Related Experiment Videos
Ca flux, contractility, and excitation-contraction coupling in hypertrophic rat ventricular myocytes
E McCall1, K S Ginsburg, R A Bassani
1Department of Physiology, Loyola University Medical Center, Maywood, Illinois 60153, USA.
Insights
Left ventricular hypertrophy in rats reduced myocyte contraction but showed normal calcium handling at physiological calcium levels. Reduced extracellular calcium revealed impaired excitation-contraction coupling, indicating altered calcium release mechanisms in hypertrophic hearts.
Area of Science:
- Cardiology
- Molecular Biology
- Physiology
Background:
- Left ventricular hypertrophy (LVH) is a common cardiac condition.
- Understanding the underlying mechanisms of contractile dysfunction in LVH is crucial for developing effective treatments.
Purpose of the Study:
- To investigate the effects of LVH on intracellular calcium handling, myocyte contraction, and excitation-contraction (E-C) coupling in a rat model.
- To identify specific molecular and functional alterations in cardiac myocytes during the development of hypertrophy.
Main Methods:
- Induction of LVH in rats by abdominal aortic banding.
- Isolation of ventricular myocytes for functional studies using indo 1 fluorescence and whole-cell voltage clamp.
- Biochemical measurements of sarcoplasmic reticulum (SR) Ca-ATPase and Na/Ca exchange mRNA levels.
- Assessment of SR Ca content, myocyte contraction, and E-C coupling efficacy under varying extracellular calcium concentrations.
Main Results:
- LVH led to depressed myocyte shortening amplitudes despite normal SR Ca content.
- SR Ca-ATPase and Na/Ca exchange mRNA levels were reduced, but this did not significantly alter cytosolic calcium decline rates.
- Excitation-contraction coupling efficacy was maintained at 1 mM extracellular calcium but significantly impaired at 0.5 mM extracellular calcium.
- Fractional SR calcium release was depressed in response to a given calcium current trigger and SR calcium load at reduced extracellular calcium.
Conclusions:
- LVH in rats impairs unloaded myocyte contraction without significant alterations in overall calcium transport at physiological extracellular calcium levels.
- Reduced extracellular calcium unmasks a defect in excitation-contraction coupling, specifically a depressed fractional SR calcium release, in hypertrophic myocytes.
- These findings highlight a critical role for altered calcium release mechanisms in the contractile dysfunction associated with left ventricular hypertrophy.
Abstract:
Left ventricular hypertrophy (approximately 40%) was induced in rats by banding of the abdominal aorta. After 16 wk, ventricular homogenates were prepared for biochemical measurements and ventricular myocytes were isolated for functional studies. In myocytes, the effects of banding on intracellular Ca handling, contraction, and excitation-contraction (E-C) coupling were determined using indo 1 fluorescence and whole cell voltage clamp. After steady-state field or voltage-clamp stimulation to load the sarcoplasmic reticulum (SR), SR Ca content assessed by caffeine-induced Ca transients was the same in sham and banded groups. Despite this, cell shortening amplitudes were significantly depressed in the banded group, suggesting altered contractile properties. In banded rats, the SR Ca-adenosinetriphosphatase (Ca-ATPase) mRNA level was reduced, as was homogenate thapsigargin-sensitive SR Ca-ATPase, but cytosolic free Ca concentration ([Ca]i) decline attributed to SR Ca-ATPase activity in intact cells was not slowed. Banding also reduced Na/Ca exchange mRNA level but did not affect either Na-dependent sarcolemmal 45Ca transport in homogenate or the rate of [Ca]i decline in intact cells attributed to Na/Ca exchange (during caffeine-induced contractures). Banding also did not change the rate of [Ca]i decline mediated by the combined function of the mitochondrial Ca uptake and sarcolemmal Ca-ATPase in intact cells. Ca current (ICa) density and voltage dependence were the same in sham and banded groups. Ryanodine receptor mRNA, protein content, and ryanodine affinity were also unchanged in the banded group. At 1 mM extracellular Ca concentration ([Ca]o), banding did not affect E-C coupling efficacy in intact cells under voltage clamp (i.e., same contraction for given ICa and SR Ca load). However, when [Ca]o was reduced to 0.5 mM, the efficacy of E-C coupling was greatly depressed in the banded group (even though ICa and SR Ca content were matched). In summary, unloaded myocyte contraction was depressed in these hypertrophic hearts, but Ca transport was little altered, at 1 mM [Ca]o. However, reduction of [Ca]o to 0.5 mM appears to unmask a depressed fractional SR Ca release in response to a given ICa trigger and SR Ca load.