Cardiac myofibrillar phosphorylation and adenosine triphosphatase activity

Area of Science:

• Cardiac Physiology
• Molecular Cardiology
• Protein Phosphorylation

Background:

• Cardiac muscle contraction is regulated by complex protein interactions.
• Protein kinase and phosphatase activities are present within cardiac myofibrils.
• Cyclic adenosine monophosphate (cAMP)-dependent protein kinase plays a role in cardiac function.

Purpose of the Study:

• To investigate the role of protein phosphorylation in regulating cardiac myofibril function.
• To examine the relationship between troponin-I phosphorylation and myofibrillar Mg2+-ATPase activity.
• To determine the in vivo relevance of observed in vitro phosphorylation effects.

Main Methods:

• Dissolution and reprecipitation of rat heart myofibrils in salt solutions.
• Assay of endogenous protein kinase and phosphatase activities.
• Phosphorylation of troponin-I using radioactive ATP and protein kinases.
• Measurement of Ca2+-stimulated myofibrillar Mg2+-ATPase activity.
• Perfusion of isolated rat hearts with L-noradrenaline.

Main Results:

• Troponin-I was identified as the primary substrate for myofibrillar protein phosphorylation.
• Increased troponin-I phosphorylation showed an inverse correlation with maximum Ca2+-stimulated Mg2+-ATPase activity.
• Higher phosphorylation levels required more calcium for half-maximum ATPase activation.
• In vitro phosphorylation effects were replicated in isolated perfused rat hearts treated with L-noradrenaline.

Conclusions:

• cAMP-dependent protein kinase-mediated phosphorylation of myofibrillar proteins desensitizes the cardiac contractile response.
• This desensitization mechanism may help cardiac cells manage increased intracellular calcium during catecholamine stimulation.
• Protein phosphorylation is a key regulator of cardiac myofilament calcium sensitivity and contractility.