Abstract
1. The aim of these experiments was to compare the time course of
changes in intracellular Ca$^2+$ concentration (Ca$^2+$i)
measured in the bulk cytoplasm with those estimated to occur near
the sarcolemma. Sarcolemmal Na$^+$-Ca$^2+$ exchange current
and Ca$^2+$i were measured in single, voltage-clamped ventricular
myocytes. 2. Spontaneous Ca$^2+$ release from the sarcoplasmic
reticulum (SR) resulted in a transient inward current. This current
developed and decayed more quickly than the accompanying changes
in Ca$^2+$i (measured with indo-1) resulting in a hysteresis
between Ca$^2+$i and current. A similar hysteresis was also
observed if Ca$^2+$i was elevated with caffeine and was removed
if the current was low pass filtered with a time constant of 132
ms. 3. Digital video imaging (using fluo-3 or calcium green-1 to
measure Ca$^2+$i) allowed measurement of Ca$^2+$i at
all points in the cell during the wave of spontaneous Ca$^2+$
release. The hysteresis between Ca$^2+$i and current remained,
even after allowing for the spatial and temporal properties of this
wave. 4. The hysteresis can be accounted for if there is a barrier
to diffusion of Ca$^2+$ ions separating the bulk cytoplasm from
the space under the sarcolemma (into which Ca$^2+$ is released
from the sarcoplasmic reticulum). The calculated subsarcolemmal Ca$^2+$
rises and falls more quickly (and reaches a higher peak) than does
the bulk Ca$^2+$. The delay introduced by this barrier is equivalent
to a time constant of 133 ms. 5. The subsarcolemmal space described
in this paper may be equivalent to the 'fuzzy space' previously suggested
to be important in controlling SR Ca$^2+$ release.
- 8576849
- animals,
- biological
- calcium,
- cell
- comparative
- compartmentation,
- dyes,
- fluorescent
- gov't,
- heart
- indoles,
- myocardium,
- non-u.s.
- patch-clamp
- rats,
- research
- sarcolemma,
- study,
- support,
- techniques,
- transport,
- ventricles,
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