Mg2+ activates the ryanodine receptor type 2 (RyR2) at intermediate Ca2+ concentrations

Area of Science:

• Cardiovascular Physiology
• Molecular Cardiology
• Biochemistry

Background:

• Ryanodine receptor type 2 (RyR2) regulates calcium release in cardiac muscle sarcoplasmic reticulum (SR).
• Understanding RyR2 activity is key to cardiac excitation-contraction coupling.
• Previous studies on RyR1 suggest reduced activity, but RyR2's behavior is less clear.

Purpose of the Study:

• To investigate the Ca(2+)-dependent activity of RyR2 in cardiac SR.
• To determine if RyR2 activity is suppressed, similar to RyR1.
• To explore the modulatory effects of Mg(2+) on RyR2 function.

Main Methods:

• Isolated SR vesicle fractions from rabbit and rat cardiac muscles were used.
• Ca(2+)-dependent [3H]ryanodine binding assays measured Ca(2+)-induced Ca(2+) release (CICR).
• Binding assays were performed with and without adenine nucleotide, caffeine, and varying Mg(2+) concentrations.

Main Results:

• Rat SR exhibited complex Ca(2+) dependence, differing from rabbit SR's biphasic pattern.
• Mg(2+) significantly increased [3H]ryanodine binding (2-3 fold) and enhanced RyR2 affinity for Ca(2+).
• Mg(2+) revealed a biphasic Ca(2+) dependence and amplified responses to other modulators, indicating a novel stimulatory effect.

Conclusions:

• Mg(2+) relieves partial suppression of RyR2 activity, acting as a substitute for Ca(2+).
• Mg(2+) demonstrates a new stimulatory role in RyR2 function, beyond its known dual inhibitory effects.
• This Mg(2+)-mediated stimulation is critical for understanding RyR2's role in cardiac excitation-contraction coupling.