biochemical level of organization.

  3. DA hearts were found to give significantly higher values than COP hearts for: (1) maximal developed tension (38.3 % greater), and rates of tension change in contraction (61 %) or relaxation (59 %) of isolated papillary muscle, (2) fractional shortening (50 %), amplitude of the Ca2+ transient (78.6 %), and caffeine-induced release of Ca2+ from the sarcoplasmic reticulum (SR; 260 %) in isolated ventricular myocytes, and (3) Na+,K+-ATPase activity of isolated myocytes (17.3 %).

  4. Our results suggest that these trait differences may prove useful for further studies into the genes responsible for natural variations in both ventricular function and aerobic endurance capacity. Understanding the genetic basis of aerobic capacity will help define the continuum between health and disease.

A long-term aim of our laboratory is to identify the genes that cause variation in mammalian aerobic exercise capacity, and to elucidate the way in which they function. Endurance exercise test performance has often been used to assess cardiovascular fitness and to estimate the magnitude of cardiovascular reserve capacity (Hammond & Froelicher, 1984; Haskell, 1994), but despite the accumulation of a vast literature dealing with the adaptational aspects of endurance capacity (Costill & Wilmore, 1994; McArdle et al. 1996), the genetic components causing high and low aerobic capacity remain undefined (Bouchard et al. 2000; Bray, 2000).

Genetic models, as described by both Jacob (Jacob, 1999) and Rapp (Rapp, 2000), have proved to be a critical substrate for dissecting the genetic origin of the allelic variations that are determinants of the differences in complex traits. Ideal models can be created by selective breeding for low and high states of a trait over many generations: this concentrates genes for the extremes of the trait in divergent lines. Once maximal divergence is attained, the low and high selected lines can themselves be inbred, creating strains that have minimal genetic variation, thereby optimizing genomic analysis for differences between the strains (Lander & Botstein, 1989).

An alternative strategy is to identify wide differences in a trait in inbred strains that are already commercially available. Numerous strains have been developed, and these, like all populations, contain individuals demonstrating wide variations in other traits. We have recently evaluated treadmill endurance running capacity in 11 inbred strains of rats, and identified several strains that differed widely enough to serve as genetic models of high and low endurance capacity (Barbato et al. 1998). Whilst endurance capacity and isolated heart capacity (Langendorff-Neely working heart) correlated well (r = 0.86) across all 11 strains, we measured a greater than 2-fold difference in running capacity between DA and COP inbred rat strains, and on average a 50 % greater cardiac output in the isolated hearts of DA rats when compared to COP rats.

Inbreeding for the DA strain was started by Odell at the Oak Ridge National Laboratory (USA) and taken through to generation 11. Wilson (Wilson, 1965) then completed inbreeding at the Wistar Institute (USA), and named the strain DA because they express the ‘d’ blood group allele and have an agouti coat colour. The COP rats were originated by Curtis (Dunning & Curtis, 1946) at Columbia University Institute for Cancer (USA) in 1921, and initial interest was in their resistance to the induction of mammary tumours. More recent work has demonstrated that DA rats also have an enhanced range of operation for autonomic control of peripheral blood flow and cardiac output, more sympathetic support of blood pressure, higher blood pressure and heart rate during exercise, and greater heart weight-to-body weight ratio compared with the COP strain (Koch et al. 1999). More information on these strains is available on-line at the Mouse Genome Informatics web site (The Jackson Laboratory; http://www.informatics.jax.org/external/festing/search_form.cgi/).

A substantial body of data (Hill & Lupton, 1923; Mitchell et al. 1958; Rowell et al. 1996) supports the notion that cardiac output might be the major limiting factor in endurance capacity. Thus, the general purpose of this study was to decompose the performance of isolated COP and DA rat hearts into simpler, intermediate phenotypes. We tested the general hypothesis that DA rat hearts are superior to their COP counterparts for intrinsic cardiac function by measuring indices of the contractile capacity of isolated papillary muscles and dispersed left ventricular cells. Our data demonstrate that DA hearts are consistently superior to COP hearts for force development, Ca2+ cycling during contraction, and Na+,K+-ATPase activity of isolated myocytes.

METHODS

Animals

Experiments were performed on hearts taken from female and male rats of the COP and DA inbred strains at 12 weeks of age. The rats were bred in-house at the Medical College of Ohio, from commercially available stock (Harlan Sprague-Dawley: Indianapolis, IN, USA). Each rat was injected with heparin sodium (300 i.u. i.p.), and then anaesthetized with pentobarbital sodium (50 mg kg−1 body weight, i.p.), 30 min prior to removal of the heart. All procedures were approved by our Institutional Animal Care and Use Committee, and were conducted in accordance with the Guiding Principles in the Care and Use of Animals, as approved by the Council of the American Physiological Society.

Isolated papillary muscle studies

Following removal, hearts were placed immediately in ice-cold oxygenated Krebs-Henseleit solution. Attempts were made to obtain two papillary muscles from each rat; 14 papillary muscles were successfully dissected from the left ventricular walls of 10 COP rats, and 20 from 11 DA rats. Muscles were placed between two stainless steel O-rings, and mounted vertically between a force transducer (Fort 10 model, WPI, Sarasota, FL, USA) and a stainless steel hook in a temperature-controlled glass tissue bath (27 °C). The bath had a volume of 25 ml and containd oxygenated Krebs solution (95 % O2, 5 % CO2) with the following composition (mm): NaCl, 100; KCl, 1.0; MgSO4, 1.5; NaHCO3, 20.0; NaH2PO4, 1.5; NaAC, 20.0; glucose, 10.0, ascorbic acid, 0.10; and CaCl2, 1.90; with insulin, 5.0 i.u. The pH of the solution was 7.4.

Initially, papillary muscle length was adjusted with a micrometer to create a passive tension of about 0.5 g (slack). Isometric contraction was stimulated by passing a current between platinum electrodes on either side of the hanging papillary muscle. Muscle was stimulated at a frequency of 0.2 Hz using a 5 ms pulse duration, and voltage was adjusted to 20 % above threshold. The length was then slowly adjusted over 45 min to a value at which the tension developed with each contraction reached a maximum. The tension between contractions was taken as the resting tension (RT), and the increment above RT that occurred with each stimulation was taken as the developed tension (DT). The analog signals from the force transducers were amplified with a Sensormedics R-611 polygraph (Anaheim, CA, USA), sampled at 250 Hz by a PO-NE-MAH digital acquisition and archiving system (Storrs, CT), and stored on disk for subsequent analysis. Besides RT and DT, estimates of the rates of change in tension (T) with each contraction were also derived. +dT/dt represents the maximum value of the first derivative of tension increase during contraction, and -dT/dt represents the maximum value of the first derivative of tension decline during relaxation. Estimates of tension development were normalized to the cross-sectional area at RT for each muscle. This was calculated as the ratio of muscle volume (estimated by weighing) to muscle length at RT. Papillary muscle cross-sectional area was 0.95 ± 0.24 mm2 for COP rats and 0.87 ± 0.20 mm2 for DA rats (P = n.s.). Baseline values of RT, DT, +dT/dt and -dT/dt for COP and DA samples were taken as the averages of the contractions recorded during approximately the last minute of a 45 min period in which the muscle was adjusted to achieve a stable level at maximum DT.

Myocyte isolation and culture

Isolation of adult left ventricular myocytes was performed as described previously (Xie et al. 1990), with minor modifications. After removal from the chest, hearts were immediately perfused retrograde, first with Jolik Modified Minimal Essential Media (MEM) containing 1.25 mm Ca2+ for 15 min, then with a nominally Ca2+-free medium for 5 min. This was followed by perfusion with the same medium containing 0.2 mg ml−1 collagenase (15 min). Myocytes were then isolated and cultured in laminin-coated Petri dishes or 6-well plates, in a medium containing Medium 199, penicillin (100 i.u. ml−1), streptomycin (100 μg ml−1) and 0.1 % bovine serum albumin (BSA).

Measurements of myocyte contraction

Myocytes cultured in Petri dishes were selected for study on the basis of their morphology (rod shaped with no hyper-contracted areas) and the absence of spontaneous contractions. Suitable samples were placed on the stage of an inverted microscope, and field stimulated with platinum electrodes at 0.5 Hz. Video images of their contractions were obtained using a Nikon inverted microscope with a ×40 objective, interfaced through a C-arm with a video imaging system as described by Qi et al. (1997). The contraction of a single cell was measured using an edge detection system developed by Crystal Biotech (Northboro, MA, USA), equipped with a Philips FTM800 image sensor module (Philips Image Technologies, The Netherlands). Edge detection data were digitized at 60 Hz, and processed using the FELIX software package that accompanies the QuantaMaster Model C-60 Spectrofluorimeter system (Photon Technologies International, Monmouth, NJ, USA), described below.

Measurements of Ca2+ transients

We used a procedure described by Freestone et al. (1996), with minor modifications. Myocytes cultured on glass coverslips were loaded with the fluorescent dye indo-1 AM (4 μm), by incubation at 37 °C for 30 min. Coverslips with cells attached were then transferred to a chamber, washed 3 times in the same medium without indo-1 AM, and incubated at 37 °C for another 10 min to ensure complete intracellular de-esterification of the dye. The chamber was then placed on the stage of an inverted microscope, and the myocytes were stimulated electrically as described above. Indo-1, internalized within the cell, was excited at 355 nm using a UV lamp. Light emitted by a single cell was selected using an adjustable iris, and detected at 405 and 485 nm using FELIX. The indo-1 fluorescence emission ratio (405 nm/485 nm) was converted to [Ca2+]i according to the formula:

graphic file with name tjp0535-0611-mu1.jpg

where Kd is the dissociation constant of the indo-1-Ca2+ complex (250 nm); R is the ratio of cellular fluorescence intensity at 485 nm/405 nm; Rmin is the ratio of fluorescence intensity in cells incubated in the Ca2+-free solution; Rmax is the ratio of fluorescence intensity in cells incubated in the Ca2+-saturated solution; Sf2 is indo-1 fluorescence intensity at 485 nm measured under Ca2+-free conditions; and Sb2 represents the intensity of indo-1 fluorescence at 485 nm measured under Ca2+-saturated conditions. EGTA and ionomycin were used to measure Rmin and Rmax. In some experiments, electrical stimulation was removed for approximately 10 s, and caffeine (10 mm) was applied to cause contraction as described by Bassani et al. (1994).

Na+,K+-ATPase activity

Na+,K+-ATPase activity was assayed by determining the initial rate of inorganic phosphate (Pi) release from ATP as described previously (Xie et al. 1989). Briefly, isolated myocytes were suspended in Tris-EGTA (pH 7.2), and made permeable with alamethicin. A 0.1 ml sample of the resultant suspension was added to 0.9 ml of a reaction mixture containing (mm): [γ32P]ATP, 2; MgCl2, 3; NaCl, 100; KCl, 25; EGTA, 1; NaN3, 5; Tris-HCl (pH 7.2), 50; and either ouabain, 1 or H2O. Incubations were conducted at 37 °C for 30 min. Assays were terminated by the addition of 1 ml of 8 % perchloric acid. Released 32Pi was converted to phosphomolybdate, extracted into 2-methylpropanol, and counted as previously described (Askari et al. 1980). To determine the activity of each sample, the assay was performed in the presence and absence of 1 mm ouabain; the difference between the two samples was taken as indicative of Na+,K+-ATPase activity.

Western blot analysis

Isolated myocytes were suspended in 10 mm Tris-EGTA (pH 7.2). Cell suspensions were assayed for protein content as described by Lowry et al. (1951), using BSA as a standard. Proteins were then separated by SDS-PAGE (50 μg per lane), and transferred to nitrocellulose membranes. These were probed with specific primary antibodies, and detected using secondary antibodies linked to horseradish peroxidase (Huang et al. 1994).

Statistical analysis

For measurements of [Ca2+]i and cell motion, three successive transients or contractions were averaged. The amplitude of the Ca2+ transient was defined as the difference between peak systolic and diastolic indo-1 fluorescence ratios. Twitch amplitude was defined as the difference in cell length before electrical stimulation and at peak contraction. For both papillary muscle and isolated cell studies, differences between COP and DA rat samples were evaluated for statistical significance using Student's t test for unpaired values. A 5 % confidence level was arbitrarily used for assigning any difference as significant, and data are presented as mean values ±s.e.m.

RESULTS

Tensions developed in isolated papillary muscles for baseline conditions are shown in Fig. 1. Measured resting tension averaged 1.80 ± 0.34 g in COP rats, and 2.06 ± 0.20 g in the DA strain: differences were not statistically significant. Developed tension averaged 2.14 ± 0.35 g in COP rats, and 2.96 ± 0.33 g in DA rats (38.3 % more developed tension in the DA rats; P < 0.05). Figure 2 shows the rates of change in tension development in isolated papillary muscle for both contraction (+dT/dt) and relaxation (-dT/dt). On average, +dT/dt was 20.2 ± 2.0 g s−1 in COP rats and 32.5 ± 4.9 g s−1 in DA rats (61% difference; P < 0.01). -dT/dt averaged 17.9 ± 1.5 g s−1 in COP rats compared to 28.5 ± 3.6 g s−1 in DA rats (59 % difference; P < 0.01).

Figure 1. Developed tension in papillary muscles from COP and DA rats stimulated at 0.2 Hz.

Figure 1

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On average, DA muscles developed 38.3 % more tension than COP muscles (*P < 0.05). In this and the following figures, data shown are means ±s.e.m. (n = 14 COP and n = 20 DA samples).

Figure 2. Maximal rate of change in contraction (+dT/dt) and relaxation (-dT/dt) with each contraction in papillary muscles from COP and DA rats.

Figure 2

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On average, both contraction and relaxation rates were greater in DA vs. COP samples (* P < 0.01; n = 14 COP and n = 20 DA samples).

Isolated ventricular myocytes from DA rats demonstrated a greater fractional shortening with stimulation compared to those from the COP strain, as shown by representative traces in Fig. 3A. On average (Fig. 3B), myocytes from COP rats shortened by 9.4 ± 0.8 %, and those from DA rats shortened by 14.4 ± 9.4 % (DA fractional shortening was 50 % greater than COP; P < 0.01) when stimulated to contract at 0.5 Hz. Figure 4A shows representative Ca2+ transients from COP and DA rat heart cells measured by indo-1 fluorescence. Measurement of basal [Ca2+]i and time to half-peak [Ca2+]i showed no discernible differences between the two strains (56 ± 9 vs. 59 ± 7 nm; 32 ± 3 vs. 33 ± 4 ms; both P = n.s.). However, as shown in Fig. 4B, both peak [Ca2+]i and the changes in [Ca2+]i (Δ[Ca2+]i) values were significantly lower in the COP rats relative to DA rats (227 ± 20 vs. 359 ± 34 and 168 ± 12 vs. 300 ± 29 nm; both P < 0.01). The change in [Ca2+]i following stimulation was therefore 78.6 % greater in the DA samples. The τ value for calcium normalization following stimulation was somewhat shorter in COP as compared to DA rats (431 ± 16 vs. 504 ± 23 ms; P < 0.05). Exposure to caffeine produced a markedly greater increase in the cytosolic [Ca2+]i of DA compared to COP myocytes (Fig. 5). Caffeine increased [Ca2+]i by 268 ± 51 nm in COP heart cells compared to 965 ± 286 nm in DA heart cells (P < 0.05).

Figure 3. Typical contractions of myocytes from COP and DA rats.

Figure 3

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A, isolated ventricular myocytes from DA rats demonstrated a greater fractional shortening with stimulation that COP rats (n = 14 COP and n = 20 DA samples). B, percentage shortening of isolated myocytes stimulated at 0.5 Hz. On average, myocytes from the DA rats (30 cells from 5 ventricles) showed a fractional shortening approximately 50 % greater than COP myocytes (29 cells from 6 ventricles; *P < 0.01).

Figure 4. Measurement of [Ca2+] transients.

Figure 4

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A, typical indo-1 fluorescence transients of electrically stimulated myocytes from DA and COP rats. B, peak [Ca2+]i. The amplitude of the peak [Ca2+] fluorescence transient was about 78.6 % greater in myocytes derived from DA rats (25 cells from 5 ventricles) relative to the COP rats (16 cells from 5 ventricles; * P < 0.01). Basal [Ca2+]i levels were not significantly different between the two strains.

Figure 5. Increase in myocyte cytosolic [Ca2+]i in the presence of caffeine.

Figure 5

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Exposure to caffeine produced a markedly greater increase in cytosolic [Ca2+]i in myocytes from DA rats (18 cells from 5 ventricles) than COP rats (17 cells from 5 ventricles).

Figure 6 shows that Na+,K+-ATPase activity averaged 2.6 ± 0.12 nmol mg−1 h−1 in COP myocytes compared to 3.05 ± 0.15 nmol mg−1 h−1 in myocytes from the DA strain (17.3 % difference; P < 0.05).

Figure 6. Myocyte Na+,K+-ATPase activity.

Figure 6

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Relative activity of the Na+,K+-ATPase was found to be about 17.3 % higher in DA (n = 6) compared to COP (n = 5) rats (* P < 0.05).

Figure 7A shows representative Western blots of the α1 and α2 subunits of Na+,K+-ATPase from both DA and COP rat myocytes. Samples from the left ventricles of DA rats showed a greater relative expression of both the α1 (24 %) and α2 subunits (41 %) of the Na+,K+-ATPase (Fig. 7B) compared to those from COP rats.

Figure 7. Relative expression of Na+,K+-ATPase protein in myocytes.

Figure 7

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A, representative immunostaining of α1 and α2 subunits of Na+,K+-ATPase in DA and COP rat myocytes. B, quantitative analysis of the relative expression levels of Na+,K+-ATPase isoforms in DA (n = 4 ventricles) and COP (n = 4 ventricles) revealed that the DA strain expressed the α1 (24 %) and α2 (41 %) subunits to a significantly higher level compared to the COP strain (* P < 0.05).

DISCUSSION

Quantitative traits such as endurance capacity are inherently complex: this study emphasizes the value of starting with simplistic theoretical models and utilizing inbred animal models in which genetic and environmental variations are minimized. Like populations of individuals, strains inbred for one trait can also demonstrate wide variation in others, and are thus useful tools in the study of genetic variation. The wide variation for many higher order physiological traits is explained by Fisher's 1930 Theorem of Natural Selection (Fisher, 1930). This theorum predicts that traits associated with evolutionary fitness (such as viability and fecundity) should demonstrate little genetic variance because of pressure from natural selection. In contrast, traits such as morphology and complex physiology are predicted to demonstrate more genetic variance because there will be less pressure from natural selection (Mousseau & Roff, 1987).

Joyner (Joyner, 1991) developed a model of endurance running capacity that is a function of three physiological variables: (1) the maximal rate at which oxygen and nutrient substrates can be utilized to produce energy in the form of ATP (Inline graphic at the threshold for lactate release Inline graphic, and (3) the efficiency of running (RE; km min−1 Inline graphic). These are related to the optimal running velocity by the formula:

graphic file with name tjp0535-0611-mu2.jpg

Overall, our hypothesis is that allelic differences in each of these three complex intermediate phenotypes account for the magnitude of the difference in running capacity between COP and DA rats (Costill et al. 1973; Anderson & Saltin, 1985). In addition, through interpretation of information from other stuides, we suggest that the three factors comprising the Joyner model do not contribute equally as determinants of running capacity. The seminal work of A. V. Hill (Hill & Lupton, 1923) strongly suggests that the ability of the heart to deliver oxygen is the predominant limiting factor in maximal endurance capacity (Mitchell et al. 1958; Rowell et al. 1996). Consideration of these ideas led us to focus on cardiac factors as the most likely source of the difference in capacity between the COP and DP strains.

Bouchard and colleagues (Bouchard et al. 2000) performed a genome search (HERITAGE family) to identify chromosomal regions linked to maximal oxygen consumption under two conditions: (1) individuals in the untrained, sedentary condition, and (2) in the same individuals after 20 weeks of endurance training on a bicycle ergometer. For the sedentary condition, microsatellite markers on chromosomes 4q, 8q, 11p and 14q were linked with maximal oxygen consumption (P < 0.01). For the response to training, markers on 1p, 2p, 4q, 6p and 11p were found to be linked with maximal oxygen consumption (P < 0.01). These results suggest that the genetic factors underlying sedentary maximal oxygen consumption (i.e. intrinsic capacity) and the responses to training are not the same. Thus, an individual's capacity can be viewed as the result of the expression of both intrinsic and adaptational genes as they interact with the environment. Although no intrinsic endurance genes have been identified, several chromosomal regions are clearly associated with the response to exercise (Williams & Neufer, 1996; Rivera et al. 1997; Montgomery et al. 1998; Rankinen et al. 2000).

From our study, it is clear that COP and DA strains have marked intrinsic differences in the function of both isolated papillary muscles and isolated myoblasts; neither COP papillary muscles nor isolated heart cells contracted as vigorously under electrical stimulation as those derived from DA rats. These parameters correspond to the observed untrained running capacity and isolated heart function previously reported (Barbato et al. 1998). Examining this further, our data strongly suggest that intrinsic differences in cardiomyocyte Ca2+ metabolism may explain the functional differences between the two strains. DA cardiomyocytes had similar diastolic Ca2+ concentrations but markedly higher peak [Ca2+]i during systole. This was associated with greater release of SR Ca2+ following stimulation with caffeine, a strategy used to assay the magnitude of SR Ca2+ stores (Bassani et al. 1994). In addition, we found that the cellular Na+,K+-ATPase activity was nearly 20 % higher in isolated DA cells than in their COP counterparts, corresponding to the significantly greater amounts of both α1 and α2 isoforms of Na+,K+-ATPase enzyme that were revealed by Western blotting.

A relationship between myocyte contractile function and Na+,K+-ATPase activity has previously been postulated for both skeletal and cardiac muscle, based on the strong relationship between Na+,K+-ATPase activity and the function of these tissues (McCutcheon et al. 1999; Verburg et al. 1999). However, the exact mechanism(s) by which the Na+ pump affects contractile function has yet to be elucidated (Clausen, 1998). We observed that the higher Na+ pump activity in DA rats was associated with a greater SR Ca2+ storage pool, as well as significantly greater increases in [Ca2+]i following stimulation; we therefore postulate that the increased Na+ pump activity may allow for greater accumulation of calcium in the SR, possibly mediated by an alteration of the transmembrane potential.

In summary, we have characterized three cardiac traits that appear to be involved in the difference between genetic inbred rat models with low (COP) and high (DA) endurance capacity: (1) isometric tension development in papillary muscle, (2) contractile function and Ca2+ cycling in ventricular myocytes, and (3) cellular activity of Na+,K+-ATPase. As such, these traits appear to be strong candidates for use in future studies aimed at establishing cause and effect at the genetic level (Rapp, 2000).

Acknowledgments

This work was supported by a grant from the United States Public Health Service (National Institutes of Health grant HL 64270).

References

  1. Anderson P, Saltin B. Maximal perfusion of skeletal muscles in man. Journal of Physiology. 1985;366:233–249. doi: 10.1113/jphysiol.1985.sp015794. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Askari A, Huang W, Antieau J. Na+,K+-ATPase: Ligand-induced conformational transitions and alterations in subunit interactions evidenced by cross-linking studies. Biochemistry. 1980;19:1132–1140. doi: 10.1021/bi00547a015. [DOI] [PubMed] [Google Scholar]
  3. Barbato JC, Koch LG, Darvish A, Cicila GT, Metting PJ, Britton SL. Spectrum of aerobic endurance running performance in eleven inbred strains of rats. Journal of Applied Physiology. 1998;85:530–536. doi: 10.1152/jappl.1998.85.2.530. [DOI] [PubMed] [Google Scholar]
  4. Bassani RA, Bassani JW, Bers DM. Relaxation in ferret ventricular myocytes: unusual interplay among calcium transport systems. Journal of Physiology. 1994;476:295–308. doi: 10.1113/jphysiol.1994.sp020131. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Bouchard C, Rankinen T, Chagnon YC, Rice T, Perusse L, Gagnon J, Borecki I, An P, Leon AS, Skinner JS, Wilmore JH, Province M, Rao DC. Genomic scan for maximal oxygen uptake and its response to training in the HERITAGE family study. Journal of Applied Physiology. 2000;88:551–559. doi: 10.1152/jappl.2000.88.2.551. [DOI] [PubMed] [Google Scholar]
  6. Bray MS. Genomics, genes, and environmental interaction: the role of exercise. Journal of Applied Physiology. 2000;88:788–792. doi: 10.1152/jappl.2000.88.2.788. [DOI] [PubMed] [Google Scholar]
  7. Clausen T. Clinical and therapeutic significance of the Na+,K+ pump. Clinical Science. 1998;95:3–17. [PubMed] [Google Scholar]
  8. Costill DL, Thomason H, Roberts E. Fractional utilization of the aerobic capacity during distance running. Medicine and Science in Sports and Exercise. 1973;5:248–252. [PubMed] [Google Scholar]
  9. Costill DL, Wilmore JH. Physiology of Sport and Exercise. Champaign, IL, USA: Human Kinetics Publishing; 1994. [Google Scholar]
  10. Dunning WF, Curtis MR. The respective roles of longevity and genetic specificity in the occurrence of spontaneous tumors in the hybrids between two inbred lines of rats. Cancer Research. 1946;6:61–81. [PubMed] [Google Scholar]
  11. Fisher RA. The Genetical Theory of Natural Selection. Oxford: Clarendon Press; 1930. [Google Scholar]
  12. Freestone NS, Ribaric S, Mason W. The effect of insulin-like growth factor-1 on adult rat cardiac contractility. Molecular and Cellular Biochemistry. 1996;163:223–229. doi: 10.1007/BF00408662. [DOI] [PubMed] [Google Scholar]
  13. Hammond HK, Froelicher VF. Exercise testing for cardio-respiratory fitness. Sports Medicine. 1984;1:234–239. doi: 10.2165/00007256-198401030-00005. [DOI] [PubMed] [Google Scholar]
  14. Haskell WL. Health consequences of physical activity: understanding and challenges regarding dose-response. Medicine and Science in Sports and Exercise. 1994;26:649–660. doi: 10.1249/00005768-199406000-00001. [DOI] [PubMed] [Google Scholar]
  15. Hill AV, Lupton H. Muscular exercise, lactic acid, and the supply and utilization of oxygen. Quarterly Journal of Medicine. 1923;16:135–171. [Google Scholar]
  16. Huang WH, Wang Y, Askari A, Zolotarjova N, Ganjeizadeh M. Different sensitivities of the Na+,K+-ATPase isoforms to oxidants. Biochimica et Biophysica Acta. 1994;1190:108–114. doi: 10.1016/0005-2736(94)90039-6. [DOI] [PubMed] [Google Scholar]
  17. Jacob HJ. Functional genomics and rat models. Genome Research. 1999;9:1013–1016. doi: 10.1101/gr.9.11.1013. [DOI] [PubMed] [Google Scholar]
  18. Joyner MJ. Modelling: optimal marathon performance on the basis of physiological factors. Journal of Applied Physiology. 1991;70:683–687. doi: 10.1152/jappl.1991.70.2.683. [DOI] [PubMed] [Google Scholar]
  19. Koch LG, Britton SL, Barbato JC, Rodenbaugh DW, Dicarlo SE. Phenotypic differences in cardiovascular regulation in inbred rat models of aerobic capacity. Physiological Genomics. 1999;1:63–69. doi: 10.1152/physiolgenomics.1999.1.2.63. [DOI] [PubMed] [Google Scholar]
  20. Lander ES, Botstein D. Mapping Mendelian factors underlying quantitative traits using RFLP linkage maps. Genetics. 1989;121:185–199. doi: 10.1093/genetics/121.1.185. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Lowry OH, Rosebrough NJ, Farr AL, Randall RJ. Protein measurement with the Folin phenol reagent. Journal of Biological Chemistry. 1951;193:265–275. [PubMed] [Google Scholar]
  22. McArdle WD, Katch FI, Katch VL. Energy, Nutrition, and Human Performance. 4. Baltimore: Williams and Wilkins; 1996. Exercise Physiology. [Google Scholar]
  23. McCutcheon LJ, Geor RJ, Shen H. Skeletal muscle Na+,K+-ATPase and K+ homeostasis during exercise: effects of short-term training. Equine Veterinary Journal Supplement. 1999;30:303–310. doi: 10.1111/j.2042-3306.1999.tb05239.x. [DOI] [PubMed] [Google Scholar]
  24. Mitchell JH, Sproule BJ, Chapman CB. The physiological meaning of the maximal oxygen intake test. Journal of Clinical Investigation. 1958;37:538–547. doi: 10.1172/JCI103636. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Montgomery HE, Marshall R, Hemingway H, Myerson S, Clarkson P, Dollery C, Hayward M, Holliman DE, Jubb M, World M, Thomas EL, Brynes AE, Saeed N, Barnard M, Bell JD, Prasad K, Rayson M, Talmud PJ, Humphries SE. Human gene for physical performance (letter) Nature. 1998;393:221–222. doi: 10.1038/30374. [DOI] [PubMed] [Google Scholar]
  26. Mousseau TA, Roff DA. Natural selection and the heritability of fitness components. Heredity. 1987;59:181–197. doi: 10.1038/hdy.1987.113. [DOI] [PubMed] [Google Scholar]
  27. Qi M, Puglisi JL, Byron KL, Ojamaa K, Klein I, Bers DM, Samarel AM. Myosin heavy chain gene expression in neonatal rat heart cells: effects of [Ca2+]i and contractile activity. American Journal of Physiology. 1997;273:C394–403. doi: 10.1152/ajpcell.1997.273.2.C394. [DOI] [PubMed] [Google Scholar]
  28. Rankinen T, Pérusse L, Gagnon J, Chagnon YC, Leon AS, Skinner JS, Wilmore JH. Angiotensin-converting enzyme Id polymorphism and fitness phenotype in the Heritage family study. Journal of Applied Physiology. 2000;88:1029–1035. doi: 10.1152/jappl.2000.88.3.1029. [DOI] [PubMed] [Google Scholar]
  29. Rapp JP. Genetic analysis of inherited hypertension in the rat. Physiological Reviews. 2000;80:135–172. doi: 10.1152/physrev.2000.80.1.135. [DOI] [PubMed] [Google Scholar]
  30. Rivera MA, Dionne FT, Simoneau JA, Pérusse L, Chagnon M, Chagnon Y, Gagnon J, Leon AS, Rao DC, Skinner JS, Wilmore JH, Bouchard C. Muscle-specific creatine kinase gene polymorphism and VO2max in the Heritage family study. Medicine and Science in Sports and Exercise. 1997;29:1311–1317. doi: 10.1097/00005768-199710000-00006. [DOI] [PubMed] [Google Scholar]
  31. Rowell LB, O'Leary DS, Kellog DLJ. Handbook of Physiology. In: Rowell LB, Shepherd JT, editors. Exercise: Regulation and Integration of Multiple Systems. Oxford: Oxford University Press; 1996. section 12. [Google Scholar]
  32. Verburg E, Hallen J, Sejersted OM, Vollestad NK. Loss of potassium from muscle during moderate exercise in humans: a result of insufficient activation of the Na+-K+-pump. Acta Physiologica Scandinavica. 1999;165:357–367. doi: 10.1046/j.1365-201X.1999.00512.x. [DOI] [PubMed] [Google Scholar]
  33. Wilson DD. Quantitative studies on the behavior of sensitized lymphocytes in vitro. Journal of Experimental Medicine. 1965;122:143–166. doi: 10.1084/jem.122.1.143. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Xie Z, Wang Y, Askari A, Huang WH, Klaunig E, Askari A. Studies on the specificity of the effects of oxygen metabolites on cardiac sodium pump. Journal of Molecular and Cellular Cardiology. 1990;22:911–920. doi: 10.1016/0022-2828(90)90122-i. [DOI] [PubMed] [Google Scholar]
  35. Xie Z, Wang Y, Ganjeizadeh M, McGee R, Askari A. Determination of total (Na+-K+)-ATPase activity of isolated or cultured cells. Analytical Biochemistry. 1989;183:215–219. doi: 10.1016/0003-2697(89)90470-3. [DOI] [PubMed] [Google Scholar]

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