Force relaxation and thin filament protein phosphorylation during acute myocardial ischemia

Area of Science:

• Cardiovascular Physiology
• Cardiac Muscle Mechanics
• Molecular Cardiology

Background:

• Ischemia impairs heart function and can advance heart failure.
• Acute ischemia in rats reduced Ca(2+)-activated force and phosphorylation of myosin-binding protein-C, titin, and troponin I (TnI).
• These proteins are targets of beta-adrenergic receptor (β-AR) activation, suggesting reduced β-AR signaling contributes to altered phosphorylation during ischemia.

Purpose of the Study:

• To test the hypothesis that decreased β-AR signaling during ischemia leads to reduced protein phosphorylation and slower force relaxation.
• To investigate the relationship between sarcomeric protein phosphorylation and the rate constants of force relaxation in cardiac muscle fibers.

Main Methods:

• Recorded force relaxation transients from permeabilized perfused and ischemic rat heart fibers after photolysis of the caged chelator diazo-2.
• Analyzed relaxation transients using double exponential functions to determine fast and slow rate constants.
• Assessed the impact of cAMP analog treatment on TnI phosphorylation and relaxation kinetics in both perfused and ischemic fibers.

Main Results:

• Rate constants of relaxation were not significantly different between perfused and ischemic fibers, despite reduced sarcomeric protein phosphorylation in ischemic fibers.
• Treatment with a cAMP analog increased TnI phosphorylation in perfused fibers but did not alter relaxation rates.
• Similar cAMP analog treatment in ischemic fibers did not affect TnI phosphorylation or force relaxation transients.

Conclusions:

• Acute ischemia does not influence the rate constants of relaxation in permeabilized cardiac muscle fibers.
• The physiological level of sarcomeric protein phosphorylation is unlikely to be the primary driver of relaxation kinetics in permeabilized cardiac muscle.
• These findings challenge the direct link between β-AR signaling-mediated phosphorylation and cardiac muscle relaxation speed under these experimental conditions.