Zusammenfassung
Although cell shortening in patch-clamped cells (current-clamp mode)
is triggered by an ordinary action potential, the trigger mechanism
in field-stimulated cells is not so obvious. The contraction characteristics
of the two methods differ, and we, therefore, examined the triggering
sequence in field-stimulated cells. Isolated rat cardiomyocytes were
plated on laminin-coated coverslips that were mounted on an inverted
light microscope and superfused with HEPES-Tyrode buffer (pH 7.4;
37 degrees C). The cells were stimulated to contract either by a
0.5-ms current injection (CC cells) through high-resistance electrodes
or a 5-ms biphasic field-stimulation pulse (FS cells), and drugs
were added to block sarcolemmal proteins involved in excitation-contraction
coupling. Time to peak contraction (TTP) was significantly longer
in FS cells and was not affected by the polarity or the length
of the stimulus pulse. Tetrodotoxin (TTX; 20 microM) blocked cell
shortening in CC cells but not in FS cells. Ni$^2+$ (5 mM) blocked
cell shortening in FS cells, whereas KB-R7943 (KB; 5 microM)
had no effect either on cell shortening or TTP. In FS cells,
nifedipine (Nif; 100 microM) and Cd$^2+$ (300 microM) reduced
fractional
shortening by 34 and 63\%, respectively, but only Cd$^2+$ affected
TTP (reduced by 48\%). A combination of Nif and KB reduced cell
shortening by 50\%, whereas a combination of Cd$^2+$ and KB almost
abolished cell shortening. We conclude that field stimulation per
se prolongs TTP and that cell shortening in FS cells is not dependent
on Na$^+$ current but is triggered by a combination of L-type
Ca$^2+$
current and reverse mode Na$^+$/Ca$^2+$ exchange.
- 15640393
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Nutzer