Department of Physiology and Biophysics, University of Nebraska
Medical Center, Omaha, Nebraska, 68198-4575
Fatty acid metabolites accumulate in the heart under
pathophysiological conditions that affect
-oxidation and can elicit marked electrophysiological changes that are arrhythmogenic. The purpose of the present study was to determine the impact of amphiphilic fatty acid metabolites on K+
currents that control cardiac refractoriness and excitability. Transient outward
(Ito) and
inward rectifier
(IK1)
K+ currents were recorded by the
whole cell voltage-clamp technique in rat ventricular myocytes, and the
effects of two major fatty acid metabolites were examined:
palmitoylcarnitine and palmitoyl-coenzyme A (palmitoyl-CoA).
Palmitoylcarnitine (0.5-10 µM) caused a concentration-dependent decrease in Ito
density in myocytes internally dialyzed with the amphiphile; 10 µM
reduced mean Ito
density at +60 mV by 62% compared with control
(P < 0.05). In contrast, external
palmitoylcarnitine at the same concentrations had no effect, nor did
internal dialysis significantly alter
IK1. Dialysis
with palmitoyl-CoA (1-10 µM) produced a smaller decrease in
Ito density
compared with that produced by palmitoylcarnitine; 10 µM reduced mean
Ito density at
+60 mV by 37% compared with control
(P < 0.05). Both metabolites delayed
recovery of Ito
from inactivation but did not affect voltage-dependent properties.
Moreover, the effects of palmitoylcarnitine were relatively specific,
as neither palmitate (10 µM) nor carnitine (10 µM) alone significantly influenced
Ito when added to
the pipette solution. These data therefore suggest that amphiphilic
fatty acid metabolites downregulate
Ito channels by a
mechanism confined to the cytoplasmic side of the membrane. This
decrease in cardiac K+ channel
activity may delay repolarization under pathophysiological conditions
in which amphiphile accumulation is postulated to occur, such as
diabetes mellitus or myocardial infarction.
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INTRODUCTION |
FREE FATTY ACIDS are the primary substrate used by the
heart to generate cellular ATP (6, 12, 18). The initial steps involved
in ATP production via
-oxidation include the formation of essential
intermediates that facilitate transport of fatty acids to the
mitochondrial matrix where the enzymes responsible for
-oxidation
are located (1, 6, 12). The two principal intermediates involved in
this process are acyl-coenzyme A (CoA), produced by acyl-CoA synthetase
at the outer mitochondrial membrane, and acylcarnitine, which is formed
at the inner mitochondrial membrane by palmitoylcarnitine transferase I
(1, 6, 12). These amphiphilic metabolites, although essential to lipid
metabolism, are potentially toxic to the cell at high concentrations
but are normally kept at low levels under physiological conditions (1, 6, 12). However, disturbances in
-oxidation can result in the
overproduction of fatty acid metabolites, particularly
palmitoylcarnitine and palmitoyl-CoA, which may have deleterious
effects on cardiac function. For example, in diabetes mellitus, it is
postulated that impaired glucose utilization by the myocardium along
with elevations in plasma free fatty acids profoundly increases the rate of
-oxidation and the subsequent formation of amphiphilic metabolites (6, 12). Moreover, under hypoxic or ischemic conditions,
inhibition of electron transport, due to lack of
O2, impairs
-oxidation and also
leads to marked accumulation of fatty acid metabolites (1, 7). Indeed,
McHowatt et al. (7) have shown that canine ventricular myocytes
subjected to hypoxic conditions for 20 min exhibit a 15-fold increase
in the total content of long-chain acylcarnitine that is confined
mainly to the sarcolemma.
The pathophysiological accumulation of fatty acid intermediates in
cardiac myocytes has been proposed to elicit a wide range of adverse
electrophysiological, biochemical, and mechanical effects. In
particular, long-chain acylcarnitines, such as palmitoylcarnitine, have
been shown to produce marked electrophysiological changes in vitro that
affect repolarization and excitability and that favor the generation of
triggered rhythms from afterdepolarizations (1). For instance,
extracellular palmitoylcarnitine applied to isolated cardiac
preparations has been shown to produce biphasic effects on action
potential duration and to reduce the maximal velocity of
phase 0, resting membrane potential,
and action potential amplitude (1). Several studies exploring the ionic
mechanisms for these electrophysiological changes have shown that
palmitoylcarnitine is capable of modulating several ionic currents,
including L-type Ca2+
(ICa; see Refs. 3
and 22), fast Na+
(INa; see Refs.
15 and 23), inward rectifier K+
(16), and transient inward (23) currents. Changes in one or more of
these currents are likely to underlie the overall electrophysiological effects of fatty acid metabolites on the myocardium and help explain the ionic mechanisms of arrhythmogenesis in diseased hearts (1).
The present investigation examined the effects of two major amphiphilic
fatty acid metabolites, palmitoylcarnitine and palmitoyl-CoA, on
K+ currents of rat ventricular
myocytes that participate in the control of refractoriness and
excitability of the heart: the transient outward
(Ito) and
inward rectifier
(IK1) currents.
Our data suggest that palmitoylcarnitine and palmitoyl-CoA selectively
downregulate Ito
density and delay recovery from inactivation by a mechanism that is
confined to the cytoplasmic side of the membrane. These amphiphiles had
no effect on IK1
and were not mimicked by internal application of palmitate or carnitine alone.
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METHODS |
Isolation of ventricular myocytes and recording
techniques. Male Sprague-Dawley rats weighing
150-200 g were given an overdose of pentobarbital sodium (100 mg/kg ip), and single ventricular myocytes were dissociated from
excised, perfused hearts by a collagenase digestion procedure described
previously (13, 14, 24). Isolated myocytes from both ventricles were
suspended in Dulbecco's modified Eagle's medium and stored in an
incubator at 37°C until used, usually within 8 h of isolation. For
electrophysiological recordings, aliquots of myocytes were transferred
to a cell chamber mounted on the stage of an inverted microscope and
perfused with standard external solution containing (in mM) 138 NaCl,
4.0 KCl, 1.2 MgCl2, 1.8 CaCl2, 0.5 CdCl2, 10 glucose, and 5 HEPES, pH
7.4.
Ionic currents were recorded using the whole cell configuration of the
patch-clamp technique. Specifically, borosilicate glass pipettes were
heat polished to an internal tip diameter of 1-2 µm and filled
with a standard pipette solution containing (in mM) 135 KCl, 3 MgCl2, 10 HEPES, 3 Na2-ATP, 10 EGTA, and 0.5 Na-GTP, pH 7.2 adjusted with KOH. Filled pipettes with resistances of 2-4
M
were coupled to a patch-clamp amplifier (Axopatch 1C, Axon Instruments), and, after whole cell recording conditions were established, series resistance was compensated and whole cell capacitance was calculated as the area under the capacitive transient divided by the amplitude (
5 mV) of an applied test pulse (13, 14, 24). A computer program (pCLAMP, Axon Instruments) controlled command potentials and acquired current signals, which were sampled at
4 kHz by an analog-to-digital converter and which were stored on the
hard disk of a 486 computer. All experiments were done at room
temperature (22-24°C).
Ito was evoked by
500-ms depolarizing pulses to test potentials
(Vm) between
40 and +60 mV (0.2 Hz). The holding potential for this protocol
was
80 mV, and a 100-ms prepulse was applied to
60 mV to
inactivate INa.
For each test pulse, the amplitude of
Ito was measured
as the difference between peak outward current and the current level at
the end of the depolarizing clamp pulse. In addition to measuring
current-voltage
(I-V)
relations of Ito, voltage-dependent steady-state parameters and kinetics of
Ito recovery from
inactivation were also determined. First, steady-state activation
parameters were derived from recorded
I-V
relations by first calculating the conductance at each
Vm, normalized to maximum conductance at +60 mV
(G/Gmax),
and plotting these values as a function of
Vm (2, 24). Data
were fitted by a Boltzmann distribution to determine the steady-state
activation parameters V1/2 and
k according to the relation
where
V1/2 is the
voltage at half-maximal activation, and
k the slope factor at
Vm = V1/2.
Steady-state inactivation was examined by applying 500-ms prepulses
from
100 to 0 mV before pulsing to +60 mV (0.1 Hz), and
inactivation curves were constructed by plotting normalized current
against prepulse voltage. These data were also fitted by a Boltzmann
distribution to derive the steady-state inactivation parameters
V1/2 and
k. Finally, recovery of
Ito from
inactivation was measured by delivering two identical 500-ms
depolarizing pulses from
80 to +60 mV and varying the interpulse
interval from 50 to 900 ms. Reactivation curves were constructed by
plotting the ratio of peak
Ito elicited by
the second pulse relative to the first against interpulse interval and
were fitted to a single exponential to derive the time constant of recovery.
IK1 was also
measured in some myocytes with 100-ms test pulses applied from a
prepulse potential of
30 mV (holding potential =
80 mV).
I-V
data were obtained by changing
Vm from
120 to
40 mV (0.2 Hz), and the amplitude of
IK1 was measured
at the end of each test pulse. Raw measurements of both
Ito and
IK1 were normalized as current densities by dividing measured current amplitude by whole cell capacitance (pA/pF). Finally, action potentials were
elicited in some myocytes in current-clamp mode using 5-ms suprathreshold depolarizing current pulses delivered at 1 Hz. For these
experiments, myocytes were superfused with room temperature external
solution without added Cd2+.
Experimental protocols and statistical
analyses. Three different experimental protocols for
voltage-clamp analyses were used in the present investigation. In most
experiments, modulation of K+
currents by intracellular compounds was examined by using pipette solutions containing different concentrations of palmitoylcarnitine, palmitoyl-CoA, palmitate, or carnitine. For each cell, a dialysis period of 15 min was allowed, after establishing whole cell recording conditions, to achieve equilibration between the pipette solution and
the cell cytoplasm. Second, to determine if extracellular amphiphiles
modulate K+ channel function,
other experiments were conducted in which palmitoylcarnitine or
palmitoyl-CoA was applied extracellularly for 15 min during whole cell
recordings. Finally, in some experiments, myocytes were preconditioned
in external solution containing amphiphile for up to 4 h before
recording K+ currents.
All results are expressed as a means ± SE. Statistical comparisons
of two groups were made using a Student's
t-test, whereas comparisons of more
than two groups were carried out by analysis of variance. When a
significant difference among groups was indicated by the initial
analysis, individual paired comparisons were made using a modified
Bonferroni t-test (19). Differences
were considered significant at P < 0.05.
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RESULTS |
Intra- versus extracellular actions of
palmitoylcarnitine. Previous experiments have shown that fatty acid
metabolites of myocardial
-oxidation are generated intracellularly
and that palmitoylcarnitine mainly accumulates in the sarcolemma under pathophysiological conditions (1, 7). Therefore, we first examined the
impact of intracellular palmitoylcarnitine by adding this
amphiphile to the pipette solution to give a final concentration from 0.5 to 10 µM. Figure
1A
compares raw traces of
Ito recorded at
potentials from
40 to +60 mV in a control myocyte
(Cm = 135 pF; traces at
top) and another dialyzed for 15 min
with 5 µM palmitoylcarnitine (Cm = 152 pF). At each test potential,
Ito amplitude
was less in the palmitoylcarnitine-dialyzed myocyte compared with
control. Figure 1B summarizes the effects of different
pipette concentrations of palmitoylcarnitine on the mean
I-V relation of Ito
after 15 min of internal dialysis compared with time controls. There
was a marked, concentration-dependent decrease in
Ito density as reflected by a downward
shift in the mean I-V relation. This is further illustrated in Fig. 1C, which plots
maximum Ito
density (at +60 mV), expressed as a percentage of the mean density of
time controls, at each pipette concentration of palmitoylcarnitine
tested. Note that, in myocytes dialyzed with 5 and 10 µM
palmitoylcarnitine, maximum
Ito density was
significantly reduced by 43 and 62% from control, respectively
(P < 0.05).

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Fig. 1.
Internal dialysis with palmitoylcarnitine.
A: superimposed traces of
transient outward current
(Ito) at
40 to +60 mV (20-mV steps) are shown for a control myocyte
(Cm = 135 pF) and a myocyte
dialyzed for 15 min with 5 µM palmitoylcarnitine added to the pipette
solution (PCp;
Cm = 152 pF).
B: mean current-voltage
(I-V)
relations of
Ito in
myocytes dialyzed for 15 min with pipette solution containing 0-10
µM palmitoylcarnitine.
[PC]p, pipette
palmitoylcarnitine concentration. No. of myocytes are in parentheses.
C: concentration-response of
Ito to
pipette palmitoylcarnitine. Maximum
Ito density
measured at +60 mV is expressed as a percentage of the mean density of
control, untreated cells (26.5 pA/pF;
n = 15 myocytes).
* P < 0.05 compared with
time control myocytes.
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To determine whether the effects of palmitoylcarnitine on
Ito were specific
to the cytoplasmic side of the membrane, two additional series of
experiments were conducted. First, palmitoylcarnitine was applied
extracellularly for 15 min during whole cell recording of
Ito with
standard, internal solution (i.e., without palmitoylcarnitine). Figure
2A
summarizes the effect of 10 µM extracellular palmitoylcarnitine on
mean
I-V
relations. For these experiments, data were recorded from the same cell
before (baseline) and 15 min after adding this amphiphile to the
external solution. As Fig. 2A
illustrates, external application of palmitoylcarnitine for 15 min did
not significantly alter the mean
I-V
relation of Ito.
In a second series of related experiments, myocytes were preconditioned
with 10 µM palmitoylcarnitine for 4 h before recording
Ito. Figure
2B shows that even external palmitoylcarnitine exposure for several hours did not significantly alter Ito
density, which contrasts markedly with data obtained during only 15 min
of internal dialysis (Fig. 1).

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Fig. 2.
External application of palmitoylcarnitine.
A: comparison of mean
I-V
relations recorded from the same cell before (baseline) and 15 min
after adding 10 µM palmitoylcarnitine to the external solution
(PCo).
B: mean
I-V
relations of Ito
of two separate groups of myocytes incubated with or without 10 µM
palmitoylcarnitine for 4 h before recording currents.
Vm, membrane
potential. No. of myocytes are in parentheses.
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To assess the significance of intracellular amphiphile on the overall
electrophysiology of myocytes, action potentials were recorded in
current-clamp mode during internal dialysis with palmitoylcarnitine. Figure 3 shows examples of action
potentials recorded at 1 Hz from a
Cm (Fig. 3,
left) and a myocyte dialyzed with 5 µM palmitoylcarnitine, illustrating a markedly longer action
potential duration in the latter. This difference is summarized in
Table 1, which compares action potential
parameters from control and palmitoylcarnitine-dialyzed myocytes. Note
that action potential duration was significantly increased at 25, 50, and 90% of repolarization in the palmitoylcarnitine-dialyzed myocytes,
whereas action potential amplitude was significantly less than control.

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Fig. 3.
Action potentials from myocytes dialyzed with palmitoylcarnitine.
Action potentials were recorded in current-clamp mode at 1 Hz from a
control myocyte (left) and a myocyte
dialyzed with 5 µM palmitoylcarnitine
(PCp; right). For
these recordings, the external solution did not contain
Cd2+. See Table 1 for summary
data.
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Effects of intracellular palmitoyl-CoA, palmitate, and
carnitine. A second major fatty acid metabolite that
accumulates under pathophysiological conditions in the myocardium,
palmitoyl-CoA (1, 6, 12), was also examined in experiments similar to those described for palmitoylcarnitine. In general, internal dialysis with palmitoyl-CoA also reduced
Ito density (Fig.
4), whereas extracellular application for
15 min had no effect (data not shown). Figure 4 summarizes the response
of Ito in
myocytes dialyzed for 15 min with 5 or 10 µM palmitoyl-CoA. When
compared on an equimolar basis with pipette palmitoylcarnitine, the
effects of internal palmitoyl-CoA were smaller in magnitude over the
same duration of dialysis. Thus, with 10 µM palmitoyl-CoA in the
pipette solution, maximum
Ito density
(measured at +60 mV) was significantly reduced but by only 37% from
control. However, when compared with either amphiphile, neither
palmitate nor carnitine alone significantly affected
Ito density when
added to the pipette solution. This is shown in Fig.
5, A and
B, which compares
I-V
relations in myocytes dialyzed for 15 min with 10 µM palmitate (Fig.
5A) or 10 µM carnitine with time
controls. Although there was a downward shift in the I-V
relation in palmitate-dialyzed cells (Fig.
5A), this change did not reach
significance under these experimental conditions.

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Fig. 4.
Internal dialysis with palmitoyl-CoA. Mean
I-V
relations of Ito
are shown for myocytes dialyzed for 15 min with pipette solution
containing 5 or 10 µM palmitoyl-CoA and time-matched controls ( ).
[PCoA]p, pipette
palmitoyl-CoA concentration. No. of myocytes are in parentheses.
* P < 0.05 compared with
control.
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Fig. 5.
Dialysis with palmitate and carnitine.
A: mean
I-V
relations of Ito
for myocytes dialyzed for 15 min with pipette solution containing 10 µM palmitate and time-matched controls ( ).
B: mean
I-V
relations of Ito
for myocytes dialyzed for 15 min with pipette solution containing 10 µM carnitine. No. of myocytes are in parentheses.
[Palmitate]P and
[L-carnitine]P,
pipette concentrations of palmitate and L-carnitine,
respectively.
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Amphiphile-mediated alterations in recovery from
inactivation. In addition to analyzing the effects of
palmitoylcarnitine and palmitoyl-CoA on
Ito density, we
also compared time- and voltage-dependent properties of this current
with and without internal amphiphile. Figure
6 summarizes the effects of intracellular
palmitoylcarnitine (A) and
palmitoyl-CoA (B) on the recovery of
Ito from
inactivation. Note that internal dialysis with both compounds delayed
recovery from inactivation compared with time control cells but that
only palmitoylcarnitine had a significant effect at the highest
concentration tested (10 µM; see Fig. 6,
insets). However, neither compound had a significant effect on the voltage dependence of steady-state activation or inactivation (Table 2).

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Fig. 6.
Delayed recovery of
Ito from
inactivation with intracellular amphiphiles. Recovery from inactivation
was measured in myocytes dialyzed with palmitoylcarnitine
(A) or palmitoyl-CoA
(B) for 15 min. Time controls are
shown by in A and
B.
Insets summarize the measured rate of
recovery from inactivation ( , in ms) at each concentration of
amphiphile tested.
I2/I1,
ratio of peak Ito
elicited by the second pulse relative to the first.
* P < 0.05 compared with the
untreated group; n, no. of myocytes.
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Table 2.
Voltage dependence of Ito steady-state activation
and inactivation in myocytes dialyzed for 15 min with
palmitoylcarnitine or palmitoyl-CoA
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Sensitivity of IK1 to
intracellular palmitoylcarnitine.
To examine whether intracellular amphiphiles influenced other
K+ currents to the same extent as
Ito, we
additionally analyzed IK1 as a function
of the concentration of palmitoylcarnitine in the pipette solution.
Figure 7A
compares raw traces of
IK1 (
120 to
20 mV) recorded from the same control (traces on
top) and palmitoylcarnitine-dialyzed
myocytes from which examples in Fig. 1A were obtained. These traces show
that a 5 µM concentration of the amphiphile added to the pipette
solution did not clearly alter the amplitude of
IK1, as measured
at the end of each test pulse. This is further illustrated in Fig.
7B, which compares mean
I-V
relations of IK1
in myocytes dialyzed with 0-10 µM palmitoylcarnitine for 15 min.
The lack of effect of palmitoylcarnitine on
IK1 contrasts markedly with the reduction in
Ito density
observed with the same pipette concentrations. Moreover, in a small
subset of myocytes (n = 5), internal
dialysis with 10 µM palmitoyl-CoA also did not change
IK1 (data not
shown).

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Fig. 7.
Effect of intracellular palmitoylcarnitine on inward rectifier
K+ current
(IK1).
A: superimposed traces of
IK1 at 120
to 20 mV (10-mV increment) are shown from the same control
(traces on top) and
palmitoylcarnitine-dialyzed myocytes from which the examples in Fig.
1A were obtained.
B: mean
I-V
relations of IK1
in ventricular myocytes dialyzed for 15 min with 0-10 µM
palmitoylcarnitine. No. of myocytes are in parentheses.
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DISCUSSION |
Modulation of cardiac ion channels by amphiphilic
metabolites. Recent studies have shown that
palmitoylcarnitine and a structurally related phospholipid
metabolite, lysophosphatidylcholine, are capable of
modulating a variety of ion channels that control refractoriness and
excitability of the myocardium. For example, action potential recordings from avian cardiomyocytes suggest that relatively high concentrations of extracellular palmitoylcarnitine (100-300 µM) increase ICa (3),
whereas voltage-clamp data from isolated guinea pig myocytes indicate
that lower palmitoylcarnitine concentrations (1-25 µM) are
inhibitory to this current whether administered extra- or
intracellularly (22). Sato et al. (15) report that external
palmitoylcarnitine (0.5-50 µM) decreases
INa in guinea pig
ventricular myocytes, whereas in rabbit myocytes Wu and Corr (23) have
shown that this same amphiphile elicits a slow-inactivating Na+ and transient inward current,
an effect that was observed with either extracellular or internal
application. These latter studies thus provide evidence that lipid
metabolites may have complex actions on
Na+ conductance that may
contribute to the genesis of arrhythmias under pathophysiological conditions.
In the present study, we show that palmitoylcarnitine and palmitoyl-CoA
(0.5-10 µM) decrease
Ito density
(Figs. 1 and 4) and prolong its recovery from inactivation (Fig. 6).
This change in K+ channel activity
in the intact cell would be expected to delay repolarization and
possibly act in concert with increased
Na+ conductance at plateau
potentials (23) to elicit automatic or triggered responses. In support
of this hypothesis, we found that internal palmitoylcarnitine increased
action potential duration (Fig. 3 and Table 1), although we did not
attempt to separate the relative contributions of
Na+ and
K+ channels to this overall
change. The decrease in action potential amplitude observed in
palmitoylcarnitine-dialyzed myocytes (Table 1) is consistent with a
decrease in INa,
as reported by Sato et al. (15) in guinea pig ventricular myocytes.
We also show that the amphiphile-mediated decrease in
Ito density was
greatest during intracellular application, a result that correlates
with findings that fatty acid metabolites are generated
intracellularly (1) and may distribute preferentially in the intact
cell, with palmitoylcarnitine accumulating mainly in the sarcolemma (7)
and palmitoyl-CoA in the mitochondrial matrix (1). Although our data
suggest that palmitoylcarnitine causes a greater reduction in
Ito density than
palmitoyl-CoA, this difference may be due to limitations of the
recording technique more than a true difference in sensitivity to these
amphiphiles. That is, given the marked difference in molecular weight
of the amphiphiles studied in our model (palmitoyl-CoA-1006;
palmitoylcarnitine-436), it is likely that the rate of internal
dialysis with palmitoyl-CoA and its intracellular concentration were
less than those for palmitoylcarnitine (10) over the 15-min dialysis
period used in our experiments. This limitation therefore precludes a
more quantitative analysis of the sensitivity of
Ito channels to
intracellular palmitoylcarnitine and palmitoyl-CoA.
The molecular mechanisms underlying the specific effects of amphiphilic
metabolites on
Ito or other ion
channels have not been defined thoroughly. One possibility is that
long-chain acylcarnitines indirectly affect ion channels by
incorporating into the membrane and altering local membrane fluidity by
changing the packing of phospholipid molecules (1, 22, 23). However, it
is equally possible that these compounds directly interact with the
channel protein to alter gating kinetics or probability of channel
opening (23). The process by which lipids directly alter protein
function, i.e., palmitoylation, has not been examined thoroughly for
cardiac ion channels but has been studied in relation to turnover of
the
-subunit of Gs, which
couples the
-adrenergic receptor to adenylyl cyclase (21). Whether
or not Ito
channels are downregulated via palmitoylation requires further study,
but our finding that palmitoylcarnitine and palmitoyl-CoA, which are
effective palmitoylating compounds (21), only reduced
Ito during
internal dialysis suggests that a specific interaction with the
Ito channel
protein is a plausible mechanism.
Our data also suggest that intracellular fatty acid metabolites may
selectively downregulate
Ito channels,
since we found no effect of either palmitoylcarnitine or palmitoyl-CoA
on IK1. This
latter finding differs however with the work of Kiyosue and Arita (5)
and Shen and Pappano (16) who show that extracellular lysophosphatidylcholine and palmitoylcarnitine reduce
IK1 in guinea pig
ventricular myocytes in addition to inhibiting the current carried by
the
Na+-K+
pump (16). The differences between our results and those of others (16)
are likely to be species-related, since all studies used the same
concentration of palmitoylcarnitine (10 µM).
Pathogenic conditions responsible for accumulation of
fatty acid metabolites. A number of pathophysiological
states are associated with altered fatty acid metabolism that
potentially can lead to intracellular accumulation of toxic
intermediates, either as a result of an excess or reduced rate of
-oxidation. For example, diabetes mellitus is a pathophysiological
state that is characterized by a marked decrease in
Ito density (17,
20, 24) and where excess
-oxidation has been proposed to cause an
accumulation of amphiphilic metabolites (6, 12). During insulin
deficiency, there are significant increases in plasma free fatty acids
and in myocardial triglyceride content plus a dramatic fall in
oxidative metabolism of glucose (6, 12). Indeed, in the diabetic heart, glucose oxidation is nearly abolished, and the contribution of fatty
acids to overall ATP production can exceed 90% of the total ATP
produced (6, 12). Therefore, under these conditions, it is proposed
that an enhanced rate of
-oxidation may not keep pace with an
oversupply of exogenous and endogenous fatty acids, thereby causing
fatty acid intermediates to accumulate (6, 12). A similar set of
conditions may also exist in the failing, nondiabetic heart, which
exhibits a profound increase in fatty acid utilization (9) and which is
also characterized by a significant decrease in
Ito density (4,
8, 11, 13, 14). However, it is uncertain to what extent fatty acid
metabolites contribute to the depressed contractile function and
electrophysiological abnormalities that characterize the diabetic and
failing heart.
Depressed
-oxidation is a second mechanism by which fatty acid
metabolites can accumulate in the myocardium and elicit deleterious effects (1, 6). For example, in the hypertrophied heart, fatty acid
oxidation is reduced compared with normal and is mediated by a decrease
in fatty acid transport in the mitochondria secondary to a reduction in
myocardial carnitine content (6). Under hypoxic or ischemic conditions,
inhibition of electron transport, due to lack of
O2, also impairs
-oxidation and
is associated with a marked accumulation of fatty acid metabolites (1,
7). This latter pathogenic condition may be worsened upon reperfusion of the myocardium where fatty acid oxidation may be greater than control (6).
In conclusion, our data suggest that palmitoylcarnitine and
palmitoyl-CoA selectively downregulate
Ito channels in
rat ventricular myocytes by a mechanism that is confined to the
cytoplasmic side of the membrane. The increased content of these
amphiphiles in the sarcolemma, particularly palmitoylcarnitine, may
underlie the contractile and electrical abnormalities in the heart
during pathophysiological states that influence fatty acid metabolism, such as diabetes mellitus and myocardial infarction.
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Address for reprint requests: G. J. Rozanski, Dept. of Physiology and
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Omaha, NE 68198-4575.