Аннотация
Stretch-activated channels (SACs) have been found in smooth muscle
and are thought to be involved in myogenic responses. Although SACs
have been shown to be Ca$^2+$ permeable when Ca$^2+$ is the
only charge carrier, it has not been clearly demonstrated that significant
Ca$^2+$ passes through SACs in physiological solutions. By imaging
at high temporal and spatial resolution the single-channel Ca$^2+$
fluorescence transient (SCCaFT) arising from Ca$^2+$ entry through
a single SAC opening, we provide direct evidence that significant
Ca$^2+$ can indeed pass through SACs and increase the local Ca$^2+$.
Results were obtained under conditions where the only source of Ca$^2+$
was the physiological salt solution in the patch pipette containing
2 mM Ca$^2+$. Single smooth muscle cells were loaded with fluo-3
acetoxymethyl ester, and the fluorescence was recorded by using a
wide-field digital imaging microscope while SAC currents were simultaneously
recorded from cell-attached patches. Fluorescence increases at the
cell-attached patch were clearly visualized before the simultaneous
global Ca$^2+$ increase that occurred because of Ca$^2+$
influx through voltage-gated Ca$^2+$ channels when the membrane
was depolarized by inward SAC current. From measurements of total
fluorescence ("signal mass") we determined that about 18\% of the
SAC current is carried by Ca$^2+$ at membrane potentials more
negative than the resting level. This would translate into at least
a 0.35-pA unitary Ca$^2+$ current at the resting potential. Such
Ca$^2+$ currents passing through SACs are sufficient to activate
large-conductance Ca$^2+$-activated K$^+$ channels and, as
shown previously, to trigger Ca$^2+$ release from intracellular
stores.
- 11983921
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