Abstract
The action potential of cardiac ventricular myocytes is characterized
by its long duration, mainly due to Ca flux through L-type Ca channels.
Ca entry also serves to trigger the release of Ca from the sarcoplasmic
reticulum. The aim of this study was to investigate the role of cell
membrane invaginations called transverse (t)-tubules in determining
Ca influx and action potential duration in cardiac ventricular myocytes.
We used the whole cell patch clamp technique to record electrophysiological
activity in intact rat ventricular myocytes (i.e. from the t-tubules
and surface sarcolemma) and in detubulated myocytes (i.e. from the
surface sarcolemma only). Action potentials were significantly shorter
in detubulated cells than in control cells. In contrast, resting
membrane potential and action potential amplitude was similar in
control and detubulated myocytes. Experiments under voltage clamp
using action potential waveforms were used to quantify Ca entry via
the Ca current. Ca entry following detubulation was reduced by ~60\%,
a value similar to the decrease in action potential duration. We
calculated that Ca influx at the t-tubules is 1.3 times that at the
cell surface (4.9 versus 3.8 ìmol/L cytosol respectively) during
a square voltage clamp pulse. In contrast, during an AP, Ca entry
at the t-tubules is 2.2 times that at the cell surface (3.0 versus
1.4 ìmol/L cytosol respectively). However, more Ca entry occurs
per microm2 of junctional membrane at the cell surface than in the
t-tubules (in nM/microm2: 1.43 versus 1.06 during an AP). This difference
is unlikely to be due to a difference in the number of Ca channels/junction
at each site because we estimate that the same number of Ca channels
is present at cell surface and t-tubule junctions (~35). This study
provides the first evidence that the t-tubules are a key site for
the regulation of action potential duration in ventricular cardiac
myocytes. Our data also provide the first direct measurements of
t-tubular Ca influx, which are consistent with the idea that cardiac
excitation-contraction coupling largely occurs at the t-tubule dyadic
clefts.
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