© 1995 American Heart Association, Inc.
Articles |
Arginine Vasopressin–Induced Potentiation of Unitary L-Type Ca2+ Channel Current in Guinea Pig Ventricular Myocytes
From the Department of Cardiovascular Diseases, Medical Research Institute, Tokyo (Japan) Medical and Dental University.
Correspondence to Yuji Hirano, MD, PhD, Department of Cardiovascular Diseases, Medical Research Institute, Tokyo Medical and Dental University, 1-5-45, Yushima, Bunkyo-ku, Tokyo 113, Japan.
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Abstract |
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Abstract The effects of arginine vasopressin (AVP) on L-type Ca2+ channels were studied by recording single-channel activity from cell-attached patches on isolated guinea pig ventricular myocytes, with 100 mmol/L Ba2+ used as the charge carrier. Bath application of AVP (100 nmol/L) reversibly increased channel open probability by a factor of 2.92±1.43 (n=15) because of the increased number of channel openings and increased open times. AVP did not change the amplitudes of single-channel currents (1.17±0.10 pA in the control condition and 1.12±0.11 pA after AVP, at +20 mV; n=6). In our experimental conditions, in which myocytes were bathed in Ca2+-free high-potassium solutions, AVP-induced potentiation was observed without changes in [Ca2+]i measured by fura 2 fluorescence signals (estimated [Ca2+]i,
80 nmol/L). The
AVP-induced increase in channel open probability was abolished by
OPC-21268 (8 µmol/L), a specific blocker of V1 receptor,
but not by a V2 blocker, OPC-31260 (5 µmol/L).
AVP-induced potentiation was also suppressed by a broad-spectrum
protein kinase inhibitor, H7 (100 µmol/L, bath application), but not
by H89 (1 µmol/L), a blocker with high specificity to protein kinase
A. AVP application after the treatment by phorbol ester (phorbol
12-myristate 13-acetate, 100 nmol/L for 1 hour) failed to potentiate
the channel activity. These results raised the possibility that protein
kinase C might be involved during signal transduction. Our
results provide direct evidence that AVP potentiates cardiac L-type
Ca2+ currents via V1 receptor
stimulation.
Key Words: arginine vasopressin • L-type Ca2+ channel • V1 receptor • protein kinase C
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Introduction |
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Arginine vasopressin (AVP) plays important roles in cardiovascular regulation, presumably through its systemic vasoconstrictor effect.1 Recently, evidence is accumulating that the heart is also a direct target for AVP,2 3 4 and receptors for AVP have been identified in the myocardium.5 Cardiac effects of AVP have been studied either in vivo or in isolated heart preparations. However, observations are variable, showing either positive or negative inotropic effects. The observed discrepancy might arise from the modification by neurally mediated reflexes, ischemia through coronary vasoconstriction, or interaction with other cardiovascular regulatory systems,1 because contributions by these factors are different, depending on experimental settings. Among controversial observations evoked by AVP are reports that describe positive inotropic effects, including experiments on the isolated heart under constant coronary flow2 and isolated papillary muscle after the functional removal of endocardial endothelium.3 In rat cardiomyocytes, AVP has been reported to increase [Ca2+]i.5 6 These results suggest that AVP may have a direct action of positive inotropism on myocardium, although its mechanism has not yet been clarified.
AVP has been shown to increase Ca2+ fluxes in hepatocyte plasma membrane7 and to potentiate Ca2+ channels in urinary bladder smooth muscle cells.8 In the heart, however, there is no experimental evidence that AVP is coupled to the modulation of Ca2+ channels. The present study was designed to examine the effect of AVP on L-type Ca2+ channels in isolated guinea pig ventricular myocytes; the preparation was free from extracardiac effects induced by AVP. To maintain the cellular metabolic condition intact, recordings were made at the single-channel level in the cell-attached patch configuration. In some experiments, changes in [Ca2+]i were monitored by the simultaneous measurement of fura 2 signals. Our results indicate that AVP exerts the upregulation of Ca2+ channels in cardiac myocytes.
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Materials and Methods |
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Preparations
Ventricular myocytes from guinea pig hearts were obtained by an enzymatic dissociation procedure similar to that described previously.9 Briefly, guinea pigs weighing 250 to 350 g were anesthetized with pentobarbital sodium (40 mg/kg IP). The chest was opened, and the aorta was cannulated in situ and perfused with Tyrode's solution before the heart was removed. Hearts were then retrogradely perfused with low-Ca2+ (30 µmol/L) Tyrode's solution with collagenase (0.4 mg/mL, type I, Sigma Chemical Co) for 20 minutes by using the Langendorff apparatus. After the enzyme was washed out, the cells were dissociated in high-K+ low-Cl- storage solution.
Electrophysiological Measurement and Data Analysis
Single L-type Ca2+ channel currents were
recorded in cell-attached configuration10 by using an
AXOPATCH-1D amplifier (Axon Instruments) at room temperature (22°C to
24°C). Pipettes were pulled from capillary tubes in a two-step
process, coated with insulating varnish, and fire-polished afterward.
The electrode had a resistance of 5 to 10 M
when the pipette was
filled with the Ba2+ solution. The membrane
potential of myocytes was depolarized to 0 mV by high-K+
solution. The electrode potential was adjusted to give a zero current
between the pipette solution and the bath solution immediately before
the seal formation. From a holding potential of -80 mV, patches were
depolarized at 1 Hz to 0, +10, or +20 mV for 180 ms. The threshold for
the activation of L-type Ca2+ channels was -10
mV in our recording conditions. After the gigaseal formation, stability
of basal Ca2+ channel activity was checked for at
least 10 minutes. Current signals were usually filtered at 1 kHz (-3
dB, eight-pole Bessel filter) and digitized at 5 kHz by using
PCLAMP software (Axon Instruments) on an IBM AT personal
computer. After digital subtraction for capacitive and leak components,
idealized recordings obtained by standard half-height criteria were
used to calculate the channel open probability (Po or NPo, where Po is
the probability of the channel opening and N is the number of
functional channels) and to obtain averaged current tracings. Fittings
for open or closed time distributions were performed with
PCLAMP software using the maximum-likelihood method and
the nonlinear least-squares fitting method. Fitted values presented
in this article were those for which both methods returned consistent
values. Where appropriate, numerical values are presented as
mean±SD. Differences in the numerical values between two groups were
evaluated by using Student's t test. A value of
P<.05 was considered significant.
[Ca2+]i Measurement
In some experiments (Fig 5
), changes in
[Ca2+]i were monitored simultaneously
during single-channel current recordings. This method has been
described in detail in our recent report,11 along with the
assessment of its limitations to obtain absolute
[Ca2+]i values. Briefly, cells were
loaded with fura 2 by exposure to 5 µmol/L acetoxymethyl ester and
were placed on a stage of an inverted microscope equipped with
epifluorescence optics (Diaphoto TMD, Nikon).
[Ca2+]i was monitored by using a
dual-wavelength fluorometer (CAM-230, Japan Spectroscopic).
Fluorescence was excited at wavelengths of 340 and 380 nm alternatively
by using a rotating sector mirror method, with a chopper frequency set
at 400 Hz. The emission of fluorescence (500 nmol/L) was sampled and
processed by the CAM-230 chopper system to yield two fluorescence
intensities separately. They were digitized at 5 kHz along with current
signals and stored in a computer (PC9801RA, NEC). By use of locally
written programs, the ratio of fluorescence intensities (R340/380) was
obtained for every sweep to elicit channel activity.
[Ca2+]i levels were then determined by
using a calibration curve obtained by in vivo calibration
technique.12
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Solutions and Drugs
The bath solution contained (mmol/L) potassium aspartate 120,
KCl 20, glucose 10, EGTA 2, and HEPES 10 (pH 7.4 by KOH). The pipette
solution contained BaCl2 100, HEPES 10 (pH 7.4 by
tetraethylammonium hydroxide). AVP was obtained from Sigma. Blockers
for vasopressinergic receptors (OPC-2126813 and
OPC-3126014 ) were gifts from Otsuka Pharmaceutical Co.
OPC-21268 was dissolved in dimethyl sulfoxide (DMSO), and OPC-31260 was
dissolved into distilled water to form a 10 mmol/L stock solution.
Isoquinolinesulfonamide protein kinase inhibitors (H715
and H8916 ) were obtained from Seikagaku Kogyo Co. H7 was
dissolved in distilled water, and H89 was dissolved in DMSO to form a
10 mmol/L stock solution. Phorbol 12-myristate 13-acetate (PMA) and
4
-phorbol 12,13-didecanoate (4PDD) were obtained from Sigma. They
were dissolved in DMSO to form a 1 mmol/L stock solution.
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Results |
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Effects of AVP on Ca2+ Channel Activity
Application of AVP to the bath solution caused an increase in Ca2+ channel activity in almost all the patches examined. Fig 1A
shows current tracings from a
single-channel patch in the control condition and during the
application of 100 nmol/L AVP. AVP increased the number of channel
openings, with their duration moderately increased. Current levels for
channel open state, however, were not discernibly affected. At +20 mV,
it was 1.17±0.10 pA in the control condition and 1.12±0.11 pA (n=6)
after AVP application. Thus, AVP increased the averaged current
amplitude (shown in the bottom tracing) through enhanced Po. The
temporal profile of Po from this patch is shown in Fig 1B. It should be
noted that effects produced by AVP took place slowly, and usually 5
to 10 minutes was required before the new steady state level was
achieved. The increase in Po was parallel with the number of channel
openings with long duration (
5 ms).
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Fig 2 depicts the results of kinetic analysis from
this patch. AVP increased the proportion of sweeps with channel
activity (availability)17 from 46.0% in the control
condition to 65.6% after the application. However, this effect was
only partially responsible for AVP-induced potentiation. In a of Fig 2,
each sweep was characterized by its Po during depolarization and
longest open time (Tmax) observed in the corresponding
sweep.18 The plot of Po versus Tmax indicated that AVP
shifted the distribution of sweeps toward increased Po and prolonged
Tmax. This was associated with changes in both open and closed time
distributions (b and c of Fig 2). With the fitting range up to 10 ms,
open time distributions during AVP application required two exponential
components for the fit. The shorter time constant during AVP
application was similar to the time constant in the control state; the
fit assumed the presence of a single-exponential
component.19 The induction or potentiation of an
extraexponential component during AVP application was a consistent
finding in an additional seven patches, including multichannel patches
from which open time constants could be analyzed reliably because of
the paucity of stacked channel openings (see also legend to Fig 3). We then tried to evaluate the amount of charges
carried by long openings induced by AVP. Because the total charge
carried during each event was calculated as open duration times open
current amplitude, we calculated the sum of durations of the events
belonging to individual bins. These values after multiplication by
current amplitude were superimposed on open time histograms (thick
lines in b of Fig 2). According to this calculation, channel openings
whose duration exceeded 5 ms carried 5.5% of the total charge in the
control state. After AVP application, long openings (5 ms) carried
13.8% of the total charges. We can also see that although the
enhancement of long openings was one of the prominent findings after
AVP application, an increase in the number of short openings (which
dominated in the control state) also significantly contributed to
the increase in overall Po. The latter effect should be explained in
terms of the changes in closed time distribution (c of Fig 2). They
were fitted by the sum of two exponentials both in the control
condition and after AVP application. In the present study, the
effect of AVP was to shorten the longer time constant. This component
corresponds to the duration of gaps between bursts.19
Similar findings were confirmed in three single-channel patches.
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The effects of AVP were persistent during continuous application for >20 minutes of the observation period (n=4) and were reversible on washout. This time course was gradual similar to the onset of potentiation. In three patches, the time periods around 15 minutes were required for the channel activity to return to the control levels.
Involvement of a Vasopressinergic Receptor
The effectiveness of bath-applied AVP (without direct access from
the pipette solution to the channel molecule) and the slow onset of
potentiation suggest that the AVP effect is mediated by intracellular
signaling systems. AVP is known to exert its effects through two
different types of receptors.20 21 22 Therefore, we tested
whether AVP-induced potentiation was affected during the blockade of
vasopressinergic receptors. Fig 3
shows a test for the effect of
OPC-21268,13 a specific blocker of V1
receptor. As seen in b of Fig 3, OPC-21268 (8 µmol/L, 20 times the
IC50 for V1 receptor blockade) itself had no
effect on channel activity. When AVP was added in the presence of
OPC-21268, it failed to potentiate the activity of
Ca2+ channels (c of Fig 3). This failure should be
explained by the blockade of specific receptor on the cell membrane,
because AVP-induced potentiation appeared when OPC-21268 was washed out
(d of Fig 3). Similar results were obtained in three additional
patches. On the other hand, OPC-31260,14 a specific
blocker of the V2 receptor, failed to suppress AVP-induced
potentiation of Ca2+ channels even at the dose of 5
µmol/L (>1000 times the IC50 for V2 receptor
blockade) in four of four cases tested. The AVP-induced changes in NPo
during receptor blockade were summarized in Fig 4. The
significance of the AVP effect was evaluated by taking the ratio of
channel open probability (NPo during AVP application/NPo in the control
condition) as an index. The values were 2.92±1.43 (n=15) without
blockers (left panel, P<.05), 1.05±0.22 (n=5) with
OPC-21268 (middle panel, P=NS), and 2.67±0.97 (n=4) with
OPC-31260 (right panel, P<.05).
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, 0 mV; 
, +10 mV; and 
,
+20 mV).
Effects on [Ca2+]i
Effects of AVP through V1 receptor activation in-
clude mobilization of intracellular Ca2+
in several tissues.5 6 20 23 24 Because limited increase
in [Ca2+]i is known to stimulate the
activity of Ca2+ channels,11 25 we
examined whether potentiation by AVP is related to changes in
[Ca2+]i in our experimental
conditions. Fig 5 shows the results of simultaneous
measurement of channel activity and the fura 2 signals. AVP increased
NPo by a factor of 2 in this patch. The ratio of fura 2 signals
(R340/380), however, was not detectably influenced. Similar disparity
between NPo and R340/380 was confirmed in two additional patches.
Effects of Protein Kinase Inhibitors on AVP-Induced
Potentiation
Ca2+ channels in muscles and neurons are under
the modulation of channel phosphorylation cycles by protein
kinases,26 whose activities are specifically affected by
various types of receptor stimulation. We observed that AVP-induced
potentiation took place via the V1 vasopressinergic
receptor but not via the V2 receptor, which is coupled to
cAMP-dependent protein kinase (PKA) activation. This raises the
possibility that protein kinase C (PKC) might be involved as a possible
signal transduction pathway. Therefore, we examined whether AVP-induced
potentiation was affected when kinase activity was suppressed. Fig 6 shows the effect of AVP in the continued presence of
H7,15 a membrane-permeant kinase inhibitor with broad
spectrum, on AVP-induced potentiation. After the bath-application of H7
(100 µmol/L), NPo usually began to decline after 10 minutes and
then settled to 50% of the control value. This was presumably due
to the change in the basal phosphorylation level of the
Ca2+ channels.26 When myocytes were
incubated with H7 for 60 minutes, AVP failed to potentiate channel
activity with the averaged NPo ratio (AVP/control) of 0.93±0.17 (n=7).
We then examined the effect of H89,16 a membrane-permeant
kinase inhibitor with high sensitivity for PKA (Fig 7).
As described below, we confirmed the efficacy of H89 under our
experimental conditions through the response to 8-bromo-cAMP. When
myocytes were incubated by H89 for 60 minutes, bath application of 1
mmol/L 8-bromo-cAMP increased the channel activity with an averaged NPo
ratio (AVP/control in the continued presence of H89) of 1.89±0.89
(n=4). This factor, however, was only approximately half of the value
without intervention (3.52±1.37, n=5), with a significant difference
(P<.05). We then compared the effects of AVP with and
without H89 treatment. When applied after incubation by H89, AVP still
exerted upregulation of the channel activity. AVP/control amounted to
2.52±1.34 (n=5), without a significant difference compared with the
control condition (see Fig 4, left panel). This was associated with the
changes in kinetic behavior that were observed in the absence of PKA
suppression, such as the increase in the number of sweeps with long
openings (b of Fig 1B).
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Effects of Phorbol Esters on AVP-Induced Potentiation
We then tested whether AVP could induce the potentiation of
Ca2+ channels when applied after the activation of
PKC by phorbol esters. In guinea pig myocytes at a dose of 100 nmol/L,
PMA caused transient increases in channel activity in three of five
cases. When cell-attached recordings were carried out from the myocytes
incubated with PMA for 60 minutes, however, the channel activity was in
a stable condition in the continued presence of PMA. Fig 8 shows the effect of AVP after myocytes were treated
with PMA and 4PDD (a phorbol ester that is ineffective in the
activation of PKC). After incubation by PMA (100 nmol/L) for 60
minutes, AVP failed to potentiate channel activity with AVP/control of
1.14±0.11 (n=4). This should be related to antecedent PKC activation
by PMA, because 4PDD failed to suppress AVP-induced potentiation
with AVP/control of 2.04±0.38 (n=5).
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Discussion |
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The present study demonstrated directly at the single-channel level that AVP caused a potentiation of the L-type Ca2+ channel via V1 receptor stimulation in guinea pig ventricular myocytes. It should be stressed that this effect should be free from extramyocardial compensatory mechanisms. Our results provide support for previous observations of positive inotropism induced by AVP2 3 when extramyocardial compensatory mechanisms were minimized.
Mechanisms of AVP-Induced Potentiation
The abolition of AVP-induced potentiation by a specific
V1 blocker, OPC-21268 (Fig 3), indicates that the cardiac
effect of AVP takes place through V1 receptor stimulation.
This is in agreement with previous observations using a different type
of V1 blocker,
d(CH2)5Tyr(Me)AVP.1 21 This agent
has been reported to suppress various types of AVP-induced effects in
the heart.2 4 6 27
Although the V2 receptor is coupled to increased adenylate
cyclase activity, leading to the activation of PKA, the V1
receptor exerts its effect through phosphatidylinositol hydrolysis,
leading to the mobilization of intracellular Ca2+
and the activation of PKC.6 20 21 Between two pathways
evoked by V1 stimulation, changes in
[Ca2+]i might be related to the
stimulatory effects on Ca2+ channels. This is
because AVP is known to increase
[Ca2+]i in several
tissues,6 20 23 24 and moderately increased
[Ca2+]i is reported to potentiate
Ca2+ channels in smooth muscle28 and
cardiac myocytes.11 25 In our recent study using an almost
identical experimental setting,11 the activity of
Ca2+ channels was potentiated when
[Ca2+]i exceeded approximately two
times the resting level. AVP-induced potentiation, however, was
observed without significant changes in
[Ca2+]i, as measured by fura 2
fluorescence (Fig 5). Thus, cellular processes following the changes in
[Ca2+]i appear to be not responsible
for the AVP-induced potentiation described in the present study.
Intracellular alkalinization through the stimulation of
Na+-H+ exchanger might be assumed to be another
candidate responsible for the potentiation.29 However,
this effect was not expected, because our bath solution contained no
Na+ for countertransport.
On the other hand, the suppressive effect of H7 (Fig 6) indicated the
involvement of channel phosphorylation cycles in AVP-dependent
potentiation. However, contribution by PKA26 was not
likely, because V2 blocker (Fig 4) and PKA inhibitor H89
(Fig 7) consistently failed to suppress the potentiation. On the other
hand, a role for PKC is supported by the observation that AVP failed to
potentiate channel activity in the continued presence of PMA, a PKC
activator (Fig 8). Thus, our results are consistent with the view that
PKC activation is at least partially responsible for the AVP-induced
potentiation.
Contribution by Intracellular Environment?
When compared with PKA-induced stimulation, PKC modulation of
L-type Ca2+ channels has been only sporadically
reported in cardiac preparations30 31 32 and therefore less
well characterized. This might be partly explained if PKC modulation of
Ca2+ channels is much more sensitive to the
intracellular metabolic environment than is PKA modulation. In
preliminary experiments, we investigated the effect of AVP by using
whole-cell recordings. In contrast to cell-attached recordings,
upregulation by AVP in this configuration was not constantly
observed when either Ba2+ or Ca2+
was used as the charge carrier. This was probably because the
intracellular milieu was largely replaced by the pipette solution. The
dependence on the intracellular environment appears to be more
prominent in the cases of L-type Ca2+ channels than
other types of channels. For example, Walsh and Kass33
observed that PKC activation by a phorbol ester modulated delayed
rectifier K+ channels but not Ca2+
channels in whole-cell experiments.
The importance of the intracellular environment in maintaining Ca2+ channel activity is well recognized.26 For example, single-channel activities always subside after patch excision. Whole-cell current amplitudes decline during the course of experiments. Although mechanisms for these observations were not yet fully clarified, it is generally ascribed to the loss of some important intracellular constituents for the modulation and functioning of Ca2+ channels. Therefore, it is reasonable that the response to external stimuli is diminished or even eliminated after the intracellular environment is replaced by artificial solutions, as encountered in whole-cell experiments. Examples include the response to thrombin,34 the effect of PMA in rat ventricular myocytes,32 and the reduced sensitivity to isoprenaline.35
Kinetic Behavior of Single-Channel Current During AVP
Application
Besides increasing the number of channel openings (or shortened
closed times), AVP moderately prolonged open time durations in the
present study. Under the simplified assumption that open time
distribution in the control state consisted of a single-exponential
component, the effect of AVP could be interpreted as the induction or
potentiation of another open state with a slightly prolonged time
constant. This effect is qualitatively similar to the observations of
Bonev and Isenberg8 regarding smooth muscle cells, where
AVP induced another open state with long openings through
unidentified mechanisms. In this case, however, the time constant of
the new component amounted to 10 times the control value, similar to
that observed in the presence of Bay K 8644. These results led them to
conclude that AVP induced "mode 2" gatings in their preparations.
On the other hand, the time constant of the new component in cardiac
myocytes was not much different from the control value.
Since its introduction during the analysis of dihydropyridine effects,36 the "mode" concept has been successfully applied to explain the complex behaviors of Ca2+ channel gating, including those during ß-adrenergic stimulation18 and high-voltage–dependent stimulation.37 In theory, it might be possible to assign the component induced by AVP as a new gating mode. This is because minimal requirements for the modal concept are well-defined sets of gating patterns, within which transitions are fast relative to conversions among sets. In practice, however, it appeared hopeless to analyze our data on the basis of modal theory. For example, without large differences in time constants for open time distri- butions, we could not follow different gating patterns on a sweep-to-sweep basis.
The kinetic behavior of cardiac L-type Ca2+ channels is highly complicated. There are observations that could not be well accommodated either in conventional multistate sequential schemes or even in mode concepts.37 38 Further studies, with special focus on biophysical aspects of channel behavior, will be required for a full understanding of gating processes.
Pathophysiological Implications
Effects of AVP on cardiovascular regulatory systems are multiple,
ranging from vasoconstriction to increased sensitivity to vasoreflex or
interaction with other humoral factors.1 Therefore,
although infusion of AVP is known to decrease cardiac performance in
vivo, assessments on the "direct" cardiac effect of AVP might be
variable, depending on experimental conditions.2 3 4 In the
present study, cell-attached recordings from isolated myocytes in
the absence of extracellular Ca2+ were chosen to
make the experimental system as simple as possible. Our results
indicate that AVP potentially is a positive inotropic agent when
extramyocardial compensatory mechanisms are excluded.
It should be noted that the dose of AVP used in the present study (100 nmol/L or 40 µU/mL) was at a level obtainable during certain pathophysiological conditions described with elevated plasma vasopressin levels.1 39 40 Potentiation of cardiac L-type Ca2+ channels is a part of the multifaceted cardiovascular regulatory systems of AVP and should be taken into consideration when we assess various types of circulatory disturbances.
Received April 18, 1994; accepted December 15, 1994.
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