Аннотация
Ca$^2+$-induced Ca$^2+$ release is a general mechanism that
most cells use to amplify Ca$^2+$ signals. In heart cells, this
mechanism is operated between voltage-gated L-type Ca$^2+$ channels
(LCCs) in the plasma membrane and Ca$^2+$ release channels, commonly
known as ryanodine receptors, in the sarcoplasmic reticulum. The
Ca$^2+$ influx through LCCs traverses a cleft of roughly 12 nm
formed by the cell surface and the sarcoplasmic reticulum membrane,
and activates adjacent ryanodine receptors to release Ca$^2+$
in the form of Ca$^2+$ sparks. Here we determine the kinetics,
fidelity and stoichiometry of coupling between LCCs and ryanodine
receptors. We show that the local Ca$^2+$ signal produced by
a single opening of an LCC, named a 'Ca$^2+$ sparklet', can trigger
about 4-6 ryanodine receptors to generate a Ca$^2+$ spark. The
coupling between LCCs and ryanodine receptors is stochastic, as judged
by the exponential distribution of the coupling latency. The fraction
of sparklets that successfully triggers a spark is less than unity
and declines in a use-dependent manner. This optical analysis of
single-channel communication affords a powerful means for elucidating
Ca$^2+$-signalling mechanisms at the molecular level.
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