Modulation of cardiac Ca2+
channels by isoproterenol studied in transgenic mice with altered
SR Ca2+ content
Hidenori
Sako1,
Stuart A.
Green2,
Evangelia G.
Kranias1, and
Atsuko
Yatani1
Departments of 1 Pharmacology
and Cell Biophysics and
2 Medicine, University of
Cincinnati College of Medicine, Cincinnati, Ohio 45267
 |
ABSTRACT |
Phospholamban
(PLB) ablation is associated with enhanced sarcoplasmic reticulum (SR)
Ca2+ uptake and attenuation of the
cardiac contractile responses to
-adrenergic agonists. In the
present study, we compared the effects of isoproterenol (Iso) on the
Ca2+ currents
(ICa) of
ventricular myocytes isolated from wild-type (WT) and PLB knockout
(PLB-KO) mice. Current density and voltage dependence of
ICa were similar
between WT and PLB-KO cells. However, ICa recorded from
PLB-KO myocytes had significantly faster decay kinetics. Iso increased
ICa amplitude in
both groups in a dose-dependent manner (50% effective concentration,
57.1 nM). Iso did not alter the rate of
ICa inactivation
in WT cells but significantly prolonged the rate of inactivation in
PLB-KO cells. When Ba2+ was used
as the charge carrier, Iso slowed the decay of the current in both WT
and PLB-KO cells. Depletion of SR
Ca2+ by ryanodine also slowed the
rate of inactivation of
ICa, and subsequent application of Iso further reduced the inactivation rate of
both groups. These results suggest that enhanced
Ca2+ release from the SR offsets
the slowing effects of
-adrenergic receptor stimulation on the rate
of inactivation of
ICa.
-adrenergic agonist; phospholamban; patch clamp; cardiac
myocytes; mouse heart; sarcoplasmic reticulum
 |
INTRODUCTION |
IN CARDIAC MUSCLE,
Ca2+ influx through the
voltage-dependent L-type Ca2+
channel (CaCh) is closely related to the initiation, maintenance, and
modulation of contractility by catecholamines. Sympathetic stimulation
results in enhancement of both contraction and heart rate (8, 15), and
modulation is clearly dependent on activation of
-adrenergic
receptors (
-ARs). It is well known that
-AR-mediated regulation
occurs through the adenosine 3',5'-cyclic
monophosphate-dependent protein kinase A (PKA) pathway. PKA activation
results in the phosphorylation of several intracellular proteins,
including CaCh (9, 15) and the regulatory protein of sarcoplasmic
reticulum (SR)
Ca2+-adenosinetriphosphatase
(ATPase), phospholamban (PLB) in the SR membrane (11). Under basal
conditions, dephosphorylated PLB is an inhibitor of SR
Ca2+-ATPase. Phosphorylation of PB
by PKA relieves this inhibition by increasing the affinity of the SR
Ca2+-ATPase for
Ca2+. These changes in SR
Ca2+-ATPase during
-AR
stimulation correlate with the increased rate of rise and fall of
contraction (7, 18). Indeed, recent studies have demonstrated that the
PLB knockout (PLB-KO) mouse heart showed significantly increased basal
contractile properties similar to those occurring with
-AR
stimulation (13). Moreover, ventricular myocytes isolated from the
PLB-KO mouse heart exhibited a much smaller
Ca2+ transient response to the
-AR agonist isoproterenol (Iso) compared with wild-type (WT)
controls (23).
The effects of
-AR agonists on the intrinsic properties of CaCh
currents have been extensively studied (15). Whole cell CaCh currents
are markedly increased by Iso, and studies performed over the course of
20 years have strongly implicated that the
-AR-mediated regulation
of the channel occurs via PKA-mediated phosphorylation. Nevertheless,
despite the extensive study of this pathway, the role of SR
Ca2+ release in modulating CaCh
activity during
-AR stimulation in intact myocytes still remains
unclear, in large degree because of the difficulty in separating the
effects of SR Ca2+ release from
those that are the result of direct agonist-promoted phosphorylation of
CaCh. Thus, in agreement with single-channel measurements (16, 21), it
has been reported that stimulation of
-AR markedly slows down the
inactivation kinetics of whole cell
Ba2+ currents
(IBa) during
depolarization (3, 20). On the other hand, when the physiologically
permeant ion Ca2+ was used as the
charge carrier,
-AR agonists did not slow the decay of
Ca2+ current
(ICa) during
depolarization (15).
Regarding the latter, inactivation of cardiac CaCh has been postulated
to be regulated by two mechanisms: one depends on membrane potential
(voltage-dependent inactivation), and the other depends on
Ca2+ entry
(Ca2+-dependent inactivation).
When Ba2+ is used as the permeant
ion, the role of Ca2+-dependent
inactivation is greatly minimized (12). It is therefore possible that
the lack of agreement regarding the effects of Iso on the inactivation
kinetics between
ICa and
IBa could be
related to the Ca2+-dependent
inactivation process of the channel. In support of this hypothesis, we
have recently demonstrated that CaCh inactivation of mouse ventricular
myocytes is controlled by membrane potential as well as by
Ca2+-dependent inactivation that
is regulated locally by the Ca2+
released from the SR in response to
Ca2+ entry through the CaCh (14).
Consistent with this idea, myocytes from PLB-KO mouse hearts with an
enhanced SR Ca2+ uptake and
content (6, 13) exhibited a significant increase in the contribution of
the Ca2+-dependent inactivation
component without a change in current density or voltage dependence of
the channel compared with WT myocytes (14).
In the present study, we sought to delineate the relative contribution
of
-AR agonist-promoted effects on CaCh modulation and SR
Ca2+ release to overall CaCh
activation and inactivation. The PLB-KO mouse model is ideal to isolate
the effects of SR Ca2+ release on
inactivation kinetics of
ICa during
-adrenergic stimulation because PLB ablation is associated with
significant attenuation of the normal increase in
Ca2+ release from the SR in
response to Iso (13, 23). Our data show that although the sensitivity
of ICa to Iso was
similar between WT and PLB-KO ventricular myocytes, there was a
profound difference in the effects on the inactivation kinetics of
ICa. Iso produced a marked prolongation of the decay of
ICa in PLB-KO but
not in WT. In contrast, Iso slowed the inactivation kinetics of both WT
and PLB-KO cells when Ba2+ was the
charge carrier. Furthermore, when SR
Ca2+ was depleted by ryanodine,
Iso prolonged the decay of
ICa in both WT
and PLB-KO cells. These results indicate that, under physiological conditions, increases in both Ca2+
entry and SR Ca2+ release induced
by
-AR stimulation can work synergistically and offset the direct
slowing effects of
-agonists on
ICa decay.
 |
METHODS |
Generation of PLB-deficient mice.
The PLB locus was disrupted in embryonic stem cells, and PLB-deficient
mice were generated as described previously (13). Adult mice
~2-4 mo of age were used in the present study.
Preparation of myocytes.
Single ventricular myocytes were isolated from the hearts of WT and
PLB-KO mice with use of a modified version of a method described
previously (14). Briefly, the heart was perfused with Ca2+-free Tyrode solution
containing collagenase type I (Worthington; 0.5 mg/ml) and bovine serum
albumin (1 mg/ml) for 30-40 min by the Langendorff method at
37°C. At the end of the perfusion period, the heart was removed,
passed through 200-µm nylon mesh, and centrifuged for 3 min at 100 g. The cells were stored in
low-Cl
,
high-K+ medium at room temperature
(20-21°C). All experiments were performed at room temperature.
Electrophysiology.
We recorded whole cell currents using previously described patch-clamp
techniques (14, 25). To measure CaCh currents, depolarizing pulses were
applied every 10 s from a holding potential of
50 mV unless
otherwise stated. Under these conditions, there was no evidence of
low-threshold T-type CaCh currents. We measured the membrane
capacitance of the cells using voltage ramps of 0.8 V/s from a holding
potential of
50 mV. PLB ablation did not alter cell capacitances
[113 ± 7 pF (n = 17) for WT
and 129 ± 11 pF (n = 13) for
PLB-KO cells]. The patch pipettes had a resistance of 2 M
or
less. The experimental chamber (0.2 ml) was placed on a microscope
stage, and external solution changes were made with the use of a
modified Y tube technique (24). The external solution contained (in
mM): 2 CaCl2 or 2 BaCl2, 1 MgCl2, 135 tetraethyl ammonium
chloride, 5 4-aminopyridine, 10 glucose, and 10 N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES), pH 7.3. The pipette solution was (in mM): 100 cesium aspartate, 20 CsCl, 1 MgCl2, 2 MgATP, 0.5 GTP, 5 ethylene glycol-bis(
-aminoethyl ether)-N,N,N',N'-tetraacetic
acid (EGTA), and 5 HEPES, pH 7.3. In some experiments, EGTA was
replaced with 10 mM
1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). These solutions provided isolation of CaCh currents from
other membrane currents; Na+ and
K+ channel currents were
completely eliminated. The omission of Na+ from external and internal
solutions also excludes Ca2+ flux
through the
Na+/Ca2+
exchanger (22).
In the initial series of experiments, we also dialyzed the myocytes
with lower concentrations of Ca2+
chelating buffer using varying amounts of EGTA (0, 0.1, and 2 mM) to
minimize interference with Ca2+
signaling from the SR. In such myocytes,
ICa decay was
indeed faster compared with myocytes dialyzed with 5 mM EGTA. However, under these conditions,
ICa exhibited
accelerated "run down" and in many cases, activation of cell
contraction occurred in the initial periods of cell dialysis, thus
rendering stable recordings difficult. We therefore performed
experiments in the presence of 5 mM EGTA, because we have previously
shown that Ca2+-dependent
inactivation properties can be reliably measured under these
experimental conditions (14).
Mean values ± SE are given in the text. Comparisons between
conditions were evaluated with the use of Student's
t-test, with significance imparted at
the P < 0.05 level.
 |
RESULTS |
Effects of Iso on CaCh currents.
Potentiation of
ICa in mouse
myocytes was examined with various concentrations of Iso. Figure
1 shows typical examples of the effects of
Iso (100 nM) on
ICa obtained from
WT (A) and PLB-KO (B) myocytes. The traces show
ICa activated at
different membrane potentials in the absence (Fig.
1A and
B,
a) and presence (Fig. 1A and
B,
b) of Iso. Peak
ICa amplitude,
normalized relative to cell capacitance (pA/pF) as a function of
voltage (I-V relationships) before and
after exposure to Iso, was also plotted (Fig.
1A and B,
c). The mean current density and the
I-V relationships were similar between
the two groups. Perfusion of Iso increased the current amplitude at all
test potentials measured and also shifted the mean
I-V relationships toward more negative
potentials. Application of Iso (100 nM) resulted in an increase in the
amplitude of ICa elicited at +10 mV by 77 ± 12% (n = 17) in WT and 80 ± 12% (n = 13)
in PLB-KO cells. The shift in the I-V
relationship was
8.8 ± 1.6 and
5.7 ± 1.1 mV for
WT and PLB-KO cells, respectively.

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Fig. 1.
Effects of isoproterenol (Iso, 100 nM) on whole cell
Ca2+ current
(ICa) recorded
in wild-type (WT, A) and
phospholamban knockout (PLB-KO, B)
cells. Traces show currents recorded from a holding potential of
50 mV to indicated test potentials in absence
(a) and presence
(b) of Iso. Times to one-half decay
(T1/2, at +10 mV)
in control and presence of Iso were 24.3 and 23.5 ms in WT cells and
10.1 and 16.5 ms in PLB-KO cells. Voltage dependence of peak
ICa in absence
(open circles) and presence (filled circles) of Iso are plotted in
c.
ICa was measured
and normalized to cell capacitance to give current densities (pA/pF).
Data points are means ± SE of WT
(n = 17) and PLB-KO
(n = 13) cells.
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|
Inactivation of
ICa was faster
and more pronounced in PLB-KO cells (Fig.
1B,
a) than in WT cells (Fig.
1A,
a). In most experiments (such as
that shown in Fig. 1A,
b), the inactivation kinetics of
ICa in WT
myocytes were not significantly altered by Iso exposure. Table
1 lists inactivation kinetic data at which
ICa reached maximum value (+10 mM) for WT and PLB-KO cells. In WT cells, the time
to one-half decay
(T1/2) was 19.7 ± 1.5 ms in the control and 19.2 ± 1.8 ms in the presence of
Iso (n = 16). In contrast, Iso markedly prolonged the decay of
ICa in PLB-KO
cells (Fig. 1B,
b), and the
T1/2 increased
from 9.2 ± 0.5 to 16.3 ± 2.1 ms
(n = 15).
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Table 1.
Kinetics of ICa inactivation in WT and PLB-KO mouse
myocytes before and after application of 100 nM isoproterenol
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Analysis of cumulative concentration-response effects of Iso on peak
ICa revealed that
both WT and PLB-KO cells showed a similar 50% effective concentration
of 57.1 nM (Fig.
2B). For
both cells, an increase in
ICa was
detectable with 10 nM Iso and reached a maximum at 1 µM (Fig.
2A). The maximum increases in
ICa amplitude were also similar: 2.1 ± 0.3-fold
(n = 12) and 2.2 ± 0.2-fold (n = 14) for WT and PLB-KO cells,
respectively.

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Fig. 2.
Concentration-dependent effects of Iso on
ICa in WT and
PLB-KO cells. A: currents recorded
from PLB-KO cell before (a) and after superfusion with 10 nM (b),
100 nM (c), and 1 µM
(d) Iso are shown. Holding potential
was 50 mV, and test pulse was +10 mV.
B: concentration-response curve of Iso
for ICa in WT and
PLB-KO cells. In each cell, relative increase of current amplitude (+10 mV) at different Iso concentrations was obtained by normalizing to
value produced by drug (1 µM). Solid line is a one-to-one binding model with a 50% effective concentration of 57.1 nM. Data are means ± SE of 6-14 cells. cont, Control.
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Effects of Iso on CaCh current decay.
To further examine the effects of Iso on CaCh currents, we analyzed the
effects of Iso on the inactivation kinetics, using Ca2+ or
Ba2+ as the charge carrier.
Decay of ICa in
both WT and PLB-KO myocytes was fitted by the sum of the two (fast and
slow) exponentials. Consistent with our previous findings (14), the
fast time constant of inactivation (
f) was significantly smaller
in PLB-KO cells than in WT cells, whereas the slow time constant of
inactivation (
s) was
comparable between the two groups (Fig. 3
and Table 1). The relative magnitude of fast inactivation
f
[
), where A is amplitude] was also considerably larger
in PLB-KO cells compared with WT cells (Table 1).

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Fig. 3.
Influence of Iso (100 nM) on rate of inactivation of
ICa during
depolarizing voltage pulse in WT (A)
and PLB-KO (B) cells. Traces show
currents recorded before (a) and
after (b) superfusion with Iso
during depolarizing steps to +10 mV from holding potential of 50
mV. Current traces in control and after Iso are scaled and superimposed
to compare inactivation time course in
c. Currents were fitted to sum of 2 exponentials (solid lines in c). WT
cell (A,
c): control fast time constant of
inactivation ( f) = 13.2 ms,
slow time constant of inactivation
( s) = 67.3 ms, and relative proportion of f = 44.9%; Iso
f = 11.1 ms,
s = 56.8 ms, and relative
proportion of f = 39.2%.
PLB-KO cell (B,
c): control f = 8.0 ms,
s = 80.4 ms, and relative
proportion of f = 73.4%; Iso
f = 8.7 ms,
s = 67.3 ms, and relative
proportion of f = 52.1%.
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In WT cells, although the values did not reach statistical
significance, Iso showed a general trend of speeding up the
inactivation time course (Fig. 3A).
In contrast, in PLB-KO cells, Iso markedly slowed the inactivation
kinetics of ICa
(Fig. 3B). The results summarized in
Table 1 suggest that Iso produced a significant decrease in the
relative proportion of
f from
72.9 to 51.8% without affecting the value of
f or
s. Note that the profound
differences in inactivation kinetics of
ICa between WT
and PLB-KO cells were abolished during Iso stimulation.
In Ba2+ solution, the current
decay was well fitted by a single exponential in both WT and PLB-KO
cells (Fig. 4). No difference was observed
in the time course of inactivation of
IBa between WT
and PLB-KO cells. Addition of Iso resulted in a significant increase in
the peak current amplitude and slowed the decay of IBa inactivation
to a similar extent in both WT and PLB-KO cells (Fig. 4). Iso slowed
the time constant from 90.3 ± 3.9 to 108.3 ± 8.4 ms
(n = 5, P < 0.05) in WT cells and from 98.7 ± 14.0 to 115.6 ± 18.4 ms in PLB-KO cells
(n = 5, P < 0.05). These results suggest
that Iso slows CaCh current decay in the absence of a Ca2+-dependent inactivation
component in mouse ventricular myocytes.

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Fig. 4.
Influence of Iso (100 nM) on rate of inactivation of currents recorded
in presence of Ba2+ in external
solution during depolarizing voltage pulse in WT (A) and PLB-KO
(B) cells. Traces show currents
recorded before (a) and after
(b) superfusion with Iso during
depolarizing steps to 0 mV from holding potential of 50 mV.
Current traces in control and after Iso are scaled and superimposed to
compare inactivation time course in c.
Currents were fitted to single exponential (solid lines in
c). WT cell
(A,
c), = 97.9 ms (control) and = 113.1 ms (Iso). PLB-KO cell (B,
c), = 98.2 ms (control) and = 107.1 ms (Iso).
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Effects of Iso on ICa
decay after depletion of SR
Ca2+ by
ryanodine or in the presence of BAPTA.
To examine whether the difference in effects of Iso on the rate of
inactivation of
ICa between WT
and PLB-KO cells was a result of the contribution of the
Ca2+ released from the SR, the SR
Ca2+ content was depleted by
ryanodine and the effects of Iso were tested. As shown in Fig.
5, after the application of ryanodine (10 µM), the rate of
ICa inactivation
was significantly reduced in both WT and PLB-KO cells. The
T1/2 of
inactivation in the presence of ryanodine was not significantly
different in WT cells and PLB-KO cells [35.7 ± 1.9 ms
(n = 5) and 30.0 ± 1.6 ms (n = 5), respectively].
Subsequent addition of Iso (100 nM) enhanced ICa and produced
additional slowing of inactivation in both cell types, with the
inactivation T1/2
significantly increased to 51.9 ± 4.9 ms
(n = 5) in WT and 49.0 ± 5.3 ms
(n = 5) in PLB-KO cells. These results
support the view that the differences between WT and PLB-KO myocytes in
the effects of Iso on
ICa inactivation are dependent on the SR Ca2+
content.

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Fig. 5.
Effects of Iso (100 nM) on inactivation of
ICa after SR
Ca2+ depletion by ryanodine (10 µM) recorded from WT (A) or PLB-KO
(B) cells during depolarizing steps
to +10 mV from holding potential of 50 mV. Current after
application of ryanodine and subsequent addition of Iso (ryanodine + Iso) were scaled to same peak current amplitude recorded before
(control) to compare wave form.
T1/2 for control,
ryanodine, and ryanodine + Iso were 18.1, 35.1, and 48.1 ms in WT cell
and 9.3, 28.2, and 52.7 ms in PLB-KO cells, respectively.
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In addition, to explore the effects of intracellular
Ca2+ buffering on the inactivation
kinetics of ICa,
myocytes were dialyzed with the faster
Ca2+ chelator BAPTA (10 mM)
through patch pipettes. Inactivation rates in the presence of BAPTA
were significantly slower compared with cells dialyzed with 5 mM EGTA
(Table 1). The
T1/2 was 44.2 ± 2.4 ms (n = 10) and 44.4 ± 3.4 ms (n = 6) in WT and PLB-KO
myocytes, respectively. Subsequent addition of Iso (100 nM) enhanced
ICa and produced
additional slowing of inactivation in both groups, with an increase in
T1/2 to 59.5 ± 6.4 ms (n = 10) in WT and 62.8 ± 6.7 ms (n = 6) in
PLB-KO cells. When taken together with the EGTA data, it is clear that
Iso indeed slows down the inactivation kinetics of
ICa as the buffer
capacity and the speed of Ca2+
buffering is increased in the experimental system. Furthermore, the
difference in inactivation rate between WT and PLB-KO cells is reduced
under these conditions. However, it should be noted that with a large
concentration of intracellular
Ca2+ chelators, the ability of the
SR to reaccumulate and release Ca2+ may be compromised (1, 19).
 |
DISCUSSION |
The modulation of CaCh activity by
-AR agonists has been extensively
studied in many cardiac cell types, but little is known about the
effects of
-AR stimulation on this channel in adult mouse
ventricular myocytes despite the potential importance of the transgenic
mouse approaches in the functional regulation of
-AR (4, 17).
Earlier studies (13) showed that ventricular myocytes isolated from the
PLB-KO mouse exhibited a markedly enhanced basal contractility compared
with WT cells. Furthermore, the response of transient
Ca2+ to Iso stimulation was
significantly reduced in PLB-KO cells (23). In cardiac myocytes, the
positive inotropic action of
-AR is caused by increased
Ca2+ influx through the CaCh and
by enhanced SR Ca2+ uptake (5).
However, it has remained unclear whether the difference in the response
to Iso between WT and PLB-KO cells was caused at the SR level or if
other mechanisms, perhaps acting at the level of the CaCh, were
involved.
In the present study, we found that potentiation of peak amplitude of
ICa by Iso in
PLB-KO cells was quantitatively similar to WT cells. However, under
control conditions, PLB-KO cells showed a faster
ICa inactivation
rate compared with WT cells and Iso prolonged the rate of inactivation.
In contrast, Iso did not significantly alter the rate of inactivation
in WT cells. When Ca2+ was
replaced by Ba2+ as the charge
carrier, or, in the presence of ryanodine (which depletes SR
Ca2+), Iso significantly slowed
the decay of the currents in both WT and PLB-KO cells. These results
suggest that Ca2+ released from
the SR is indeed responsible for the differential effects of Iso on the
inactivation kinetics of
ICa between WT and PLB-KO cells.
Localized Ca2+ signaling between
CaCh and SR Ca2+ has been proposed
in cardiac myocytes (18). We have recently shown that, in mouse
myocytes, inactivation kinetics of
ICa exhibit two
(fast and slow) components, similar to other mammalian species.
Inactivation of
ICa was
controlled by the amount of Ca2+
released from the SR, because the decay of
ICa was
significantly slowed in the presence of ryanodine or when
Ca2+ was replaced by
Ba2+ (14). Consistent with this,
PLB-KO cells with increased SR Ca2+ content had a significantly
faster inactivation rate compared with WT cells (14). These results
supported the hypothesis that the SR
Ca2+ load is the major source of
the regulator Ca2+ and that
elevation of Ca2+ near the CaCh
strongly modulates the inactivation kinetics of CaCh (1, 18, 19).
The involvement of Ca2+-dependent
inactivation during
-AR stimulation has long been proposed (15). For
example, Iso increases whole cell current amplitude and markedly slows
the decay of the current when Ba2+
is used as the charge carrier. This is in agreement with the single-channel measurements, in which Iso increases average
Ba2+-carried current amplitude and
slows down the time course of decay. This slowing has been suggested to
reflect a change in the slow-gating kinetics (an increase in the
proportion of nonblank sweep) caused by Iso (16). However, Iso does not
slow the time course of
ICa; if anything,
it rather accelerates inactivation (15). This lack of agreement on the
changes in inactivation kinetics has been suggested to result in part
from the offsetting effects of
Ca2+-dependent inactivation and
Iso-promoted increases in
ICa amplitude.
In this respect, we also found that, in WT mouse ventricular myocytes,
Iso does not significantly alter the inactivation kinetics of
ICa, but, in the
absence of SR Ca2+ release, Iso
significantly slowed the decay of the currents in both WT and PLB-KO
cells. Because current densities and Iso-promoted current amplitudes
between WT and PLB-KO cells were comparable, these results suggest that
the SR Ca2+ load is the major
factor in the regulation of
ICa inactivation rate. In WT cells, Iso enhances
Ca2+-dependent inactivation by
increasing both
ICa and SR
Ca2+ load. Thus the slowing effect
of Iso was offset by enhanced
Ca2+-dependent inactivation. In
contrast, in PLB-KO cells in which the SR
Ca2+ content is already enhanced
under basal conditions, Iso had little effect on the amount of SR
Ca2+ release (23) and subsequent
Ca2+-dependent inactivation.
Consistent with this, recent studies (2, 10) have shown that the
fraction of SR Ca2+ release
activated by ICa
is highly dependent on the SR Ca2+
load. Although our experimental conditions do not permit quantitative measurement, our results provide strong support for the idea that the
enhanced Ca2+ content in the SR
plays a significant role in the modulation of the time course of
ICa during
-AR
stimulation. In the physiological context, this negative feedback of
CaCh activity may provide a potentially important mechanism for
regulation of Ca2+ overload,
because catecholamines induce increases in both
Ca2+ influx and SR
Ca2+ load, leading to subsequent
increased SR Ca2+ release during
depolarization.
 |
ACKNOWLEDGEMENTS |
This work was supported by a National Science Foundation Career
Advancement Award (A. Yatani), National Institute of General Medical
Sciences Grant GM-54169 (A. Yatani), and National Heart, Lung, and
Blood Institute Grants HL-26507, HL-52318, and HL-22619 (E. G. Kranias).
 |
FOOTNOTES |
Address for reprint requests: A. Yatani, Dept. of Pharmacology and Cell
Biophysics, Univ. of Cincinnati College of Medicine, Cincinnati, OH
45267-0575.
Received 19 May 1997; accepted in final form 22 July 1997.
 |
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