Changes in luminal pH caused by calcium release in sarcoplasmic reticulum vesicles

Area of Science:

• Muscle Physiology
• Cellular Biophysics
• Ion Transport

Background:

• Muscle contraction relies on rapid calcium (Ca2+) release from the sarcoplasmic reticulum.
• This Ca2+ release creates a charge imbalance that requires counter-ion fluxes for compensation.
• The specific role of protons (H+) as counter-ions during this process remains incompletely understood.

Purpose of the Study:

• To investigate the potential involvement of proton fluxes in charge compensation during Ca2+ release from the sarcoplasmic reticulum.
• To quantify the contribution of proton movements to the electrical events accompanying Ca2+ release in muscle cells.

Main Methods:

• Utilized triad-enriched sarcoplasmic reticulum vesicles from rabbit fast skeletal muscle.
• Measured intra-vesicular pH changes using a trapped fluorescent pH indicator (pyranin) in a stopped-flow fluorimeter.
• Inducted Ca2+ release using controlled pCa and pH conditions, with and without ATP analogs and Mg2+.

Main Results:

• Observed significant acidification of the sarcoplasmic reticulum lumen (approx. 0.2 pH units) concurrent with Ca2+ release (t(1/2) = 0.75 s).
• ATP and AMPPNP accelerated Ca2+ release and lumen acidification rates (>20-fold).
• Mg2+-induced blockade of Ca2+ release abolished lumen acidification, confirming the link between Ca2+ and H+ fluxes.

Conclusions:

• Proton fluxes partially compensate for the charge deficit generated by Ca2+ release from the sarcoplasmic reticulum (5-10%).
• These findings highlight the physiological relevance of proton movements in regulating muscle cell electrical activity during contraction.
• The study provides direct evidence for proton participation in the complex ion dynamics of muscle excitation-contraction coupling.