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1Center for Perinatal Biology, 2Department of Physiology and Pharmacology, Loma Linda University School of Medicine, Loma Linda, California 92350
Submitted 13 February 2003 ; accepted in final form 8 April 2003
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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) yielded
linear and parallel relationships, and the change in
V for a 10-fold change in
[Ca2+] was also similar. Channel activity increased
e-fold for a 19 ± 2-mV depolarization for adult myocytes and
for an 18 ± 1-mV depolarization for fetal myocytes (P >
0.05). However, the relationship between BKCa open probability and
membrane potential had a relative leftward shift for the fetal compared with
adult myocytes at different [Ca2+]i. The
[Ca2+] for half-maximal activation (i.e., the calcium
set points) at 0 mV were 8.8 and 4.7 µM for adult and fetal myocytes,
respectively. Thus the increased BKCa current density in fetal
myocytes appears to result from a lower calcium set point.
patch clamp; perforated whole cell; inside-out patch; calcium set point; calcium-activated potassium channel
VSM contraction is determined largely by free intracellular calcium concentration ([Ca2+]i), and the regulation of [Ca2+]i changes dramatically during development (2, 6, 24, 29). Developmental changes that may affect the regulation of [Ca2+]i involve the different calcium sources (6), the inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]-releasable store (24), the ryanodine-sensitive store (24, 29), and the membrane potential (12). Furthermore, basilar arterial smooth muscle cells from neonatal rats exhibited a decreased Ca2+ spark frequency compared with that of adult VSM myocytes (14). In addition, the sensitivity of contraction to calcium differs between adult and fetal VSM (24, 29). In the ovine middle cerebral artery, the fetal smooth muscle cells depend more on the influx of extracellular Ca2+ during smooth muscle contraction, whereas the adult cells rely more on the mobilization of intracellular Ca2+ from the sacroplasmic reticulum (2, 24).
In general, membrane potential and [Ca2+]i are closely related, such that depolarization is associated with increased [Ca2+]i. Thus the regulation of membrane potential may be one of the determinants of developmental differences in VSM activity. In VSM cells, slight changes in the K+ channel activity dramatically affect the membrane potential (30). Four major types of K+ channels have been identified in cerebral arterial smooth muscle cells, including large conductance Ca2+-activated K+ (BKCa) channels, voltage-dependent K+ (KV) channels, ATP-sensitive K+ (KATP) channels, and inward rectifier K+ (KIR) channels (30). The BKCa channels, in particular, are prime candidates for developmental regulation, because they respond to changes in both [Ca2+]i and membrane potential, both of which vary during development.
In one of our previous studies (23), the activation of BKCa channels by NS-1619, a potent BKCa channel opener, profoundly inhibited vascular tension in fetal middle cerebral arteries, but decreased tension to a lesser extent in adult arteries. This result, and the recognized role of K+ channels in regulating membrane potential, L-type Ca2+ channel conductance, and vascular tone, lead us to hypothesize that known developmental differences in the contractile characteristics of cerebral blood vessels may result, in part, from age-related changes in K+ channel activity.
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METHODS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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For these studies, we used basilar arteries from near-term fetal (
140
days) and nonpregnant adult sheep (2 yr) obtained from Nebeker Ranch
(Lancaster, CA). All surgical and experimental procedures were performed
within the regulations of the Animal Welfare Act, the National Institutes of
Health Guide for the Care and Use of Laboratory Animals, "The
Guiding Principles in the Care and Use of Animals" approved by the
Council of the American Physiological Society, and the Animal Care and Use
Committee of Loma Linda University.
Artery and Cell Isolation
We selected arteries from the same anatomic segments of adult and fetal
basilar arteries to approximate segments of similar function and embryonic
origin. Consequently, the adult and fetal arteries were of different size
(300 vs. 200 µm). To determine whether arteries of different size
within age groups have the same current densities, we sampled current
densities from proximal and distal segments of both adult and fetal basilar
arteries. We found no significant differences in current densities within age
groups for arteries of different diameter.
Smooth muscle cells were enzymatically dissociated from dissected basilar arteries. The cell isolation procedure was modified from that previously described for rat cerebral arteries (34). Briefly, basilar arteries from adult (200–400 µm diameter) and fetal sheep (150–200 µm diameter) were cut into 1-mm2 pieces and placed in HEPES-buffered solution containing 0.3 mg/ml papain (Worthington Biochemical; Lakewood, NJ) and 1 mg/ml dithioerythritol for 20 min (adult) or 10 min (fetus) at 37°C. The tissues were then transferred to a second incubation in HEPES-buffered solution containing 100 µM CaCl2, 0.6 mg/ml collagenase type F, and 0.3 mg/ml collagenase type H (Sigma Blend Collagenase, Sigma; St. Louis, MO) and incubated for 12 min (adult) or 8 min (fetus) at 37°C. All of the enzymatic solutions also contained 1 mg/ml bovine serum albumin to minimize enzymatic effects on the cells. The digested tissue was triturated with a fire-polished, siliconized (Sigmacote, Sigma) glass Pasteur pipette to yield intact, separated smooth muscle cells. The cells were kept on ice and used within 6 h. To ensure that incubating the fetal and adult tissues in proteolytic solutions for the isolation procedure did not significantly alter the recorded current densities of the isolated cells, we incubated adult and fetal tissues in both papain and collagenase for varying times. We incubated adult arteries in papain for 20–40 min and in collagenase for 12–25 min and fetal arteries in papain for 10–20 min and in collagenase for 8–25 min. During these regimens of incubation, we observed no significant changes over time of incubation in the magnitude of outward current densities recorded from either the adult or fetal cells. Thus the proteolytic treatments we used were optimized to have no significant effect on the recorded outward currents in either age group.
Whole Cell Current Recordings
VSM cells adherent to glass coverslips were mounted in a perfusion chamber
for 15 min containing HEPES-buffered solution on the stage of an inverted
microscope (Axiovert 35M, Carl Zeiss Instruments), where they were viewed to
have characteristic elongated shapes with axial ratios of 10:1 for the
adult myocytes and 5:1 for the fetal myocytes. The HEPES-buffered solution was
then exchanged for the bathing solution.
Positive outward currents were measured in the perforated-patch configuration of the whole cell, voltage-clamp technique (17) using an Axopatch 200B amplifier (Axon Instruments; Foster City, CA). Whole cell currents were recorded with Clampex 8 while the cells were held at a steady membrane potential of –60 mV. Currents were filtered at 1 kHz, using an Axopatch 200B internal four-pole low-pass Bessel filter, and digitized at 2 kHz. We normalized whole cell currents with cell capacitance to give current density, due to differences in cell size between the adult and fetus, as well as within a given age group. An agar salt bridge was used to minimize the solution junction potential differences.
Single-Channel Recordings
Single-channel currents were recorded from inside-out membrane patches of
isolated arterial myocytes
(15). Patch pipettes were
fabricated using a programmable Flaming-Brown pipette puller and standardized
fire-polishing procedures. Because the patch pipettes so produced had very
similar tip resistances (
15 M
), the area of contact with each
membrane was most probably also very similar
(36). Currents were filtered
at 2 kHz and digitized at 10 kHz. The number of channels present in any given
excised patch (N) is estimated from all-point histograms. Channel
activity (NPo) was calculated using
,
where i is the number of open channels (0 for closed state) and
Ai is the area associated with each channel state
as determined from the curve-fit individual peak areas. The single-channel
open probability (Po) was then calculated from
NPo/N. The values for N were obtained by
using high [Ca2+] and/or depolarization to ascertain
that less than three coincidental open events occurred during long recordings
(>20 s) at a Po higher than 0.8. Preparations with more
than three channels present were discarded.
Reagents and Solutions
Papain was obtained from Worthington Biochemical. Calcium standards and
fura 2 were obtained from Molecular Probes (Eugene, OR). All other chemicals
were obtained from Sigma. For cell isolation, the HEPES-buffered solution
contained (in mM) 55 NaCl, 80 Na+-glutamate, 5.6 KCl, 10 HEPES, 2
MgCl2, and 10 glucose; pH 7.3 with NaOH. For whole cell recording,
the bathing solution contained (in mM) 134 NaCl, 6 KCl, 1 MgCl2, 10
HEPES, 10 glucose, and 2 CaCl2; pH 7.4 with NaOH. The pipette
solution contained (in mM) 110 K+-aspartate, 30 KCl, 10 NaCl, 1
MgCl2, 10 HEPES, and 0.05 EGTA (pH 7.2 with KOH), containing 200
µg/ml amphotericin B. The single-channel bathing solution contained (in mM)
140 KCl, 10 HEPES (pH 7.2), 1 Mg2+, and 5 EGTA with
several different free Ca2+ concentrations (300 nM,
1 µM, 3 µM, and 10 µM), which were measured fluorometrically using
fura 2. The single-channel pipette solution had the same composition as the
bathing solution with 3 µM free Ca2+.
Data Analysis and Statistics
All values were calculated as means ± SE. In all cases, n refers to the number of replicate cells. All statistical comparisons were performed at the 95% confidence level using two-sample, unpaired Student's t-tests. All sample populations were verified to be normally distributed. For comparisons of values that were not significantly different, power analyses were performed to confirm that statistical power was 0.7 and the probability of type II errors was acceptably small. Curve fitting was performed with GraphPad Prism 3 (GraphPad Software; San Diego, CA).
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RESULTS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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To compare the total outward K+ current in isolated adult and
fetal basilar arterial myocytes, we used the perforated whole cell,
voltage-clamp technique. We measured cell capacitance as 15.7 ± 0.6 pF
(n = 24) for adult myocytes and 9.1 ± 0.6 pF (n = 16)
for fetal myocytes (P < 0.01). Thus arterial smooth muscle
myocytes from the adult present 72% more plasma membrane surface area to
the bathing medium than cells from fetal arteries. We confirmed this by
microscopic measurements of the cell dimensions. Because of the mean
differences in cell size, we normalized the outward currents in terms of
membrane capacitance and present these data as current densities
(Fig. 1A).
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We recorded total outward currents from cells held at –60 mV and applied a series of +10-mV depolarizing steps (from –60 to +60 mV), each for a duration of 450 ms. We plotted steady-state current density and found it to be 37.9 ± 1.8 pA/pF (n = 6) for the adult myocytes and 57.9 ± 6.7 pA/pF (n = 10) for the fetal myocytes at +60-mV activation (Fig. 1B). Because the current-voltage relationships exhibit rectification, the difference in total outward current between adult and fetal myocytes is likely due to voltage-gated channels.
Voltage-Gated, Outward Currents in Adult and Fetal Myocytes
To distinguish pharmacologically between the two types of voltage-gated
K+ channels, e.g., BKCa and KV channels, and
assess their contributions to the differences in outward currents between
adult and fetal arteries, we employed iberiotoxin (IbTX) and 4-aminopyridine
(4-AP). IbTX specifically inhibits the current of BKCa channels,
whereas 4-AP broadly inhibits most KV channels
(30). IbTX and 4-AP produced
typical, dose-dependent suppression of the outward currents recorded from
adult and fetal myocytes (Fig. 2,
A and B). We detected no significant differences
in sensitivity to either of the drugs between adult and fetal cells. The
half-inhibitory concentration (IC50) for IbTX was
10–8 M and the IC50 for 4-AP was
10–4 M.
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IbTX- and 4-AP-Sensitive Outward Current Density in Adult and Fetal Myocytes
We applied 10–7 M IbTX externally to both adult (Fig. 3A) and fetal (Fig. 3B) myocytes to inhibit the BKCa channel current, followed by application pf 10–3 M 4-AP in the presence of IbTX to also inhibit KV channels. The total outward current density of the untreated fetal myocytes (Fig. 3B) was about one-third greater than that of the adult myocytes (Fig. 3A).
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The IbTX-sensitive current density was 43% greater in fetal myocytes compared with adult myocytes (Figs. 3 and 4A). On the other hand, the IbTX-insensitive currents were not significantly different between the two age groups (Fig. 4B). Indeed, in the presence of IbTX, neither the 4-AP-sensitive (Fig. 4C) nor the 4-AP-insensitive (Fig. 4D) currents were significantly different between the two age groups. These data strongly suggest that the primary cause of the increased positive, outward current in fetal myocytes compared with adult myocytes is due to the increased activity associated with BKCa channels.
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Calcium and Voltage Sensitivity of Adult and Fetal Myocytes
To determine whether the sensitivity of BKCa channels to
[Ca2+]i and/or to membrane potential account
for the increased activity of BKCa currents in fetal myocytes, we
used inside-out, excised patch preparations of adult
(Fig. 5A) and fetal
(Fig. 5B) arterial
muscle cell membrane. The bath solutions, representing cytosolic conditions,
contained different concentrations of free Ca2+, namely,
0.35, 1.3, 2.1, and 8.1 µM (Fig.
6). The recorded data were fitted to the following Boltzmann
equation: Po/Po,max = 1/{1 +
exp[(V –
Vm)/K]}, where Po is the open
probability of a single BKCa channel, Po,max is
the maximum Po of the BKCa channel,
V is the membrane potential
(Vm) required for 50% activation of the channels, and
K is the logarithmic voltage sensitivity (change in voltage required
for an e-fold increase in activity;
Fig. 6, A and
B). The voltage sensitivities estimated from the curve
fits were similar for all concentrations of Ca2+ tested
and indicated that channel activity increased e-fold for a 19
± 2-mV depolarization for adult myocytes (n = 4) and for a 18
± 1-mV depolarization for fetal myocytes (n = 4). The slopes
of the curve fits were 10.1 ± 0.1 for adult myocytes (n = 4)
and 10.5 ± 0.8 for fetal myocytes (n = 4). The
V for the different free
[Ca2+] for adult and fetal myocytes are given in
Table 1. In general, the
V values of the fetal BKCa channel at
different free [Ca2+] are leftward shifted –18.1
± 1.5 mV relative to those of the adult.
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We plotted the V values against
log[Ca2+] for both adult and fetal myocytes
(Fig. 6C). The plot
for both adult and fetal values yielded a linear relationship. The data on the
linear fits for both adult and fetal preparations are listed in
Table 2. From the equation for
the line fit through these data, we estimated that the change in
V for a 10-fold change in
[Ca2+] (
V) equaled
67.1 ± 2.5 and 67.6 ± 2.7 mV for adult and fetal myocytes,
respectively. Here, we define the calcium set point (Ca0) as the
[Ca2+] required for half-maximal activation at 0 mV. The
Ca0 was estimated to be 8.8 and 4.7 µM for the adult and
fetus, respectively. Despite the difference in the Ca0 of the
BKCa channels in adult and fetal myocytes, the unitary conductances
of these channels are not significantly different from each other
(Fig. 6D). The unitary
conductance for adult BKCa channels was 221 ± 8 pS
(n = 16) and for fetal BKCa channels was 229 ± 5 pS
(n = 12).
We calculated the Hill coefficient (nH) for the
Ca2+-dependent activation of BKCa channels
using nH =
V/(S ln 10), where S is
the slope of the voltage-dependent activation in the Boltzmann function. The
values of nH for each [Ca2+] were
2.9 ± 0.1 and 2.9 ± 0.2 for adult and fetal myocytes,
respectively. This indicates parallel shifts of the Ca2+
response curves induced by voltage changes.
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DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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Specifically, our findings show that 1) fetal myocytes have significantly greater whole cell voltage-dependent, outward current density than those of the adult; 2) KV channel activity is not significantly different between fetal and adult myocytes; 3) BKCa channel activity is significantly higher in fetal than adult myocytes; 4) the unitary conductances of BKCa channels in adult and fetal myocytes do not significantly differ; and 5) the BKCa channel set point is lower in the fetus than in the adult. In summary, these findings suggest that at any specified [Ca2+]i, the BKCa channels in fetal smooth myocytes are likely to be more active than those in adult, because fetal resting levels of [Ca2+]i in the intact vessel have been reported to be lower than those of the adult (2). To ensure that our procedure for isolating smooth muscle cells does not disrupt [Ca2+]i, we measured the basal or resting [Ca2+]i levels of smooth muscle cells isolated from the adult and fetal basilar artery using microfluorometric imaging with fura 2. We found that the basal level of [Ca2+] in fetal cells was 149 ± 19 nM (340/380 nm = 0.424 ± 0.027, n = 7) and in adult cells was 157 ± 15 nM (340/380 nm = 0.438 ± 0.020, n = 10). VSM cells from fetal arteries appear to be somewhat smaller than those from adult. Furthermore, there are variations in myocyte size between cells from the same and different individuals of the same age. To compare currents elicited from adult and fetal myocytes, as well as from different cells of the same age group, we took into account the differences in cell size. Cell surface is directly proportional to membrane capacitance (4, 11). By expressing the measured currents in terms of cell capacitance, current density was obtained and used by us to normalize measured currents from different cells within an age group and to compare measurements between the age groups.
The total outward current density at a highly depolarized membrane potential was significantly higher in the fetal myocytes than in the adult myocytes (Fig. 1). Because 4-AP-sensitive currents, mediated by KV channels, were not significantly different (Figs. 2 and 3), and the outward currents remaining after treatment with both IbTX and 4-AP were not significantly different between the fetal and adult myocytes, whereas the IbTX-sensitive currents in the fetal cells were significantly greater than in the adult cells, we conclude that the increased current density in the fetal myocytes is predominately due to increased BKCa channel currents.
Altered BKCa Channel Properties
Ovine middle cerebral arteries from both the adult and fetus respond to IbTX, a BKCa inhibitor, with increased tension and increased [Ca2+]i (23, 39). Although the tension increased to a similar extent in arteries from both age groups, the fetal vessels exhibited twice the increase in [Ca2+]i as the adult (23). On the other hand, in adult rat basilar arteries, IbTX increased [Ca2+]i and tension to a greater extent than in the neonate (14). In rat neonatal vessels, IbTX had almost no effect on either [Ca2+]i or vessel tension. The nearly opposite results of these two studies may be due to differences between species and/or the use of different arteries. Nonetheless, the findings of both studies may be explained in terms of altered BKCa channel properties during development.
Possible Causes of Higher BKCa Channel Current in Fetal Cells
The relatively higher BKCa channel current in the fetal myocytes
was not due to differences in unitary conductance or to channel protein
expression. The mean unitary conductances of BKCa channels in adult
and fetal myocytes did not differ significantly
(Fig. 5C).
Furthermore, our preliminary Western immunoblotting studies show that the
-subunit of BKCa channels in the fetal and adult myocytes
are similarly expressed (L. D. Longo and Y. Zhao; data not shown). The
similarity in the number of functional BKCa channels present on
adult and fetal myocytes was reflected in membrane patches of inside-out
experiments, where the average number of BKCa channels present on
the patches from adult and fetal myocytes was not significantly different.
Thus the cause of the difference in BKCa channel current density is
most likely due to intrinsic difference in the BKCa channel
activity.
If both BKCa channel protein expression and BKCa unitary conductance (Fig. 6D) are not significantly different between fetal and adult myocytes, then Po is the likely cause for the difference in BKCa channel currents. The total current carried by a specific channel type (I) is equal to N · Po · i, where N is the number of channels expressed, Po is the open probability of the channel, and i is the unitary conductance of the channel. In other words, if the total current carried by BKCa channels (I) is increased in fetal myocytes while N and i remain constant, then Po must be increased in the fetus. Indeed, in fetal cells, we found that at any specific [Ca2+]i, the Po of the BKCa channels is increased relative to cells from adults (Fig. 6). Increased Po values of BKCa channels have also been reported by Pérez et al. (33) and Gollasch et al. (14) in the rat basilar arterial smooth muscle cells during development from the neonate to adult. The higher Po of BKCa channels in the fetal myocytes could be due to either a lower voltage sensitivity, a higher Ca2+ sensitivity, or a lower Ca0 of the BKCa channels (10).
Voltage sensitivity of BKCa channels. In ovine basilar arterial smooth muscle cells, the depolarizing voltages required to produce an e-fold increase in channel activity, 19 ± 2 mV for the adult and 18 ± 1 mV for the fetus, were not significantly different. These voltage sensitivities are within the range of values estimated in other types of smooth muscle cells, namely, from 10 to 20 mV (10, 30). Thus both the adult and fetal myocytes possess similar voltage sensitivities.
Calcium sensitivity of BKCa channels.
The sensitivity of the BKCa channels to calcium relates to the
affinity of calcium ion binding to the channels and the average number of
calcium ions able to bind to a BKCa channel at any given time. As
suggested by Carl et al. (10),
we can estimate the calcium sensitivities of BKCa channels for
adult and fetal ovine basilar myocytes from our inside-out preparations. The
equation nH =
V/(S ln 10) suggests a linear
relationship between the shift in V for a 10-fold
increase in Ca2+ (V)
and nH, the Hill coefficient for activation of
BKCa channels by Ca2+. S is the
steepness of the voltage-dependent activation in the Boltzmann function. The
value of V for the adult was calculated to
be 67.1 ± 2.5 mV and for the fetus was 67.6 ± 2.7 mV. This
suggests that the Ca2+ sensitivity of these channels did
not differ significantly from one another. These values are similar to the
V values observed in other smooth muscle
cells (10). We also calculated
the average nH as 2.9 ± 0.1 and 2.9 ± 0.2
for adult and fetal myocytes, respectively, which are similar to the values
reported in other smooth muscle cells
(10,
27).
Ca0 of BKCa channels. On the other hand, from the Ca0 estimations, fetal BKCa channels require only 4.7 µM to activate one-half of the BKCa channels at 0 mV, whereas adult BKCa channels require 8.8 µM to half-activate. Ca0 of BKCa channels from different VSM cells have been estimated to be 0.5 µM for guinea pig mesenteric arteries (5), 1 µM for rabbit portal veins (18), 1.5 µM for rat pulmonary arteries (3), and 9 µM for hamster cremasteric arterioles (19). Thus our estimations of the Ca0 fit within the higher range of values from previous studies. The relatively lower fetal Ca0 implies that at a given [Ca2+]i and voltage, the BKCa channels of fetal myocytes are more activated than those of the adult. We propose that the lower Ca0 of BKCa channels in fetal cells is either an adaptation to lower resting [Ca2+]i, which in fetal myocytes is lower than in adults (1), or to higher resting potentials, which in the fetal myocyte is more depolarized (12), or to compensate for differences in Ca2+ entry.
Nauli et al. (28) suggested that in ovine basilar arteries, maturation decreases Ca2+ release from ryanodine receptors; however, in neonatal rat cerebral smooth muscle cells, the ryanodine receptors do not act in synchronized fashion to produce Ca2+ sparks, as in the adult (14). To counter the less frequent Ca2+ sparks of the fetal myocytes, it appears that the BKCa channels of fetal cells have a lower Ca0. This allows the fetal myocytes to respond to smaller changes in subplasmalemmal [Ca2+] than the adult cells. In this manner, the lower Ca0 of the BKCa channel may act as a protective mechanism to govern the membrane potential of fetal myocytes.
Possible causes of lower Ca0. The difference
in Po of the BKCa channels of fetal and adult
myocytes appears to be due to an altered Ca0. The underlying cause
for the lower Ca0 of BKCa channels in fetal myocytes can
only be speculated at present. Several mechanisms are possible. The
-subunits of the fetal and adult BKCa channels might be of
different isoforms. Although it has been suggested that in VSM the
-subunits of the BKCa channels come from a single gene, it
is possible that there are several variations in the expressed gene due to
posttranscriptional modifications, perhaps involving the "calcium
bowl" residing on the carboxyl end of the -subunit
(22,
40,
46). Dynamic association of
the -subunits of the BKCa channel with accessory
-subunits provides another possible molecular mechanism for channel
plasticity. To date, eight -subunits have been identified and shown to
differ in their effects on BKCa channels, and they may come from
different genes or splice variants
(21,
42,
45). In VSM, the
-subunit is associated specifically with the 1-subunit
(8,
9,
13,
21,
38). Experiments monitoring
binding of monoiodotyrosine-charybdotoxin (125I-ChTX) to membranes
derived from COS-1 cells transiently transfected with either - or
+ -subunits of the BKCa channel indicate that the
-subunit enhances ligand affinity by 50-fold under low ionic
strength conditions (16). It
has also been reported that 1-subunits can increase the
Po of the BKCa channels as much as 40-fold
(35,
41). Thus variations in
associated -subunits may explain the difference in Ca0.
Recent studies also have shown that dynamic lipid-protein interactions and lateral phase separations (i.e., lipid "rafts") among plasma membrane lipids can modulate K+ channel activities (7, 20, 26). Location within different raft compartments and/or differences in raft lipid composition may account for our observed difference in Ca0 of BKCa channels in adult and fetal myocytes. Furthermore, BKCa channel activity can be regulated posttranslationally by different protein kinases and phosphatases (32, 37, 44). Protein kinases and phosphatases are regulated differently in adult and fetal myocytes (25, 29). Thus the posttranslational modulation of BKCa channels has the potential to differentially modulate the Ca0 of BKCa channels during development. These possibilities and others are important considerations for future research.
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DISCLOSURES |
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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TOPABSTRACTMETHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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M. T. Lin, L. D. Longo, W. J. Pearce, and D. A. Hessinger Ca2+-activated K+ channel-associated phosphatase and kinase activities during development Am J Physiol Heart Circ Physiol, July 1, 2005; 289(1): H414 - H425. [Abstract] [Full Text] [PDF]
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