SR Ca loading in cardiac muscle preparations based on rapid-cooling contractures

Area of Science:

• Cardiovascular Physiology
• Cardiac Excitation-Contraction Coupling
• Cellular Calcium Handling

Background:

• Cardiac muscle contractility is regulated by intracellular calcium (Ca) dynamics, primarily involving the sarcoplasmic reticulum (SR).
• The influence of rest periods on cardiac contractility and SR Ca content remains incompletely understood across different species.

Purpose of the Study:

• To investigate the impact of varying rest durations on cardiac twitch force and rapid-cooling contractures (RCCs) in different mammalian and amphibian cardiac tissues.
• To utilize RCCs as an index of sarcoplasmic reticulum (SR) Ca content to understand species-specific differences in Ca metabolism.

Main Methods:

• Isolated trabeculae from rabbit, rat, guinea pig ventricle, and rabbit atrium were subjected to varying rest periods.
• Twitch force and rapid-cooling contractures (RCCs) were measured to assess contractility and relative SR Ca content.
• Experiments also explored the effects of stimulation resumption, extracellular Ca concentration ([Ca]o), and rewarming during RCCs.

Main Results:

• Rabbit and guinea pig ventricles displayed 'rest decay' (decreased twitch and RCC force after rest), suggesting Ca loss from SR during quiescence.
• Rat ventricle and rabbit atrium showed 'rest potentiation' (increased twitch and RCC force after rest), indicating Ca replenishment during quiescence.
• Differences in twitch recovery kinetics and rewarming spike characteristics suggest variations in SR properties, sarcolemmal transport, and myofilament Ca sensitivity across species.

Conclusions:

• Significant species-dependent differences exist in cardiac Ca metabolism, particularly concerning SR Ca handling during rest and stimulation.
• These variations in rabbit, rat, guinea pig, and frog cardiac tissues may stem from fundamental differences in SR or sarcolemmal transport mechanisms.
• Understanding these differences is crucial for comprehending cardiac function and dysfunction in various species.