Intracellular calcium binding and release in frog heart.

Area of Science:

• Cardiology
• Cell Biology
• Muscle Physiology

Background:

• Intracellular calcium ions (Ca++) are critical regulators of cardiac muscle contraction.
• Understanding the binding sites and capacities of intracellular calcium is essential for elucidating cardiac function.

Purpose of the Study:

• To investigate the capacities and affinities of intracellular calcium-binding sites in frog ventricles.
• To determine how sarcoplasmic calcium concentration affects muscle tension and calcium movement.

Main Methods:

• Controlled manipulation of sarcoplasmic Ca++ concentration using EDTA treatment in frog ventricles.
• Measurement of total muscle calcium content at rest and during tension generation.
• Observation of net calcium movement into or out of cells at different sarcoplasmic Ca++ levels.
• Analysis of data to identify characteristics of intracellular calcium sinks.

Main Results:

• Two distinct intracellular calcium sinks were identified, competing with contractile proteins.
• The first sink has a capacity of approximately 0.6 micromol/g and an affinity constant > 10(7) M(-1).
• The second sink has a capacity of 4.0 micromol/g and an affinity constant of about 2 x 10(6) M(-1).
• Net calcium movement occurred without tension changes at sarcoplasmic Ca++ levels < 10(-7) M or ~5 x 10(-7) M.
• Higher affinity calcium is released by anoxia, oligomycin, or abrupt Ca++ changes.
• Electron densities were observed in the sarcoplasmic reticulum and mitochondria of Sr-Ringer's treated muscles.

Conclusions:

• Frog ventricular muscle possesses at least two groups of intracellular calcium sinks with differing capacities and affinities.
• These calcium sinks play a significant role in modulating sarcoplasmic calcium levels and thus, cardiac contractility.
• The release of higher affinity calcium under specific conditions suggests dynamic intracellular calcium regulation mechanisms.