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Updated: May 6, 2026
Ex Vivo Assessment of Contractility, Fatigability and Alternans in Isolated Skeletal Muscles
Published on: November 1, 2012
Constitutively active phosphatase inhibitor-1 improves cardiac contractility in young mice but is deleterious after
Katrin Wittköpper1, Larissa Fabritz, Stefan Neef
1Institute of Experimental and Clinical Pharmacology and Toxicology, Cardiovascular Research Center Hamburg, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
Abstract:
Phosphatase inhibitor-1 (I-1) is a distal amplifier element of beta-adrenergic signaling that functions by preventing dephosphorylation of downstream targets. I-1 is downregulated in human failing hearts, while overexpression of a constitutively active mutant form (I-1c) reverses contractile dysfunction in mouse failing hearts, suggesting that I-1c may be a candidate for gene therapy. We generated mice with conditional cardiomyocyte-restricted expression of I-1c (referred to herein as dTGI-1c mice) on an I-1-deficient background. Young adult dTGI-1c mice exhibited enhanced cardiac contractility but exaggerated contractile dysfunction and ventricular dilation upon catecholamine infusion. Telemetric ECG recordings revealed typical catecholamine-induced ventricular tachycardia and sudden death. Doxycycline feeding switched off expression of cardiomyocyte-restricted I-1c and reversed all abnormalities. Hearts from dTGI-1c mice showed hyperphosphorylation of phospholamban and the ryanodine receptor, and this was associated with an increased number of catecholamine-induced Ca2+ sparks in isolated myocytes. Aged dTGI-1c mice spontaneously developed a cardiomyopathic phenotype. These data were confirmed in a second independent transgenic mouse line, expressing a full-length I-1 mutant that could not be phosphorylated and thereby inactivated by PKC-alpha (I-1S67A). In conclusion, conditional expression of I-1c or I-1S67A enhanced steady-state phosphorylation of 2 key Ca2+-regulating sarcoplasmic reticulum enzymes. This was associated with increased contractile function in young animals but also with arrhythmias and cardiomyopathy after adrenergic stress and with aging. These data should be considered in the development of novel therapies for heart failure.
Insights
Phosphatase inhibitor-1 (I-1) enhances cardiac contractility but can cause dangerous arrhythmias and cardiomyopathy under stress or with aging. Conditional I-1 expression offers potential for heart failure gene therapy but requires careful consideration of side effects.
Area of Science:
- Cardiology
- Molecular Biology
- Genetics
Background:
- Phosphatase inhibitor-1 (I-1) amplifies beta-adrenergic signaling by blocking dephosphorylation.
- I-1 downregulation is observed in human heart failure.
- A constitutively active mutant (I-1c) can reverse contractile dysfunction in mouse models.
Purpose of the Study:
- To investigate the effects of cardiomyocyte-restricted expression of I-1c in mice.
- To assess the therapeutic potential and risks of I-1c in cardiac function.
- To evaluate the impact of I-1c on cardiac contractility, stress response, and aging.
Main Methods:
- Generation of conditional transgenic mice (dTGI-1c) with cardiomyocyte-specific I-1c expression on an I-1 deficient background.
- Assessment of cardiac function via catecholamine infusion and telemetric ECG.
- Analysis of protein phosphorylation (phospholamban, ryanodine receptor) and Ca2+ sparks in isolated myocytes.
- Evaluation of aged mice for spontaneous cardiomyopathy and confirmation in a second transgenic line (I-1S67A).
Main Results:
- Young dTGI-1c mice showed enhanced contractility but developed severe dysfunction and dilation upon catecholamine challenge.
- Catecholamine infusion led to ventricular tachycardia and sudden death in dTGI-1c mice.
- Hyperphosphorylation of phospholamban and ryanodine receptor was observed, correlating with increased Ca2+ sparks.
- Aged dTGI-1c mice developed spontaneous cardiomyopathy, confirmed by a second transgenic line.
- Doxycycline withdrawal reversed all observed abnormalities.
Conclusions:
- Conditional expression of I-1c or I-1S67A enhances phosphorylation of key Ca2+-regulating enzymes.
- While beneficial for contractility in young animals, it leads to adverse effects under adrenergic stress and aging.
- These findings highlight the dual role of I-1 in cardiac function and inform the development of heart failure therapies.
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