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Updated: Apr 24, 2026
Assessment of Myofilament Ca2+ Sensitivity Underlying Cardiac Excitation-contraction Coupling
Published on: August 1, 2016
Balanced changes in Ca buffering by SERCA and troponin contribute to Ca handling during β-adrenergic stimulation in
Sarah J Briston1, Katharine M Dibb1, R John Solaro2
1Unit of Cardiac Physiology, Manchester Academic Health Science Centre, Core Technology Facility, 46 Grafton St, Manchester M13 9NT, UK.
Insights
Beta-adrenergic stimulation alters calcium buffering in cardiac cells by affecting SERCA and troponin. These buffers counterbalance each other, maintaining stable calcium levels during stimulation, which is vital for heart function.
Area of Science:
- Cardiovascular Physiology
- Cellular Biology
- Biochemistry
Background:
- Cardiac myocytes regulate intracellular calcium (Ca) for contraction, with most Ca bound to buffers like SERCA and troponin C.
- Beta-adrenergic stimulation, a key regulator of heart function, involves protein phosphorylation that can alter Ca-binding affinities.
- Understanding how these changes affect Ca buffering is crucial for comprehending cardiac response and dysfunction.
Purpose of the Study:
- To investigate the impact of beta-adrenergic stimulation on Ca buffering in cardiac myocytes.
- To distinguish the specific contributions of SERCA and troponin to Ca buffering changes during stimulation.
Main Methods:
- Utilized cardiac myocytes from wild-type, phospholamban-knockout (PLN-KO), and slow skeletal troponin I (ssTnI) expressing mice.
- Administered the beta-adrenoceptor agonist isoproterenol (ISO) to stimulate beta-adrenergic signaling.
- Employed thapsigargin to selectively block SERCA activity for further analysis.
Main Results:
- Isoproterenol (ISO) did not alter Ca buffering in wild-type cells, suggesting counterbalancing effects.
- ISO decreased Ca buffering in PLN-KO myocytes, highlighting troponin's role.
- ISO increased Ca buffering in ssTnI cells, indicating SERCA's response to altered phosphorylation.
Conclusions:
- SERCA and troponin play distinct, counterbalancing roles in modulating Ca buffering during beta-adrenergic stimulation.
- The net effect maintains stable Ca buffering, which is physiologically significant for cardiac function.
- Altered Ca buffering, especially in disease states affecting SERCA or myofilament Ca binding, warrants careful consideration.
Aims:
During activation of cardiac myocytes, less than 1% of cytosolic Ca is free; the rest is bound to buffers, largely SERCA, and troponin C. Signalling by phosphorylation, as occurs during β-adrenergic stimulation, changes the Ca-binding affinity of these proteins and may affect the systolic Ca transient. Our aim was to determine the effects of β-adrenergic stimulation on Ca buffering and to differentiate between the roles of SERCA and troponin.
Methods And Results:
Ca buffering was studied in cardiac myocytes from mice: wild-type (WT), phospholamban-knockout (PLN-KO), and mice expressing slow skeletal troponin I (ssTnI) that is not protein kinase A phosphorylatable. WT cells showed no change in Ca buffering in response to the β-adrenoceptor agonist isoproterenol (ISO). However, ISO decreased Ca buffering in PLN-KO myocytes, presumably unmasking the role of troponin. This effect was confirmed in WT cells in which SERCA activity was blocked with the application of thapsigargin. In contrast, ISO increased Ca buffering in ssTnI cells, presumably revealing the effect of an increase in Ca binding to SERCA.
Conclusions:
These data indicate the individual roles played by SERCA and troponin in Ca buffering during β-adrenergic stimulation and that these two buffers effectively counterbalance each other so that Ca buffering remains constant during β-adrenergic stimulation, a factor which may be physiologically important. This study also emphasizes the importance of taking into account Ca buffering, particularly in disease states where Ca binding to myofilaments or SERCA may be altered.
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