Ca2+-induced Ca2+ release from fragmented sarcoplasmic reticulum: Ca2+-dependent passive Ca2+ efflux.

Area of Science:

• Muscle Physiology
• Calcium Signaling
• Molecular Biology

Background:

• The sarcoplasmic reticulum (SR) plays a crucial role in muscle contraction by regulating intracellular calcium levels.
• Calcium-induced calcium release (CICR) is a critical mechanism for muscle excitation-contraction coupling.
• The precise mechanism of CICR mediated by the SR Ca2+-gated Ca2+ channel remains incompletely understood.

Purpose of the Study:

• To elucidate the mechanism of Ca2+-induced Ca2+ release (CICR).
• To characterize the putative Ca2+-gated Ca2+ channel in the sarcoplasmic reticulum.
• To determine the quantitative relationship between Ca2+ efflux and extravesicular Ca2+ concentration.

Main Methods:

• Isolation of heavy and light fractions of fragmented sarcoplasmic reticulum (FSR) from rabbit skeletal muscle.
• Passive Ca2+ loading of FSR vesicles.
• Measurement of passive Ca2+ efflux under varying conditions, including different Ca2+ concentrations and the presence of caffeine, Mg2+, and procaine.

Main Results:

• A fast phase of Ca2+ efflux was identified and attributed to the Ca2+-gated Ca2+ channel, dependent on extravesicular Ca2+ concentration.
• Ca2+ binding within FSR was observed, exceeding estimates based on vesicle volume and equilibrium.
• The Ca2+ efflux rate constant was determined, showing a maximum of 0.75 s-1 at 3 microM extravesicular free Ca2+.
• Caffeine enhanced channel affinity for Ca2+, Mg2+ inhibited Ca2+ binding, and procaine appeared to block the channel's ionophore function.

Conclusions:

• The study provides a quantitative characterization of the SR Ca2+-gated Ca2+ channel's function in Ca2+ efflux.
• Ca2+ binding within the SR influences the release mechanism.
• Modulators like caffeine, Mg2+, and procaine affect channel activity through distinct mechanisms, offering insights into excitation-contraction coupling regulation.