Abstract
Mg2+, an important constituent of the intracellular milieu in cardiac
myocytes, is known to inhibit ryanodine receptor (RyR) Ca$^2+$
release channels by competing with Ca$^2+$ at the cytosolic activation
sites of the channel. However, the significance of this competition
for local, dynamic Ca$^2+$-signaling processes thought to govern
cardiac excitation-contraction (EC) coupling remains largely unknown.
In the present study, Ca$^2+$ stimuli of different waveforms
(i.e., sustained and brief) were generated by photolysis of the caged
Ca$^2+$ compound nitrophenyl (NP)-EGTA. The evoked RyR activity
was measured in planar lipid bilayers in the presence of 0.6-1.3
mM free Mg2+ at the background of 3 mM total ATP in the presence
or absence of 1 mM luminal Ca$^2+$. Mg2+ dramatically slowed
the rate of activation of RyRs in response to sustained (> or =10-ms)
elevations in Ca$^2+$ concentration. Paradoxically, Mg2+ had
no measurable impact on the kinetics of the RyR response induced
by physiologically relevant, brief (<1-ms) Ca$^2+$ stimuli. Instead,
the changes in activation rate observed with sustained stimuli were
translated into a drastic reduction in the probability of responses.
Luminal Ca$^2+$ did not affect the peak open probability or the
probability of responses to brief Ca$^2+$ signals; however, it
slowed the transition to steady state and increased the steady-state
open probability of the channel. Our results indicate that Mg2+ 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$^2+$ channels and RyRs in heart
cells.
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