Regulation of dynamic behavior of cardiac ryanodine receptor by Mg[sup 2+] under simulated physiological conditions.

Mg[sup 2+], an important constituent of the intracellular milieu in cardiac myocytes, is known to inhibit ryanodine receptor (RyR) Ca[sup 2+] release channels by competing with Ca[sup 2+] at the cytosolic activation sites of the channel. However, the significance of this competition for local, dynamic Ca[sup 2+]-signaling processes thought to govern cardiac excitation-contraction (EC) coupling remains largely unknown. In the present study, Ca[sup 2+] stimuli of different waveforms (i.e., sustained and brief) were generated by photolysis of the caged Ca[sup 2+] compound nitrophenyl (NP)-EGTA. The evoked RyR activity was measured in planar lipid bilayers in the presence of 0.6-1.3 mM free Mg[sup 2+] at the background of 3 mM total ATP in the presence or absence of i mM luminal Ca[sup 2+]. Mg[sup 2+] dramatically slowed the rate of activation of RyRs in response to sustained (≥10-ms) elevations in Ca[sup 2+] concentration. Paradoxically, Mg[sup 2+] had no measurable impact on the kinetics of the RyR response induced by physiologically relevant, brief (<1-ms) Ca[sup 2+] stimuli. Instead, the changes in activation rate observed with sustained stimuli were translated into a drastic reduction in the probability of responses. Luminal Ca[sup 2+] did not affect the peak open probability or the probability of responses to brief Ca[sup 2+] signals; however, it slowed the transition to steady state and increased the steady-state open probability of the channel. Our results indicate that Mg[sup 2+] is a critical physiological determinant of the dynamic behavior of the RyR channel, which is expected to profoundly influence the fidelity of coupling between L-type Ca[sup 2+] channels and RyRs in heart cells. [ABSTRACT FROM AUTHOR]