Mechanisms of hypoxia-induced decrease of load dependence of relaxation in cat papillary muscle

Area of Science:

• Cardiovascular Physiology
• Muscle Biology
• Biophysics

Background:

• Cardiac muscle relaxation is load-dependent under normal conditions.
• Hypoxia alters cardiac muscle mechanics, making relaxation less load-dependent.
• Previous hypotheses attributed this to delayed calcium removal.

Purpose of the Study:

• To investigate the mechanisms underlying hypoxia's effect on cardiac muscle relaxation.
• To compare hypoxia's impact on load dependence with agents affecting calcium handling.
• To differentiate the contributions of various factors to the observed changes in relaxation.

Main Methods:

• Studied relaxation in 26 cat papillary muscles under varying conditions.
• Compared load dependence of relaxation during hypoxia with low extracellular calcium, verapamil, and nifedipine.
• Quantified load dependence by analyzing force and time at relaxation onset.

Main Results:

• Hypoxia, low extracellular calcium, verapamil, and nifedipine all decreased load dependence of relaxation.
• Hypoxia caused a more pronounced decrease in load dependence than other agents.
• The hypoxia-induced decrease had two phases: an early, fast drop and a later, slower decrease.
• The early phase was sensitive to low extracellular calcium and calcium channel blockers, suggesting involvement of calcium influx.
• The second phase was potentially related to impaired sarcoplasmic reticulum calcium reuptake.

Conclusions:

• Hypoxia significantly reduces load dependence of cardiac muscle relaxation through a biphasic mechanism.
• The early phase of reduced load dependence is primarily linked to altered isometric relaxation dynamics, potentially involving calcium influx.
• The later phase may involve impaired calcium reuptake by the sarcoplasmic reticulum.