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Background
Model of cardiac hypertrophy induced by swimming
training
Chronic conditioning by swimming in mice results in an increase in left
ventricular mass (Kaplan et al., 1994; Geisterfer-Lowrance
et al., 1996; Miller et al.,
2000). It was previously reported, that mice swum 5 days a week for 4
weeks had increased HW/BW ratios (increase of "16%, Kaplan et al., 1994). In
an attempt to induce a larger degree of hypertrophy, we employed a similar
protocol except that mice were swum daily for 4 weeks.
Experimental design:
FVB mice were obtained at 8 weeks of age from Charles River
Laboratories. Mice were swum in tanks with a diameter of 50 cm and a surface area of 2000 cm2. Physical interaction between the mice discouraged floating, and unlike
rats (Kaplan et al., 1994), mice do not dive. The water temperature was
maintained at 30-32șC to avoid thermal stress induced by cold water. Mice
were swum for 10 min sessions twice a day separated by a 4 hour
break. Sessions were increased by 10 min each day until 90 min sessions were
reached.
Mice were sacrificed at 10 min, 2.5 days, one week, two weeks, three weeks,
or 4 weeks of exercise training, and after 4 weeks of swimming and one week
of rest. As controls, age-matched mice were used that did not excercise,
e.g. 8 week old mice served as controls for the 10 min, 2.5 day, 1 week, and
2 week time points, and 12 or 13 week old mice were used as controls for the
3 and 4 week time points. The ventricular portion of the heart was collected
and snap frozen. Phenotypic data including heart, lung, liver, and body
weight, and tibial length were collected at the time of sacrifice.
At each of the time points examined, heart size was markedly increased
(P<0.05) in swimming mice compared to age and sex matched non-swimming
mice. In mice swum 1 week, 2 weeks, 3 weeks and 4 weeks, HW/BW ratios were
increased 29, 39, 45 & 49%, respectively. To assess whether the change in
heart size was due to hypertrophy of myocytes, the cell size of individual
isolated adult myocyte preparations was measured. The cell area of cardiac
myocytes isolated from hearts of mice swum for 4 weeks was approximately 45%
greater than cardiac myocytes from non-swimmers. Mice swum for 4 weeks had
lower basal heart rates (398 ± 23 bpm) than non-swimming mice (557 ± 23 bpm),
suggesting a similar chronic physical conditioning as seen in human
athletes. Echocardiographic examination (performed before and after the
completion of swimming training) of mice swum for 4 weeks showed increased
diastolic anterior and posterior wall thicknesses (P<0.05), as well as an
increase in left ventricular diastolic diameter compared to non-swimming
mice. Fractional shortening was unchanged between the 2 groups. Histological
sections of hearts from swimming mice showed no evidence of necrosis,
fibrosis or myocardial disarray (not shown).
Page last modified: 11-Jun-2003
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