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Original Paper
Dev Neurosci 1994;16:337-351
Department of Pharmacology and the
Stroke Research Centre, University of
Saskatchewan, Saskatoon, Canada
Key Words
Astrocytes
Brain
Glutamatergic neurons
Glycogen
Glycolysis
Monoamines
Oxidative metabolism
Potassium
Signalling Effect of Elevated
Potassium Concentrations and
Monoamines on Brain Energy
Metabolism at the Cellular Level
Abstract
The effects of elevated K+ concentrations and monoamine transmitters on
different cell types in the CNS and on different subcellular structures in these
cells are reviewed. Pronounced differences exist in the metabolic processes
that are stimulated by excess K+ and by adrenergic agonists, e.g., noradrena­
line. An elevation in the extracellular K+ concentration appears to enhance
neuronal-astrocytic interaction by stimulating metabolic processes involved
in (1) the promotion of supply of precursors for transmitter glutamate, and
(2) reestablishment of resting ion distribution following neuronal excitation.
The monoamine transmitters stimulate energy production and Na+,K+ATPase activity in astrocytes in a complex manner and, in so doing, facilitate
their role in ion regulation. However, in contrast to excess K+, they do not
enhance the production of astrocytic precursors for neuronal glutamate pro­
duction. Emphasis is placed on possible profound differences in metabolic
effects on excitatory and inhibitory neurotransmission and on the importance
of stimulation of glycolytic metabolism in astrocytes versus oxidative metab­
olism in neurons.
Introduction
Intrathecal administration of noradrenaline (NA) in
the brain in vivo [1], exposure of locally perfused brain
tissue to excess potassium [2] and afferent stimulation
[3,4] all lead to an increase in energy metabolism. Func­
tional activity increases deoxyglucose utilization in the
neuropil (consisting of neuronal and glial processes), but
not in the cell bodies, but the spatial resolution is not
high enough to distinguish between metabolic events in
dendrites, axons, oligodendrocytic processes and astro­
cytic processes [3, 4]. We have, therefore, attempted to
obtain further information about the localization of the
metabolic stimulation during CNS activation by measur­
ing the effects of exposure to excess potassium ion (K+)
or NA on energy metabolism and/or Na+,K+-ATPasc
activity in primary cultures of either astrocytes, normal
cerebral cortical neurons (a GABAcrgic preparation),
normal cerebellar granule cells (a glutamatergic prepara­
tion) or cerebellar granule cells displaying massive
dendritic degeneration. In the case of the neuronal cul­
tures it should be kept in mind that a glutamatergic cul­
ture is a system in which glutamate is the only transmit­
ter operating during development. Therefore, postsynap-
This paper was presented at the Satellite meeting entitled 'Functional Aspects of
Dr. L. Hertz
Energy Metabolism in Neural Tissue’ which was sponsored by the International
Society for Ncurochcmistry and held from August 28th to September 1st. 1993. in
Carcassonne. France. This paper has undergone the Journal’s usual peer review.
Department of Pharmacology
University of Saskatchewan
Saskatoon. Sask. S7N OWO (Canada)
© 1995 S. Karger AG. Base!
0378 - 5866/94/0166-0337
S $ .0 0 / 0
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Rong Huang
Liang Peng
Ye Chen
Ivan Hajek
Zhong Zhao
Leif Hertz
tic effects, mimicked by exposure to an elevated K+
concentration, glutamatergic, i.e. excitatory. In GABAergic cultures, postsynaptic events are GABAcrgic, i.e.
inhibitory.
Before discussing the effects of the stimulatory agents
wc will briefly review the pertinent aspects of energy
metabolism. Since the adult brain as an organ under nor­
mal conditions exclusively utilizes glucose as its meta­
bolic fuel, only glucose metabolism will be dealt with.
Moreover, since K+ effects to a large extent are exerted
on the Na+,K+-ATPase and stimulation of Na+,K+ATPase activity accounts for most of the increase in
energy metabolism during functional activity in the CNS
[4], we will describe differences in binding of ouabain, a
ligand which binds to the Na+,K+-ATPase [5], in neurons
and astrocytes; since NA exerts many of its effects by an
action on the phosphoinositol second messenger system
and thereby on cytosolic and potentially also on intramitochondrial calcium ion |Ca2+) concentration [6], we
will also briefly describe cellular Ca2+ homeostasis.
Energy Metabolism
338
of glucose-6-phosphate because deoxyglucose accumulation is
enhanced, hut the stimulated reaction(s) might be further ‘down­
stream'. The compounds marked with an asterisk are thought to be
precursors for transmitter glutamate.
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Fig. 1. Diagram of glucose metabolism to glucose-6-phosphate,
which can be either further metabolized during glycolysis in the
cytosol to pyruvate/Iactate or incorporated into the carbohydrate
energy store, glycogen. Under anaerobic conditions, lactate formation
is a necessity in order to reconvert the NADH formed during produc­
tion of glycerate from glyceraldchyde, but under aerobic conditions
NADH is reconverted aerobically to NAD* via the malatc-aspartate
shuttle (MAS: indicated in an oversimplified manner) and pyruvate is
available for further metabolism in the mitochondrial tricarboxylic
acid (TCA) cycle, transamination to alanine or resynthesis of glyco­
gen. Pyruvate can be introduced via two different reactions into the
TCA cycle: it can be oxidized to acctyleocnzymc A (acctylCoA) and
condensed with oxaloacclaie, which is cycled with concomitant pro­
duction of oxidatively derived energy, ultimately to regenerate
oxaloacetate, available for condensation with another molecule
acetylCoA (in both neurons and astrocytes) or it can be condensed in
astrocytes with CO: (carbon dioxide fixation) in order to provide net
synthesis of oxaloacetate. from which de novo synthesis can occur of
other TCA cycle intermediates or their derivatives, including gluta­
mate, GAB A and glutamine. TCA cycle constituents can also be used
for malic enzyme-catalyzed regeneration of pyruvate front malatc.
Here, extracellular malate is shown as being derived from citrate
which is known to be released from astrocytes, although mainly after
exposure to labeled acetate [8], but other pathways for formation of
extracellular malatc should also be considered. Processes stimulated
by an elevated potassium concentration ((KJ). noradrenaline ([NA])
or specifically by the noradrenergic a 2 agonist clonidine ([os]) are
indicated together with their astrocytic (A) or neuronal (N) localiza­
tion. The | K ] and NA effect on glycolysis is indicated on the formation
Glycolysis
As illustrated in figure l, glucose metabolism is initi­
ated by glycolysis, leading to the formation of pyruvate.
Although some glucose is metabolized via the hexose
monophosphate shunt |7], most of the glucose degrada­
tion in brain occurs via the Embden-Meyerhof pathway
illustrated in figure l. It can be seen that this pathway
includes one oxidative process, i.e., formation of l ,3biphosphoglycerate from glyceraldehyde-3-phosphate,
leading to the formation of NADH. In the presence of
oxygen, this NADH can be oxidized in the cytosol,
mainly by reacting with oxaloacetate, generated from
aspartate, which is reduced to malate and returned to the
mitochondria, where it is re-oxidized and the resulting
oxaloacetate returned to the cytosol via the malate-aspartate shuttle. In the absence of oxygen (during cerebral
ischemia), cytosolic NADH must be reconverted to
NAD by conversion of pyruvate to lactate. In the brain,
pyruvate can also be transaminated to alanine, an amino
acid which can be released from astrocytes [8] and sup-
2.5
Fig. 2. Incorporation of radioactivity from pyruvate (H. 3 mM),
glucose (■ . 3 mM ) or glutamate (□ . I mM) into glycogen, measured
as described by Subbarao et al. [68] during a 60-min period. Results
arc the m eans! SEM of 6 individual experiments from two batches of
cultures [Peng L, Zhang X, Hertz L. unpublished experiments].
Fig. 3. Contents of glycogen in the forebrains of 1-day-old
untrained chicks (□ ) or in trained chicks at different time periods after
one-trial passive avoidance learning (■ . B) in the absence of drug
administration. B Corresponds to the ITMB stage of the Gibbs-Ng
model. Results are the means of 3 experim ents! SEM from O'Dowd
et al. [ 18].
ply the amino group for synthesis of transmitter gluta­
mate in cerebellar granule cells [9], Lactate readily exits
across cell membranes [10].
The amount of ATP produced during glycolysis is
much less than the energy yield during oxidative metab­
olism of pyruvate in the tricarboxylic acid (TCA) cycle.
Nevertheless, a multitude of experimental data indicate
that glycolytically derived energy is essential to carry out
important energy-requiring processes, including active
uptake of glutamate and K+, both in brain slices, retina
and cultured astrocytes [ 11 -14], Part of the K+ clearance
from the extracellular space in the brain in vivo also
appears to be dependent upon glycolytically derived
energy [15]. Dependence of K+ clearance specifically on
the glycolytic part of glucose metabolism raises the
question as to how metabolic reactions, providing a rela­
tively small fraction of the total energy in the glucose
molecule, can provide sufficient amounts of energy for
active processes which account for a major part of the
total energy consumption in the CNS. One possible
answer to this question is that pyruvate or lactate in an
energy-requiring process may be converted back to
glycogen, a mainly astrocytically located carbohydrate
energy store [16]. In accordance with this concept, we
have recently found an equally intense production of
glycogen from pyruvate as from glucose in astrocytes in
primary cultures (fig. 2), confirming previous findings by
Dringen el al. [16] that lactate is incorporated into glyco­
gen. If oxidatively derived energy can be utilized for
resynthesis of glycogen from pyruvate/lactate, a glyco­
gen-pyruvate shuttle may operate, in which reutilization
of glycogen formed from pyruvate would allow a much
larger fraction of the total amount of energy residing in
glucose to be generated during glycolysis. Such a twoway trafficking between pyruvate and glycogen would
render the increased turnover of glycogen during func­
tional activity in the CNS [17] functionally much more
meaningful than would a simple detour of glucose equiv­
alents to glycogen en route to pyruvate. A rapid turnover
of glycogen has been demonstrated not only during
afferent activity evoked by sensory stimulation but also
in the chick forebrain. During a specific stage of a onetrial aversive learning task, during which no motor activ­
ity or sensory stimulation occur, there is a pronounced
decline in glycogen content (fig.3), but immediately
after the termination of this stage the previous glycogen
level is rapidly reestablished [18]. These findings raise
the question as to which signalling mechanisms trigger,
respectively, glycogenolysis and resynthesis of glyco­
gen. These questions will be discussed below.
Under normal conditions, brain glycogen is present in
astrocytes, but not in neurons, as is its degrading
enzyme, phosphorylase [17, 19, 20]. In liver, glycogen
can be released as glucose after degradation (a means of
increasing the blood concentration of glucose and
thereby preventing large diurnal changes in blood glu-
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Time after learning (min)
Pyruvate Metabolism
Pyruvate, which is not used for glycogen synthesis, is
further metabolized in the TCA cycle. For complete
oxidative degradation, it initially undergoes an oxidative
decarboxylation to form acetyl coenzyme A (acetylCoA), which is condensed in the TCA cycle with
oxaloacetate to form citrate. Citrate is reconverted to
oxaloacetate by one turn of the cycle, resulting in the
production of C 0 2, water, a considerable amount of ATP
and no net synthesis of any TCA cycle intermediate
(fig. 1). Citrate can also leave the mitochondria [8, 22]
and might possibly be used for production of lipids.
In addition to being converted to acetylCoA, pyruvate
can become condensed with one molecule of C 0 2 to
form oxaloacetate (CO: fixation), a process which in the
CNS is of considerable quantitative importance [23].
C 0 2 fixation, which in brain mainly or exclusively is cat­
alyzed by the pyruvate carboxylase, is confined to glial
cells, specifically astrocytes, and according to all avail­
able information is absent in neurons [24-26]. This
reaction does not generate energy (in contrast, it con­
sumes one molecule of ATP), but it leads to net synthe­
sis of a TCA cycle intermediate and is essential not only
to replace wear and tear in the TCA cycle but also for de
novo synthesis of TCA cycle intermediates and their
derivatives, including the amino acid transmitters gluta­
mate and GABA, which can be formed from the TCA
cycle intennediate a-ketoglutarate (a-KG; fig. 1). Such a
synthesis of glutamate is crucial for brain function
because after release from neurons, both glutamate and
GABA as transmitters are partly accumulated into astro­
cytes rather than into neurons [27]. In spite of formation
of glutamine from the accumulated transmitters in the
astrocytes (fig. 1) and a partial return of glutamine to
neurons and subsequent hydrolysis to glutamate, a sub­
340
stantial synthesis of other glutamate precursor molecules
in astrocytes is essential for brain function, and a-KG is
effectively accumulated into synaptosomes [28] and can
function as a direct precursor for transmitter glutamate in
cultures of cerebellar granule cells [9]. One reason that
de novo synthesis of glutamate precursors is essential is
that a considerable fraction of the accumulated glutamate
is oxidatively degraded in astrocytes, rather than used for
synthesis of glutamine [9, 27, 29].
In order to utilize glutamate or glutamine as metabolic
substrates for energy production, glutamate is initially
converted to a-KG. At least in primary cultures of
mouse astrocytes this process mainly occurs as an oxida­
tive deamination [8, 27, 30]. For maximum energy yield
the carbon skeleton is eventually converted to pyruvate/lactate [8, 30], possibly via the reaction catalyzed
by malic enzyme, another astrocyte-specific enzyme [31,
32], and reintroduced into the TCA cycle via acetylCoA.
It cannot be excluded that a glutamate molecule, initially
formed from pyruvate by C 0 2 fixation, may eventually
be reconverted to pyruvate in the malic enzyme-cat­
alyzed reaction. Such a trafficking in and out of the TCA
cycle must be confined to astrocytes since both the pyru­
vate carboxylase and the malic enzyme are found only in
these cells. Since pyruvate is also a substrate for glyco­
gen synthesis it can be expected that net synthesis of
glycogen can occur from glutamate and glutamine, an
expectation that has been verified experimentally (fig. 2).
Sodium/Potassium Transport Sites
Two mechanisms for active uptake of K+ exist in
brain tissue. One is the ‘classical’ Na+,K+-ATPase,
which is inhibited by ouabain, and the other is the co­
transport mechanism for K+,Na+and chloride (Cl- ). The
Na+,K+-ATPase is energetically driven by utilization of
ATP and this reaction accounts for a major fraction of
energy metabolism in brain preparations and for the
stimulation of energy metabolism in the neuropil follow­
ing neuronal stimulation [3, 4, 33, 34],
The cotransport system accumulates in combination 1
molecule K+, 1 molecule Na+ and 2 molecules Cl" [35],
and it is inhibited by furosemide, ethacrynic acid, or
bumetanide. Energetically it is driven by the Na+ gradi­
ent, allowing Na+ to enter along its electrochemical gra­
dient and to bring with it the two other ionic species.
Once inside the cell, accumulated Na+ stimulates
Na+,K+-ATPase activity at the intracellular, sodium-sen­
sitive site of the enzyme, leading to an active, energy-
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cose concentration), whereas muscle glycogen cannot be
released as glucose, but exclusively as pyruvate. This
applies also to astrocytes, at least in primary culture [19],
indicating that the energy obtained during conversion of
glycogen to pyruvate is for the exclusive use by the
astrocytes themselves, and cannot be utilized by neu­
rons. This is consistent with the conclusion by most, but
not all, authors that the brain does not express glucose-6phosphatase activity [19]. In contrast, the larger amount
of energy generated during subsequent oxidative degra­
dation of pyruvate or its metabolites, alanine and lactate
is available for both glial cells and neurons, and a trans­
fer of alanine from glial cells to photoreceptors has been
demonstrated in the honey-bee retina [21],
Ouabain concentration (¡j/W)
Fig. 4. Ouabain binding (□ , *, O. left ordinate) and ouabaininduced inhibition of 42K uptake into astrocytes (A , right ordinate) as
a function of the concentration of ouabain. The latter values are from
Walz and Hertz [38] (results are means of 4 - 6 individual experiments
from at least two different batches of cultures) and the former are
unpublished experiments by R. Huang, T. Clausen and L. Hertz
(results arc means of 3 - 6 individual experiments, and each of the 3
kinelically distinct binding sites has been demonstrated in cells from
at least 2 different batches of cultures).
not necessarily indicate the presence of this uptake
mechanism in neurons. We have been unable to demon­
strate any K+ uptake by the cotransporl system in cul­
tured cerebral cortical neurons |40|, but it cannot be
excluded that it may be present in cerebellar granule cell
neurons; the presence of this system is well established
in astrocytes [35, 36, 40-42],
Calcium Signalling
The concentration of free cytosolic Ca2+ ([Ca2+];)
serves as an essential intracellular messenger in virtually
any type of cell, and [Ca2+]j signalling is important in
several of the metabolic effects exerted in astrocytes by
an elevated K+ concentration or monoamines [431. This
concentration is regulated by Ca2+ entry and exit across
the cell membrane and by Ca2+ release and/or binding to
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dependent extrusion of Na+ and a concomitant uptake of
K+ [36]. The combined operation of the Na+,K+-ATPase
and the cotransport system brings about a cellular accu­
mulation of K+ and Cl~, without any net Na+ extrusion.
In order to obtain information about possible differ­
ences between neuronal and astrocytic Na+,K+-ATPases,
ouabain binding [5] to intact primary cultures of either
astrocytes (treated with dibutyryl cyclic AMP in order to
achieve morphological and probably also functional dif­
ferentiation [37]) or cerebellar granule cell neurons was
studied. The astrocytes displayed three binding sites, one
with very low affinity for ouabain and the two others
with much higher but distinctly different affinities
(fig. 4); the two latter may correspond to the a, and a 2
isozymes known to be present in these cells [Sweadner
K, Huang R, Hertz L, unpublished experiments]. The
site showing the lowest affinity (Kd>5 \xM) probably
accounts for about two thirds of the total binding (>200
pmol/mg protein), but the kinetics of the specific binding
to this site cannot be accurately determined because the
nonspecific binding is large and the dissociation of
bound ouabain rapid due the high ouabain concentra­
tions used. The site with the highest affinity accounts for
only a small fraction of the binding sites. The presence
of more than one binding site in astrocytes is reflected in
the dose-response curve for inhibition of K+ uptake into
astrocytes by ouabain [38], shown in figure 4 as a
log/probit analysis. About one third of the uptake is
potently inhibited by ouabain and is abolished at 1 \xM. A
further tenfold increase in ouabain concentration has lit­
tle, if any, effect on K+ uptake, but at ouabain concentra­
tions above 10 \iM the remaining part of the K+ uptake is
inhibited with a Kd value of close to 100 pA/. Cultured
cerebellar granule cell neurons expressed one binding
site only with a K(, value of 0.8 pM and a Bmax of =50
pmol/mg protein, i.e., they lack the low-affinity, highcapacity binding site observed in astrocytes. Binding of
ouabain has to our knowledge not been studied with
other types of neurons, but the observation that there is
no apparent compartmentation of the curve showing
inhibition of K+ uptake as a function of ouabain concen­
tration in cerebral cortical neurons [38J suggests that
there is also only one binding site for ouabain in these
cells.
Furosemide binding has not yet been studied in astro­
cytes, but a very modest binding to a neuronal tumor cell
line and to synaptosomes has been reported by Babila et
al. [39]. Since synaptosomes may be contaminated with
astrocytic membranes and since cell lines may show
non-cell-type-specific characteristics, this finding does
intracellular structures, resulting in an average [Ca2+]j in
mammalian cells of about 1x 10-4 mM. This value is
much lower than the extracellular Ca2+ concentration
(approximately 1 mM) and therefore easily dramatically
altered. The steep Ca2+ gradient across the cell mem­
brane is maintained by control of channel-mediated Ca2+
entry into the cell, energy-dependent, carrier-mediated
transport of Ca2+ out of the cell, and controlled seques­
tration of Ca2+ in the cell interior by binding to or release
from intracellular organelles. Channel-mediated Ca2+
entry into the cell may occur either by transmitter activa­
tion of receptor-operated channels, or through one of at
least four different voltage-sensitive Ca2+ channels,
physiologically described as L, N, P and T channels.
342
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Fig. 5. Effects of elevated K* concentrations on astrocytes,
expressed on a logarithmic axis, with the parameters at resting K* con­
centration (5 niAi) indicated as 100%. Parameters shown are [Ca2+]„
( • ) ; Na+.K+-ATPase activity (A): dcoxyglucose accumulation (A)
|62|: mC0 2 production from either [NC]lactate (■ ) [52] or [l'1C]glutamine (O) measured during 30 min [Huang R, Hertz L. unpublished
experiments], and CO: fixation (□ ) [26]. The values for [Ca’+], arc
unpublished experiments by Z. Zhao and L. Hertz; they are, with one
exception, the means of 3 - 6 individual experiments from 2 - 3 differ­
ent batches of cultures and the effect is statistically significant from
18 mM K* and onwards. The values for Na*.K+-ATPase activity are
from Hajek el al. [5 1] and they are the means of at least 6 different
determinations using cultures from at least 2 different batches and the
effect is statistically significant at all elevated K* concentrations. The
values for deoxyglucose accumulation are the means of 12 different
experiments from 4 different batches of cultures and the effect is sta­
tistically significant. The values for C 0 2 production are the means of
6 individual experiments from 2 different batches of cultures and the
effects are statistically significant.
The P channel [44] has mainly been described in
Purkinje cells and may convey generation of dendritic
action potentials mediated by Ca2+ entry. The Ca2+ chan­
nel which generally is assumed to play the major role in
depolarization-induced transmitter release is the N chan­
nel, and the absence of L channels has been reported in a
multitude of neuronal preparations. In contrast, differen­
tiated cerebral cortical astrocytes in primary cultures
express voltage-sensitive L channels as well as a potas­
sium-induced uptake of 45Ca and increase in [Ca2+]j,
which both are potently inhibited by the dihydropyridine, nimodipine [43]. Thus entry of Ca2+ through this
channel is the basis for the large increase in [Ca2+]j in the
presence of excess extracellular K+ in differentiated
astrocytes (fig. 5).
Release of Ca2+ from its intracellular binding sites
occurs in response to the second messenger inositol
trisphosphate, which is released together with another
messenger, diacylglycerol, as a result of receptor activa­
tion (e.e., by a-adrenergic and some serotonergic, cho­
linergic or peptidergic agonists). In the presence of free
Ca2+ ions, diacylglycerol activates protein kinase C
(PKC). In turn, PKC activity regulates events in or on
the plasma membrane, including active extrusion of Ca2+
ions and closing or opening of K+ channels. PKC stimu­
lation also exerts a multitude of effects in different subcellular domains, e.g., protein synthesis on the endoplas­
mic reticulum, glycogenolysis in glycogen particles and
phosphorylation of proteins and protein kinases, result­
ing in protein kinase ‘cascades’, capable of exerting pro­
found effects even in distal areas of the cell [43].
Propagation of waves of increased [Ca2+]i through
glial cells has recently been described by several differ­
ent groups [45-48] and may be elicited by neuronal
activity, e.g., release of glutamate. What remains to be
firmly established is the consequences of the increased
[Ca2+]j in astrocytes and to what extent neurons encoun­
tered by the wave are affected. Evidence that the
increase in [Ca2+]( may actually spread to adjacent neu­
rons has recently been obtained in experiments using
neuronal-astrocytic cocultures [49].
The increased [Ca2+]i resulting from receptor activa­
tion by, e.g., NA, has been found to cause an increase in
intramitochondrial Ca2+ concentration in muscle and in
liver cells and regulate mitochondrial functions by acti­
vating specific dehydrogenases [6]. In addition, an
increase in utilization of glutamine as a metabolic sub­
strate has been found in these tissues during receptor
activation [50]. The Ca2+ entry into the mitochondria can
be prevented by specific drugs, e.g., ruthenium red,
which accordingly can be used to establish potential cor­
relations between an increase in mitochondrial Ca2+ con­
centration and metabolic events. The functional advan­
tage of a direct stimulation of enzyme activities, com­
pared to the increase in metabolic activity evoked by uti­
lization of ATP and the resulting increase in ADP, is that
there is no decrease in the cellular content of ATP which,
in contrast, may be elevated [6].
Mechanisms of Action for Elevated Extracellular
K* Concentrations
The level of the extracellular K+ concentration may
Potassium concentration (m M )
have a direct effect at the extracellular potassium-sensi­
tive site of the Na+,K+-ATPase or the intracellular K+
concentration may exert direct effects, e.g., on enzymatic
Fig. 6. Effects of elevated K’ concentrations on cerebellar gran­
activities. In addition, elevated extracellular K+ leads to ule cell
neurons, expressed on a logarithmic axis, with the parameters
depolarization of both neurons and astrocytes, a depolar­ at resting K+ concentration (5 mM) indicated as 100%. Parameters
ization which may trigger opening of channels for Na+ shown are [Ca2*], ( • ) [Zhao Z. Hertz L. unpublished experiments]
(in neurons, but not in astrocytes, which are non- and COi production from [ MC]glucose in cerebellar granule cells (A)
excitable cells) and Ca2+ (in both astrocytes and neu­ and in cerebral cortical neurons (■ ) [52], The values for [Ca3*], are the
means of 2 - 6 individual experiments, using cultures from at least 2
rons), resulting in influx of Ca2+ (fig.5, 6) and Na+ along different
batches and the effect is statistically significant from 18 mM
their concentration gradients, and thus provides the pos­ K* and onwards. The difference between the two types o f neurons in
sibility for effects of increased intracellular Na+ and their response to elevated K* is statistically significant.
Ca2+ concentrations. Some distinction between these dif­
ferent mechanisms of action can be obtained by adminis­
tration of inhibitors, e.g., specific Ca2+ channel antago­ mM K+ will be regarded as the normal resting concen­
nists, by depletion of extracellular Ca2+ or by adminis­ tration. This might be an oversimplification since the
tration of drugs causing similar effects, e.g., veratrine, a cells may be genetically programmed to consider 3 mM
mixture of alkaloids causing opening of Na+ channels.
K+ as their natural environment.
From figure 5 it can be seen that a moderate increase
in extracellular K+ concentration to 12 mM causes an
Metaholic Stimulation Resulting from Enhanced
increase in Na+,K+-ATPase activity [51] and in
Na+,K+-ATPase Activity
It has long been known that a large fraction of energy deoxyglucose utilization [52], at least in cells that have
metabolism in brain is utilized for active transport of Na+ been treated with dibutyryl cyclic AMP to enhance func­
and K+ by the Na+,K+-ATPase and therefore is inhibited tional differentiation [37], and it also substantially
increases the intracellular K+ concentration in cultured
in the presence of ouabain [33, 34].
During functional activity in the CNS, it is the astrocytes [53]. All of these effects are statistically sig­
restoration of the resting ion distribution, i.e., normaliza­ nificant. A larger increase in the extracellular K+ con­
tion of elevated extracellular K+ and/or intracellular Na+ centration exerts only little additional stimulation and
concentrations, that is metabolically relevant, not the there is no further increase in the intracellular K+ content
maintenance of normal transmembrane ion distribution up to =50 mM K+ [53]. However, the potassium-induced
under resting conditions [4]. Therefore, only metabolic increase in [Ca2+]j, which is modest at 10 mM K+, con­
effects of K+ concentrations exceeding the normal extra­ tinues so that [Ca2+]j is 5- to 10-fold increased at
cellular level will be discussed. In the mammalian CNS 40-50 mM K+. In contrast, the oxidative metabolism of
the resting [K+]„ is approximately 3 mM, but tradition­ both lactate [52] and glutamine [Huang R, Hertz L,
ally most cell culture studies are carried out using a cul­ unpublished experiments] in astrocytes is decreased by
turing medium containing 5 mM K+, in which case 5 excess extracellular K+ (fig. 5). Nevertheless, a potas-
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Potassium Effects on Metabolism
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