Endocrine Research Unit, Department of Veterans Affairs Medical Center, Department of Medicine, University of California, San Francisco, California 94121
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ABSTRACT |
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Parathyroid cells
express Ca2+-sensing receptors
that couple changes in the extracellular
Ca2+ concentration
([Ca2+]o)
to increases in the intracellular free
Ca2+ concentration
([Ca2+]i)
and to the suppression of parathyroid hormone secretion. Using whole
cell patch clamping, we previously identified voltage-independent Ca2+-conducting currents in bovine
parathyroid cells that increased with rising
[Ca2+]o
and were blocked by Cd2+ and
nifedipine. Because cAMP-dependent phosphorylation regulates dihydropyridine-sensitive Ca2+
channels in other systems, we tested whether cAMP modulates these currents. At 0.7 mM Ca2+,
nonselective Ca2+-conducting
currents were suppressed by 30-50% when the recording pipette was
perfused with cAMP.
High-[Ca2+]o-induced
increases in membrane currents were also abrogated. The effects of cAMP
were reversible and dose dependent (3 × 109 to 3 × 10
3 M) and required ATP in
the pipette solution. Perfusion of the cell interior with the catalytic
subunit of protein kinase A mimicked the effects of cAMP, as did
perfusion of the bath with the adenylate cyclase activator forskolin.
These findings support the idea that cAMP-dependent phosphorylation
suppresses
high-[Ca2+]o-induced
cation currents and may play a role in regulating ion fluxes in
parathyroid cells.
parathyroid hormone secretion; nonselective cation currents; calcium receptor; dihydropyridine; calcium channel; calcium influx
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INTRODUCTION |
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EXTRACELLULAR Ca2+ concentration ([Ca2+]o) determines the rate of parathyroid hormone (PTH) secretion in vivo. Low [Ca2+]o maximally stimulates secretion, whereas high [Ca2+]o suppresses PTH release (2, 25). Parathyroid cells respond to changes in [Ca2+]o by altering the levels of critical second messengers, including cAMP, inositol trisphosphate, diacylglycerol, and intracellular Ca2+ concentration ([Ca2+]i) (2, 19, 25, 29). The accumulation of inositol 1,4,5-trisphosphate induced by raising [Ca2+]o is likely to play a key role in the initial mobilization of Ca2+ from intracellular stores (1). Sustained Ca2+ mobilization, however, requires membrane Ca2+ influx, presumably through the opening of ion channels. The nature of the channels responsible for the Ca2+ influx due to the raising of [Ca2+]o in the parathyroid is controversial, and their molecular identity is unknown.
Certain pharmacological studies have implicated
dihydropyridine-sensitive ion channels in the regulation of PTH
secretion (11, 26). Other studies, using antisera directed against
1-subunits of L-type
Ca2+ channels, support a role for
L-type channels in modulating Ca2+
influx and PTH release (12). Using whole cell patch clamping, we
recorded dihydropyridine-sensitive, nonselective cation currents that
conduct Ca2+ and are activated by
raising
[Ca2+]o
(6). This membrane conductance may be a component of the mechanism
responsible for Ca2+ influx in
this system.
The activity of the Ca2+ channel is regulated by protein kinases. cAMP-dependent phosphorylations, initiated by protein kinase A (PKA), are one pathway implicated in the control of ion channels, including the dihydropyridine-sensitive Ca2+ channels of many excitable cells (15-17, 31). In the present study, we found that internal perfusion of parathyroid cells with cAMP or the catalytic subunit of PKA (PKA-CS) and perfusion of the bath with forskolin blocked Ca2+-conducting currents. These effects required the presence of ATP in the pipette, suggesting that blockade of these currents involves phosphorylation of either the channel protein and/or a critical regulator. These findings raise the possibility that ion influx in the parathyroid via these channels is controlled by cAMP-induced phosphorylation.
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MATERIALS AND METHODS |
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Materials
Chemicals in the electrode and bath solutions, channel blockers, the sodium salts of cGMP and cAMP, forskolin, and PKA-CS were purchased from Sigma (St. Louis, MO). The acetoxymethyl ester of fura 2 was from Molecular Probes (Eugene, OR). Culture media were prepared by the Cell Culture Facility of the University of California, San Francisco. Recording micropipettes were pulled from thin-walled borosilicate glass capillaries (World Precision Instruments, Sarasota, FL) by an electrode puller (model P-87, Sutter Instrument, Novato, CA) and polished using a Narishige MF-9 microforge (Technical Instruments, San Francisco, CA) as previously described (6, 7).Cell Preparation
Dispersed bovine parathyroid cells were prepared (4) for patch clamping as described (6, 7). After the cell pellet was washed three times with Ca-, Mg-, and NaHCO3-free Eagle's medium containing HEPES (20 mM, pH 7.4) (cell medium) supplemented with CaCl2 (1 mM) and MgSO4 (1 mM), the cells were plated on no. 1 round microscope coverglasses (Fisher Scientific, Santa Clara, CA) and incubated for 30 min at 37°C before recording. For the measurement of [Ca2+]i, cells were equilibrated in the cell medium noted above supplemented with 0.5 mM CaCl2, 0.5 mM MgSO4, and 0.2% BSA (wt /vol) .Solution Composition
Pipette solutions. Recording pipettes in most experiments were first filled with a whole cell electrode solution (WCES) containing (in mM) 140 cesium methanesulfonate, 5 MgCl2, 10 EGTA, 10 HEPES (pH 7.4), 4 MgATP, 0.3 GTP, and a nucleotide-regenerating system [NRS; 14 units/ml phosphocreatine and 50 units/ml creatine phosphokinase (6, 7)]. In this solution, Cs+ was the major charge carrier for outward currents. In certain experiments, we excluded both the NRS and ATP (NRS-free WCES) to examine the effects of these components on membrane currents.
Bath solutions.
In the bath solution (BS), acetate was the major anion present to
minimize the recording of
Cl currents.
Ca2+ was the major carrier of the
inward currents. Tetraethylammonium ion
(TEA+; 10 mM) was included in
these solutions to block
K+-selective currents and also to
act as a charge carrier (6, 7). All solutions were buffered with HEPES
(10 mM, pH 7.4), and osmolarity was adjusted to
330
mosM with sucrose as needed. Each BS is designated by the
major cations that it contains. Their compositions are specified in
Table 1.
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Patch-Clamping Protocols and Data Analysis
Whole cell patch clamping was conducted as described (6, 7). Briefly, thin-walled borosilicate glass pipettes with an electrical resistance ofMembrane potential (Vm) was controlled, and membrane current (Im) values were detected by an Axopatch amplifier (Axon Instruments, Foster City, CA) as described (6). Data were digitized (100 kHz), recorded onto a microcomputer, and analyzed with the use of pCLAMP and AxoGraph software.
The voltage-clamping protocol for acquiring the membrane
current-voltage relationship
(Im-Vm)
plots is shown in Fig.
1A and was as follows. Each run of voltage clamping was initiated with the
resting membrane potential held at 60 mV. Test voltage pulses of
150-ms duration were applied to the recording pipette every 2 s in
increments of 20 mV (from
100 to +120 mV). The presented current
traces were recorded from 20 ms before to 25 ms after each applied
voltage pulse. Inward (+) currents, from the bath to the cytoplasmic
compartment of the cell, are presented as downward deflections in the
current traces. Current traces represent the arithmetic means of the
currents recorded during three consecutive runs of the same voltage
protocol. Im was
calculated as the average current during a given test voltage pulse for
determining the Im-Vm.
The membrane conductance, designated as
Cm, was
calculated from the slope of the linear regression of the
Im-Vm.
Each experiment was performed on at least three cells from different
preparations.
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Micropipette Perfusion
Micropipette perfusion techniques adapted for parathyroid cells were described previously (7) and are illustrated in Fig. 2. After the whole cell configuration was established, the cell was gently detached from the coverslip and lifted up by raising the micropipette holder (World Precision Instruments) with a micromanipulator (Technical Instruments, San Francisco, CA) (Fig. 2). This provided the spatial freedom (0.2 cm in vertical direction) for manually uncapping the tubing from the suction port of the micropipette holder (Fig. 2). The slender drawn tip (<50 µm in diameter) of a 1-ml plastic syringe was placed inside the hub of the micropipette to replace the electrode solution (ES). To ensure complete replacement of the ES, the tip of perfusing syringe was placed close to the tip of recording micropipette (<200 µm in distance), and the volume of the new ES was 10 times greater than the volume of micropipette. Using this technique, we were able to deliver cAMP successfully into the pipette by replacing the original WCES with a WCES containing cAMP. This maneuver suppressed both inward and outward currents (Fig. 1, B and C). Removal of cAMP by subsequent perfusion with a new WCES restored these currents (Fig. 1D). These manipulations did not alter the membrane-pipette seal, which was verified by perfusing the bath with the channel blocker Gd3+ (Fig. 1E) (6, 7). In some experiments, we successfully perfused the cell interior with six different ESs. We completely replaced the ES in 2-4 min and started our recording ~10 min after each ES was delivered.
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Measurement of [Ca2+]i
[Ca2+]i of cell populations was measured using fura 2 as previously described (30). After cells (5-10 × 106/ml) were loaded with the acetoxymethyl ester of fura 2 and washed, they were resuspended (2-8 × 106/ml) and preincubated in cell medium with 0.5 mM CaCl2, 0.5 mM MgSO4, and 0.2% BSA with or without forskolin (10 ![]() |
RESULTS |
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Intracellular cAMP Blocks Im and Cm Induced by Raising [Ca2+]o
Because cAMP-dependent phosphorylation regulates Ca2+-channel activity in other cells (10, 13, 34, 35), we tested the effects of cAMP on parathyroid cell currents. Perfusion of the cell interior with a WCES that contained cAMP (3 × 10We next assessed the effects of perfusing the cell with ES containing
different concentrations of cAMP (3 × 109 to 3 × 10
4 M). cAMP dose
dependently suppressed the baseline
Im and
Cm at 0.7 mM
Ca2+ and blocked the increase in
Cm observed with
raising
[Ca2+]o
from 0.7 to 90 mM (Fig. 3,
A and
B, and data not shown).
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We found that 7 of 12 cells tested could respond to a maximal dose of
cAMP (3 × 104 M), and
the potency of cAMP varied among cells. In one cell, basal
Im at 0.7 mM
Ca2+ and
high-[Ca2+]o-induced
Im were
suppressed >95% by 3 × 10
9 M cAMP (data not
shown). In six other cells, both baseline
Cm and the
increase in Cm
due to raising
[Ca2+]o
to 90 mM were maximally blocked by >95% by 3 × 10
4 M cAMP (Fig.
3B). Typically, cAMP concentrations
in the pipette of 3-30 × 10
6 M significantly lowered
Cm in these cells
compared with control recordings without added cAMP. In three cells
that were successfully perfused with multiple ES containing various
cAMP concentrations, the dose dependence of the inhibition by this
molecule was clearly demonstrated (Fig.
3B).
To assess the specificity of the response to cAMP, we perfused the
pipette with cGMP, which regulates nonselective cation channels in
other systems (15). Baseline
Im and
Im induced by raising
[Ca2+]o
were not suppressed by intracellular perfusion with 3 × 104 M cGMP (Fig.
4,
A-C, and data not shown). These
currents could be blocked (
70%) by the subsequent perfusion with a
WCES containing 3 × 10
4 M cAMP (Fig.
4D), confirming the specificity of
the response to cAMP. Similar results were observed in two other cells
(data not shown).
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We next assessed whether the effects of cAMP were dependent on phosphorylation and whether they were reversible. Whole cell currents were recorded from isolated parathyroid cells perfused sequentially with the following pipette solutions: 1) WCES, 2) WCES+cAMP, 3) NRS-free WCES+cAMP, and 4) WCES+cAMP. Initial recordings were made in a BS containing 0.7 mM Ca2+. Before pipette solutions were changed, the BS was changed to vary [Ca2+]o stepwise from 0.7 to 90 mM Ca2+ to assess the effects of different pipette constituents on the responsiveness of the cell to raising [Ca2+]o.
When the pipette solution contained cAMP (3 × 103 M; see Fig.
5Aii),
Im and
Cm were markedly
suppressed compared with controls as was the response to perfusion of
the bath with high
[Ca2+]o
(Fig. 5,
Ai,
Aii, and
B). When the NRS and ATP were
removed from the pipette solution (Fig. 5,
Aiii and
B), the inhibition by cAMP of
Im and
Cm was reversed,
and the responsiveness of
Im to high
[Ca2+]o
was restored. When the next ES containing the NRS, ATP, and cAMP was
perfused (Fig. 5Aiv), the ability of
cAMP to block the baseline
Im at 0.7 mM
Ca2+ and the response to high
[Ca2+]o
were again observed (Fig. 5, A and
B). Addition of ATP and the NRS
(which regenerates ATP) back to the WCES restores the ability of cAMP
to block Im and
lower Cm (Fig. 5,
Aiv and
B). The inhibitory effects of cAMP,
therefore, require ATP.
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Perfusion of the Pipette With PKA-CS Mimics the Effects of cAMP on Membrane Currents
Because cAMP activates PKA and because the ability of cAMP to lower Im and Cm in parathyroid cells depends on the presence of ATP, we examined whether perfusion of the patch pipette with the PKA-CS mimicked the effects of cAMP in whole cell recordings. In the experiment shown in Fig. 6 and in three other cells, cAMP reversibly suppressed Im and Cm both at 0.7 mM Ca2+ (by >50%) and after raising [Ca2+]o (by >80%) (Fig. 6, Ai-Aiii and B). Perfusion of the same cell internally with a WCES supplemented with PKA-CS (5 × 10
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We next determined whether either the NRS, ATP, or both were required for the inhibition by PKA-CS. As shown in Fig. 7, pipette perfusion with WCES plus PKA-CS again dramatically lowered Im (Fig. 7, A and B). When the NRS and ATP were deleted from the pipette solution, there was no inhibition by PKA-CS (Fig. 7C). Instead, the addition of ATP alone back to the pipette solution restored the ability of the PKA-CS to inhibit membrane currents (Fig. 7D and data not shown). The NRS was not essential for the inhibition of membrane currents by PKA-CS in parathyroid cells.
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Effects of Forskolin on cAMP Content and Whole Cell Membrane Currents in Parathyroid Cells
We next tested whether manipulations that stimulate increases in intracellular cAMP reproduce the effects of perfusion of the cell interior with cAMP. Voltage-clamped parathyroid cells were perfused with a BS containing the adenylate cyclase activator forskolin. Forskolin at concentrations
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Effects of cAMP on Nifedipine- and Cd2+-Resistant Currents in Parathyroid Cells
We previously reported that nonselective cation currents in parathyroid cells have two major components (6). Type I currents are blocked by Cd2+ and nifedipine and increase with raising of [Ca2+]o. Type II currents, in contrast, are not activated by raising [Ca2+]o and are Cd2+ and nifedipine insensitive. The present studies support the idea that cAMP, PKA-CS, and forskolin mainly affect type I currents. To examine whether these agents could also modulate type II currents, we recorded from cells in a BS containing 3 mM Cd2+. Cd2+ blocks type I currents (>95%) but not type II currents (6). In the presence of a Cd2+-containing BS, intracellular perfusion of cAMP (3 × 10
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Effects of Forskolin on [Ca2+]i in Parathyroid Cells
Because type I currents are activated by high [Ca2+]o and inhibited by forskolin, we examined the ability of forskolin to suppress the high-[Ca2+]o-induced increases in [Ca2+]i. As shown in Fig. 10, preincubation of parathyroid cells without or with forskolin (10
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DISCUSSION |
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PTH plays an important role in the maintenance of systemic Ca2+ homeostasis. Changes in [Ca2+]o regulate the rate of PTH release and biosynthesis (2, 25). [Ca2+]o affects these changes by interacting with parathyroid Ca2+-sensing receptors (CaRs), which couple changes in [Ca2+]o to sustained increases in [Ca2+]i and other second messengers. Increases in [Ca2+]i are likely due to mobilization of Ca2+ from intracellular stores and Ca2+ influx across the membrane. Previous studies have implicated dihydropyridine-sensitive Ca2+ channels in both Ca2+ influx and the regulation of PTH secretion (26). The expression of such channels has been inferred on the basis of functional studies with pharmacological probes and antibodies against Ca2+-channel subunits (12, 18). Little direct evidence exists, however, regarding the nature of the Ca2+ influx mechanism(s) activated by high [Ca2+]o in these cells.
We have previously identified and characterized voltage-independent and dihydropyridine-sensitive cation currents by whole cell patch clamping. These currents may provide a mechanism for Ca2+ influx in parathyroid cells (6). In the present study, we further demonstrate by several lines of evidence that these currents, which are responsive to changes in [Ca2+]o, are regulated by cAMP-dependent phosphorylation.
Phosphorylation is a common mechanism for regulating Ca2+ channels. In both cardiac and skeletal muscle cells, PKA-mediated phosphorylation activates Ca2+ channels and thereby enhances Ca2+ influx (5, 31). In contrast, cAMP-dependent phosphorylation blocks voltage-dependent L-type Ca2+ channels in vascular smooth muscle cells (35). We found that cAMP and PKA-CS, through ATP-dependent mechanisms, suppress high-[Ca2+]o-induced, dihydropyridine-sensitive Ca2+ currents in parathyroid cells. Thus the nature of the regulation of channel activity by cAMP varies depending on the cell type.
The marked suppression of membrane currents by cAMP in the perfusion
pipette was dose dependent and reversible. These effects are likely to
be specific for several reasons. 1)
They were detectable at concentrations of 3 × 109 to 3 × 10
5 M cAMP in the pipette
solution. This concentration range is comparable to those at which cAMP
activates PKA in vitro (10
9
to 10
5 M) (33).
2) cGMP did not reproduce these
effects. 3) The effects observed
with cAMP were also observed when the PKA-CS was in the perfusion
pipette or when the bath was perfused with forskolin. Thus
manipulations that would either increase intracellular cAMP content or
enhance cAMP-dependent phosphorylation produced the same inhibition of
the nonselective currents we monitored.
There are a variety of mechanisms by which cAMP- and PKA-mediated phosphorylation can regulate channel currents. Cyclic nucleotides can modify the gating properties of channels directly by binding to the channel protein (10, 13, 36, 37) or indirectly by modifying the channel protein through cAMP- or cGMP-dependent phosphorylation (15, 24, 32, 34).
The effect of cAMP on
high-[Ca2+]o-induced
Ca2+ currents in parathyroid cells
is likely through a pathway involving phosphorylation. This is based on
the dependence of the effect of cAMP on the presence of ATP. There are
many candidate substrates for cAMP-dependent phosphorylation in the
parathyroid that could modulate ion channel activity. These include the
channel subunits themselves, other channel regulators, and even the
CaRs. Any of these molecules could be affected by cAMP-dependent
phosphorylation that could then alter the activation of membrane
currents. The CaR may be a less likely target of PKA-mediated
phosphorylation, since the bovine parathyroid CaR lacks consensus sites
for PKA-mediated phosphorylation (3). It has been shown, however, that
the 1- and
-subunits of
dihydropyridine-sensitive Ca2+
channels are targets of cAMP-dependent phosphorylation in other systems
(9, 14, 20, 27). Therefore, it is a real possibility by analogy that
cAMP-dependent phosphorylation suppresses membrane currents in the
parathyroid via the phosphorylation of a channel protein. Studies in
Xenopus laevis oocytes injected with
mRNA encoding cardiac Ca2+-channel
subunits suggest that phosphorylation of other as yet unidentified
proteins can also be responsible for the suppression of channel
activity (8). Detailed understanding of the mechanism by which cAMP and
PKA-CS regulate type I nonselective cation currents in parathyroid
cells awaits the molecular identification of the channel protein(s)
responsible for this conductance.
Our studies showed that internal perfusion of parathyroid cells with cAMP suppressed Ca2+-conducting currents, presumably through the activation of PKA. Therefore, in the whole cell configuration, these cells clearly have endogenous PKA activity. This idea may explain our observations that removal of the NRS and ATP not only restored the Im and Cm to levels recorded before addition of cAMP but often enhanced Im and Cm slightly. In addition, switching the ES directly from the normal WCES to an NRS-free WCES increased both basal and high-[Ca2+]0-induced Im and Cm. One possible explanation for this enhancement is that endogenous cAMP may be involved in setting the baseline Im and Cm in parathyroid cells.
In our studies, not all cells were responsive to patch pipette perfusion with cAMP. This may be due to inherent heterogeneity in the cell population in terms of their expression of the relevant effector proteins, either channel subunits or other channel regulators. In previous studies, we (6) and others (22, 23) have observed that there is heterogeneity in the intracellular Ca2+ responses of parathyroid cells exposed to the changes in the [Ca2+]o. Alternatively, our pipette perfusion techniques may alter either the responses or the viability of less robust cells.
The inhibition of Im by perfusion of the cell interior with cAMP or the PKA-CS may explain previous observations regarding high-[Ca2+]o-mediated signal transduction in parathyroid cells. It is known that high [Ca2+]o decreases forskolin-stimulated cAMP content (28) and increases [Ca2+]i in these cells (2, 25). Results of the present study indicate that an increase in intracellular cAMP is associated with inhibition of cation influx. Thus the decrease in cAMP content due to high [Ca2+]o might be predicted to promote cation (and possibly Ca2+) influx in parathyroid cells. By this line of reasoning, the reduction in cAMP induced by high [Ca2+]o could contribute to cellular Ca2+ mobilization and the inhibition of PTH secretion. This hypothesis is supported by observations that forskolin (28) and other agents that raise cAMP content (2) increase PTH secretion in intact parathyroid cells. Whether the increase in PTH secretion due to forskolin is related to the suppression of the cation currents we studied or to other mechanisms is unknown.
Arguing against the paradigm that changes in
Ca2+ mobilization mediate the
effects of forskolin on PTH secretion are our measurements of
[Ca2+]i.
These determinations fail to show an effect of forskolin on this
parameter. There are several explanations, however, for differing results between patch-clamp recordings and intracellular
Ca2+ determinations. 1) Patch clamping is
inherently a more sensitive, direct, and better controlled way to
detect changes in ion flux than is the measurement of steady state
[Ca2+]i. 2)
Recordings from single cells selected on the basis of
responsiveness to cAMP allowed us to study only those cells with this
response in the population. Fluorescence determinations from a cell
population are inherently less sensitive to changes that may be
occurring in a subpopulation, particularly if the changes are small.
3) The levels of critical intracellular regulators may
differ under the two different experimental conditions. In whole cell
recordings, cells were perfused with a solution of defined composition.
A continuous supply of ATP, GTP, and an NRS was provided. In contrast, [Ca2+]i
was determined in intact cells under more physiological conditions. Under these conditions, cells may maintain different basal levels of
cyclic nucleotides, ATP, and GTP than those we provided in whole cell
recordings. Furthermore, these levels may vary during the course of the
experiments. 4) The electrophysiological conditions we used
favor isolation of and recording from nonselective cation currents, for
which Ca2+ and other cations can
be the charge carriers. We blocked
K+ currents with
TEA+ and
Cs+ (6), and
Cl was omitted from our
bath and pipette solutions. In contrast, determinations of
[Ca2+]i
are difficult to make under such rigidly controlled conditions. For all
of the above reasons, differing intracellular
Ca2+ and electrophysiological
results are likely due to inherent differences in the two assay
systems.
The role of these nonselective cation currents in the overall function of the parathyroid cell is unknown. Our previous studies suggested that these currents might play a role in intracellular Ca2+ mobilization, since Gd3+, which blocks these currents, also reduced [Ca2+]i (6). The present study, however, does not implicate a role for the type I nonselective cation conductance, which is responsive to high [Ca2+]o, in the determination of [Ca2+]i, at least under the conditions employed in this study. Whether these cation currents are conducted by Ca2+ or another cation(s) under physiological circumstances is unknown. These issues, as well as the functional role played by this current in parathyroid physiology, await the molecular identification of the responsible channel protein. This will greatly facilitate an investigation into the role of this current in extracellular Ca2+-regulated parathyroid function.
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ACKNOWLEDGEMENTS |
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These studies were supported by National Institutes of Health Grant R01-43400, a Merit Review from the Research Service, Department of Veterans Affairs, and the Northern California Chapter of the Arthritis Foundation. D. Shoback was a Clinical Investigator of the Department of Veterans Affairs during these studies.
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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. §1734 solely to indicate this fact.
Address for reprint requests: D. Shoback, 111N, Endocrine Research Unit, Veterans Affairs Medical Center, 4150 Clement St., San Francisco, CA 94121.
Received 3 February 1998; accepted in final form 24 April 1998.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Berridge, M. J.
Inositol trisphosphate and calcium signalling.
Nature
361:
315-325,
1993[Medline].
2.
Brown, E. M.
Extracellular Ca2+ sensing, regulation of parathyroid cell function, and role of Ca2+ and other ions as extracellular (first) messengers.
Physiol. Rev.
71:
371-411,
1991
3.
Brown, E. M.,
G. Gamba,
D. Riccardi,
M. Lombardi,
R. Butters,
O. Kifor,
A. Sun,
M. A. Hediger,
J. Lytton,
and
S. C. Hebert.
Cloning and characterization of an extracellular Ca2+-sensing receptor from bovine parathyroid.
Nature
366:
575-580,
1993[Medline].
4.
Brown, E. M.,
S. Hurwitz,
and
G. D. Aurbach.
Preparation of viable isolated bovine parathyroid cells.
Endocrinology
99:
1582-1588,
1976[Abstract].
5.
Chang, C. F.,
L. M. Gutierrez,
W. C. Mundina,
and
M. M. Hosey.
Dihydropyridine-sensitive calcium channels from skeletal muscle. II. Functional effects of differential phosphorylation of channel subunits.
J. Biol. Chem.
266:
16395-16400,
1991
6.
Chang, W.,
T. H. Chen,
P. Gardner,
and
D. Shoback.
Regulation of Ca2+-conducting currents in parathyroid cells by extracellular Ca2+ and channel blockers.
Am. J. Physol.
269 (Endocrinol. Metab. 32):
E864-E877,
1995
7.
Chang, W.,
T. H. Chen,
S. A. Pratt,
B. Yen,
M. Fu,
and
D. Shoback.
Parathyroid Ca2+-conducting currents are modulated by muscarinic receptor agonists and antagonists.
Am. J. Physiol.
273 (Endocrinol. Metab. 36):
E880-E890,
1997
8.
Charnet, P.,
P. Lory,
E. Bourinet,
T. Collin,
and
J. Nargeot.
cAMP-dependent phosphorylation of the cardiac L-type Ca channel: a missing link?
Biochimie
77:
957-962,
1995[Medline].
9.
De Jongh, K. S.,
B. J. Murphy,
A. A. Colvin,
J. W. Hell,
M. Takahashi,
and
W. A. Catterall.
Specific phosphorylation of a site in the full-length form of the 1 subunit of the cardiac L-type calcium channel by adenosine 3',5'-cyclic monophosphate-dependent protein kinase.
Biochemistry
35:
10392-10402,
1996[Medline].
10.
Firestein, S.,
and
F. Zufall.
The cyclic nucleotide gated channel of olfactory receptor neurons.
Semin. Cell Biol.
5:
39-46,
1994[Medline].
11.
Fitzpatrick, L. A.,
M. L. Brandi,
and
G. D. Aurbach.
Control of PTH secretion is mediated through calcium channels and is blocked by pertussis toxin treatment of parathyroid cells.
Biochem. Biophys. Res. Commun.
138:
960-965,
1986[Medline].
12.
Fitzpatrick, L. A.,
H. Chin,
M. Nirenberg,
and
G. D. Aurbach.
Antibodies to an alpha subunit of skeletal muscle calcium channels regulate parathyroid cell secretion.
Proc. Natl. Acad. Sci. USA
85:
2115-2119,
1988[Abstract].
13.
Goulding, E. H.,
G. R. Tibbs,
and
S. A. Siegelbaum.
Molecular mechanism of cyclic-nucleotide-gated channel activation.
Nature
372:
369-374,
1994[Medline].
14.
Haase, H.,
S. Bartel,
P. Karczewski,
I. Morano,
and
E. G. Krause.
In-vivo phosphorylation of the cardiac L-type calcium channel beta-subunit in response to catecholamines.
Mol. Cell. Biochem.
163-164:
99-106,
1996.
15.
Hescheler, J.,
and
G. Schultz.
Nonselective cation channels: physiological and pharmacological modulations of channel activity.
Exper. Suppl.
66:
27-43,
1993.
16.
Hoffman, F. J.,
and
R. A. Janis.
Effects of calcium channel antagonists on the phosphorylation of major protein kinase C substrates in the rat hippocampus.
Biochem. Pharmacol.
46:
677-681,
1993[Medline].
17.
Ismailov, I. I.,
and
D. J. Benos.
Effects of phosphorylation on ion channel function.
Kidney Int.
48:
1167-1179,
1995[Medline].
18.
Jones, J. I.,
and
L. A. Fitzpatrick.
Binding of [125I]iodipine to parathyroid cell membranes: evidence of a dihydropyridine-sensitive calcium channel.
Endocrinology
126:
2015-2020,
1990[Abstract].
19.
Kifor, O.,
I. Kifor,
and
E. M. Brown.
Effects of high extracellular calcium concentrations on phosphoinositide turnover and inositol phosphate metabolism in dispersed bovine parathyroid cells.
J. Bone Miner. Res.
7:
1327-1336,
1992[Medline].
20.
Leach, R. N.,
K. Brickley,
and
R. I. Norman.
Cyclic AMP-dependent protein kinase phosphorylates residues in the C-terminal domain of the cardiac L-type calcium channel alpha1 subunit.
Biochim. Biophys. Acta
1281:
205-212,
1996[Medline].
21.
Membreno, L.,
T. H. Chen,
S. Woodley,
R. Gagucas,
and
D. Shoback.
The effects of protein kinase-C agonists on parathyroid hormone release and intracellular free Ca2+ in bovine parathyroid cells.
Endocrinology
124:
789-797,
1989[Abstract].
22.
Miki, H.,
P. B. Maercklein,
and
L. A. Fitzpatrick.
Effect of magnesium on parathyroid cells: evidence for two sensing receptors or two intracellular pathways?
Am. J. Physiol.
272 (Endocrinol. Metab. 35):
E1-E6,
1997[Abstract].
23.
Miki, H.,
P. B. Maercklein,
and
L. A. Fitzpatrick.
Spontaneous oscillations of intracellular calcium in single bovine parathyroid cells may be associated with the inhibition of parathyroid hormone secretion.
Endocrinology
136:
2954-2959,
1995[Abstract].
24.
Ochi, R.
Single-channel mechanism of beta-adrenergic enhancement of cardiac L-type calcium current.
Jpn. J. Physiol.
43:
571-584,
1993[Medline].
25.
Pocotte, S. L.,
G. Ehrenstein,
and
L. A. Fitzpatrick.
Regulation of parathyroid hormone secretion.
Endocr. Rev.
12:
291-301,
1991[Abstract].
26.
Pocotte, S. L.,
G. Ehrenstein,
and
L. A. Fitzpatrick.
Role of calcium channels in parathyroid hormone secretion.
Bone
16:
365S-372S,
1995[Medline].
27.
Rotman, E. I.,
B. J. Murphy,
and
W. A. Catterall.
Sites of selective cAMP-dependent phosphorylation of the L-type calcium channel alpha 1 subunit from intact rabbit skeletal muscle myotubes.
J. Biol. Chem.
270:
16371-16377,
1995
28.
Shoback, D. M.,
and
E. M. Brown.
Forskolin increases cellular cyclic adenosine monophosphate content and parathyroid hormone release in dispersed bovine parathyroid cells.
Metabolism
33:
509-514,
1984[Medline].
29.
Shoback, D. M.,
T. H. Chen,
B. Lattyak,
K. King,
and
R. M. Johnson.
Effects of high extracellular calcium and strontium on inositol polyphosphates in bovine parathyroid cells.
J. Bone Miner. Res.
8:
891-898,
1993[Medline].
30.
Shoback, D. M.,
and
J. M. McGhee.
Fluoride stimulates the accumulation of inositol phosphates, increases intracellular free calcium, and inhibits parathyroid hormone release in dispersed bovine parathyroid cells.
Endocrinology
122:
2833-2839,
1988[Abstract].
31.
Sperelakis, N.
Regulation of calcium slow channels of heart by cyclic nucleotides and effects of ischemia.
Adv. Pharmacol.
31:
1-24,
1994[Medline].
32.
Sperelakis, N.,
Z. Xiong,
G. Haddad,
and
H. Masuda.
Regulation of slow calcium channels of myocardial cells and vascular smooth muscle cells by cyclic nucleotides and phosphorylation.
Mol. Cell. Biochem.
140:
103-117,
1994[Medline].
33.
Traugh, J. A.,
and
R. R. Traut.
Characterization of protein kinases from rabbit reticulocytes.
J. Biol. Chem.
249:
1207-1212,
1974
34.
Walter, U.,
M. Eigenthaler,
J. Geiger,
and
M. Reinhard.
Role of cyclic nucleotide-dependent protein kinases and their common substrate VASP in the regulation of human platelets.
Adv. Exp. Med. Biol.
344:
237-249,
1993[Medline].
35.
Xiong, Z.,
and
N. Sperelakis.
Regulation of L-type calcium channels of vascular smooth muscle cells.
J. Mol. Cell. Cardiol.
27:
75-91,
1995[Medline].
36.
Yau, K. W.
Cyclic nucleotide-gated channels: an expanding new family of ion channels.
Proc. Natl. Acad. Sci. USA
91:
3481-3483,
1994
37.
Zufall, F.,
S. Firestein,
and
G. M. Shepherd.
Cyclic nucleotide-gated ion channels and sensory transduction in olfactory receptor neurons.
Annu. Rev. Biophys. Biomol. Struct.
23:
577-607,
1994[Medline].