Abstract
In cardiac muscle, excitation-contraction (E-C) coupling is determined
by the ability of the sarcoplasmic reticulum (SR) to store and release
Ca$^2+$. It has been hypothesized that the Ca$^2+$ sequestration
and release mechanisms might be functionally linked to optimize the
E-C coupling process. To explore the relationships between the loading
status of the SR and functional state of the Ca$^2+$ release
mechanism, we examined the effects of changes in SR Ca$^2+$ content
on spontaneous Ca$^2+$ sparks in saponin-permeabilized and patch-clamped
rat ventricular myocytes. SR Ca$^2+$ content was manipulated
by pharmacologically altering the capacities of either Ca$^2+$
uptake or leak. Ca$^2+$ sparks were recorded using a confocal
microscope and Fluo-3 and were quantified considering missed events.
SR Ca$^2+$ content was assessed by application of caffeine. Exposure
of permeabilized cells to anti-phospholamban antibodies elevated
the SR Ca$^2+$ content and increased the frequency of sparks.
Suppression of the SR Ca$^2+$ pump by thapsigargin lowered Ca$^2+$(SR)
and reduced the frequency of sparks. The ryanodine receptor (RyR)
blockers tetracaine and Mg$^2+$ transiently suppressed the frequency
of sparks. Upon washout of the drugs, sparking activity transiently
overshot control levels. Low doses of caffeine transiently potentiated
sparking activity upon application and transiently depressed the
sparks upon removal. In patch-clamped cardiac myocytes, exposure
to caffeine produced only a transient increase in the probability
of sparks induced by depolarization. We interpret these results in
terms of a novel dynamic control scheme for SR Ca$^2+$ cycling.
A central element of this scheme is a luminal Ca$^2+$ sensor
that links the functional activity of RyRs to the loading state of
the SR, allowing cells to auto-regulate the size and functional state
of their SR Ca$^2+$ pool. These results are important for understanding
the regulation of intracellular Ca$^2+$ release and contractility
in cardiac muscle.
- 11463625
- animals,
- antibodies,
- atpase,
- caffeine,
- calcium
- calcium,
- cell
- channel,
- electrophysiology,
- fluorescence,
- gov't,
- heart
- ion
- magnesium,
- male,
- membrane
- monoclonal,
- non-u.s.
- p.h.s.,
- patch-clamp
- permeability,
- rats,
- receptor
- release
- research
- reticulum,
- ryanodine
- sarcoplasmic
- sprague-dawley,
- support,
- techniques,
- tetracaine,
- thapsigargin,
- transport,
- u.s.
- ventricles,
- {c}a$^{2+}$-transporting
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