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Receptor-secretion coupling in mast cells
TIPS - December 1980
460
Receptor-secretion coupling in imast cells
of IgE
John Foreman The cross-linking of membrane receptors in m4st cells and barophi: leucocytes inidates a series of bt?&emical
changes, involving methpIations in membrane
lipids. The restdt
of
r&c events ir the increase of membrane permeability to calcium which leads to the secretion oj hbtamine.
mast cell is a secretory cell which liberates its histamine-containing granules in responw to a number of specific stimuli. It has proved to be a pwerful and interesting model in the s!udy of secretion, and the release of hijstamine from mast cells following an immunological stimulus to the membrane is a basic pathological mechanism of allergic diseac :.
The
The membrane fithmhs Mast cells anld tt&ir circulating analogues, basophil leucocytes. possess membrane receptors which bind specifitally the so-called homocytotropic antibody, now known to be IgE in most species’. There are about I to 5 X 1(r IgE receptors per cell and the equi!ibrium dissociation constant for the binding ‘ofIgE to its cellular receptor is about lO_” M. The forward rate constant of the binding reaction is estimated to be of the order of 1CPbf-’ s-l and the reverse reaction is relatively slow with a raue constant of lo-’ smi. The bincling of IgE antibody to its c&ular receptrzr does not in itself bring about activation of the cell. Indeed, mast cells have varying degrees of saturation of their IgE receptors. depending on the circulating IgE! level. The signal to the mast cell or basophil leucocyte to secrete histaminecontaining granules is the union of the &-bound IgE antibody with antigen present in the surrounding milieu. The antigen must. of course. be that to which the IgE has specifically-directed binding sites (Fig, I). It has long been appreciated that monovalent antigens can combine with only one of the two antigen biding sites that each IgE molecule possesses (Fig. Ia) and this does not Ctig about activation of the cell for secretion. To be active, an antigen must be divalent or multivalent, an observation which can lx interpreted in terms of bridging of two antigen biding sites either witbin a sbgle IgE molectde (Fig. I b) or between adjacent IgE mole&es (Fig. Ic). Other evidence suggests that it is indeed s EbnicrlNtiHo&lld BimlXd&l pm ,980
and can cross-link adjacent receptors for lgE on the cell membrane. Antireceptor antibody stimulates histamine secretion (Fig. lg). Whilst the evidence for the cross-linking receptors in the cell membrane being the stimulus to secretion is compelling. an alternative hypothesis should be mentioned. It is based on the observation that certain peptides derived from the lgE antibody molecule are capable of eliciting h’tstaminc secretion, and the hypothesis states that binding of antibody to the receep tor does not itself activate secretion, but conformational change in the antibody brought about. say. by antigen binding, can expose these peptide fragments which induce release. Although the hypothesis does not easily explain the results of experiments such as those in Fig. la and lg. there is no solid evidence against it. The IgE receptors. IO0 to 500 x I@’of them per mast cell appear to be monovalent with respect to IgE binding, but. so far, no biochemical activity has been associated with the receptor. even in its purified form. It is a glycoprotein of molecular weight about 80,000 and there is some dispute about whether it consists of two subunits. Some elegant experiments employing photobleaching have demonstrated that the receptor is freely mobile in the lateral plane of the membrane’.
the bridging of adjacent IgE molecules which activates the secretory processof the mast cell or basophil leucocyte. The evidence may be summarized as follows: (1) Antibody (anti-IgE) directed against the Fc portion of the IgE molecule cross-links adjacent IgE molecules and induces histamine secretion (Fig. Id). (2) Concanavalin A binds to the carbohydrate associated with the R region of the IgE molecule. thereby cross-linking adjacent molecules and inducing histamine secretion (Fig. le). (3) Chemically dimetised IgE molecules added to the extracellular fluid bind to unoccupied IgE receptors on the cell surface. inducing cross-linking of IgE receptors. and histamine secretion occurs (Fig. If). (4) The IgE receptor. obtainled from a rat basophil leukaemia. has been isolated and purified. It is a glycoprotein with a molecular weight of about 80,000 and an antibody has been prepared against it. The antireceptor antibody (an IgG) is divalent
cakIum It has been known for some time that histamine secretion from mast cellsstimulated by the cross-linking of antibody on the membrane requires the presence of extracellular calcium, and it was suggested
fig. 1. Diagrammuic repnsrrumionofantigen-lgE&mtingon dmmawcell membme and other mechanism of cm&inking JgE receptonin Ihe cell membrane. Those reoctiotukaahg to ceIlactiwion andsecnkm we indicatcdby +.
TIPS - December I MO bl
wcta?ion
of antigen from
I@: OF of I#:
from 11s cellular receptor Furrhcsrmclre. rhcrc i\ no app:ircnt reJl\trihutlt)n. ‘capping’ (Jr en&c!
W&of
rcieptclrx Jurinp tnz
time when in.Jcti\aricrn is occurring recent experiment<
calcium into the mii\t cell tnduco phosphoplation
S,kmc
shg)u that the cntn
of
>pcittr
of a membrane
prrllcin.
and it is interesting to speculate that thl\ ma) be rclatrd to the formanon
and inacrt-
vation of calcium charm&.
A role for phospholipid in histamine XXrerion
was identified
before
the current
awareness of the phenomena of membrane activation lb4
Imkl
Fi#. 2. (a} TLme-counr o.fthedecay of mast ceil response to calcium /DUOM mg an anttgen-IgE stun&s HLvamme scretion ir measured as a @wtion of increawtginterval between stunt&ton in the abwnce of calrtum and the addittim ofralcium. (b) Time-course ofthe change of “Co uptake a&r antigen stunulat~on CeILr &WI the rame pool were eitherchallenged with antigen in the presence of Va: t = 0, or ceils were chal&nged wtth antigen In the rmsence ofnonlbelled
in theabsence of
calciwn
and ?a
nw addpdat
t min ajier stimulation.
The coonnc~lrepresenn
Va
uptake
stimulation.
>lrch as phosphatid>l
and before the inrerpretatlon
receptor
function
mosaic
model for Immunologically-tntg~r~J
retion from m&cell\ potrntiatcd
stimulus
and
histamine
sozretion.
The
evidence for this hypothesis came from experiments with the calcium ionophore A23 187* which transports calcium across the mast cell membrane from extracellular to intracellular compartments and thereby induces histamine secretion. Experiments with the ionophore demonstrated a major role for extracellular calcium ions in the initiation of histamine secretion, although it has been suspected for some time that release of calcium from intracellular stores may induce secretion in certain circumstances. and recent experipriricipal
secretion. but it is known that injection of calcium into mast cells using a micropipette will induce the exocytosis of granules’. A further piece of sumed to initiate histamine
evidence
consistent
with
:he hypothesis
that a rise in intracellular calcium concentration triggers histamine secretion comes from experiments employing phospholipid
of
ill terms elf rhc fluidmembrane\ hi\taminc WC-
has been shlrsn to I-W
%pccific;~ll) b!
scrine or its I!%.)-analogue.
by analogy with muscle that a rise in intracellular calcium ion concentration might be the link between membrane
inosit,4
turnover.
pho\phariJll rhe lipid &elf
not being capat:le of stimulating 5ecrefi0n Furthermore. this acuon of phusphariJ!l serine ua\ clear& related to rhe role of Calcium in histamine secretion anJ it has hen shobn that phosphatidtl
serinr
increases
the membrane permrabilit! to calcium in cells stimulated b> a cross-linhinp stimulus. The potentiar:ng effect of phosphatidvl serine in immu~~ologicall~ mediated Wcrc‘-
of
non is assoclatcJ with the binding of vesi-
these vesicles with mast cells induces his-
cles of rhe phospholipiJ 10 the mast cell?;. II
tamine secretion presumably
is interesting that a ai number N-substiruteit Jer;vati\rs of phosphatlJ>l
vesicles loaded
calcium ment’.
into
with calcium.
the
Fusion
by liberating
intracellular
compart-
serine. which are not metabolizrci.
are not
active on m;lst cell5’ IlMCtiV8tiOIl
These observations
is reasonable to expect that any cell employing a rise of intracellular calcium
scheme
concentration
It
proposed
fir vep
wrll
b> Axelrod
into 3
and WI-
leagues (see Fig. 3). Cross-linking
of IgE
thisa.
sess a mechanism for sequestering calcium.
receptors on the mast cell initiates a sequence of membrane phospholipici
Even before the ionophore was available it had been suggested that the immunoloy:ical reaction on the mast cell membrane opened channels to allow calcium to p21ss from the extracellular mmpartment to the intracellular compartment. ?Jsing radiolabelled calcium, it has been shown that the resting permeability of the mast cell membrane to calcium is about 8 fmol cm-’ s-l and that this rises to a peak of 380 fmol cm-’ s-’ after an optimal immunological stimulus. It has recently been demonstrated that direct cross-linking of IgE receptors themselves, using an antirecep tor antibody, is sufficient to increase the membrane permeability to calcium and thereby induce histamine secretion4. There has been no measurement of the rise in free cytosolic calcium which is pre-
so that the activity does not continue in-
mctah>lism
ments with A23187
are in keeping wir h
definitely.
for activation
Mast
will also pos-
and enr>me
acrivation
which
cells appear to possess a resuhs rn !he formanon ~),f calcium chan-
mechanism for turning off the increase in membrane permeability to calcium which the immunological stimulus brings about. Fig. 2 shows that after immunological stimulation of the membrane. the mem-
nels. I‘he initial cve~rt is Jecarboxylation
brane permeability
[iv=. A second meth~ttrdnsferasc! c~XIWrts
to calcium and the sec-
ethanolamme nhich is then methklated b> a nagnesium-dependent meth! Irransfera.% l to produce an l-munomrth>I
retion of histamine decline with a half-time
the
Since secretion is targel? complete 30 s after the stimulus, the deca) of membrane calcium permeabilit) cannot be the only means of terminating or limiting secretion and it is assumed that the action of calcium is rapidly terminated by pumping into mitochondria and other organelles or out of the cell. It is worth noting that the inactivation of membrane permeability to calcium is not the result of dis-
ethanola&e
of about
1 min.
of
phosphatiJ>-I serine to Corn1 phasphatiJ!l
methtl
derivatitr into
of
Jenv;r-
ph<)sphdtid>i
phosphatid!I
choline
which serves as a substrate for a membrane phospholipax It. The I!,sophosphariJ)
I choline produced is considered t<) be responsible for the opening t?f a ca!ciUnl channel. High concentrations *>flysophosphatidyl
choline
release histamine
frclm
mast cells bv a mechaniJm independent calcium and-in which the cells are Ipd. &is
effea
of but
on calcium channels is con-
Coltium chonnsl
require a source of ATP. It Seems fairly clear that microtubulrs are not involved in the mechanism of histamine secretion”, but actin has been demonstrated in mast cells and indirect evidence for a role for microfilament contraction in secretion has been presented.
Read@ IW I Metzger. W. (lY77)
it\ Re~vptours und Recognirinn series A. Vol. 4 (Cuatrwasas, P. and Greaves. M. E. eds), pp. 75-l If?. Chapman Halt, London
2 Foreman, J. C., Mungar. J. L. and Gump&s, B. D. (1973) fValure (t.oRdon) 254.249-251 Fatty acid
3 Johansen. T. (19811) Eur. 1. Phurmuco/. 62, 329-334
ln0dd
4 Ishizaka. T.. Foreman. J. C.. Sterk. A. R. and lshizaka. K. (197Y)Pmc. ,Vcul.Acad.Sci. U.S.A. 76.5858-586: S Kannv. T.. Cochrane. D. E. and Dougtas, W. W. (1973) ciin. f. PhvsioI. Phurmocol. 51 I Fig. 3. A dd.gtanmakswtmutyofri’mmbtatt~t~niv ronsia%re~ftu beiwolvcdin the acfiwtion ofmosf ce& to secr~%dtcir htatmine. Rewptor cro.sr&tking initiates o seqtuwe ofpharphalipid methyhatkmsand the activation ofo ~pho+ae which rogerher bring about the opening of aL+wn channeZs.C$rlic AMP inot shown) IS believed to be nsrnnsibk f21 Luuti~min~ the &vmel. twssib& by o protein kitwe media&d phosphoryltttion. II
s&red to k a more selective and specific effect. Tbt, methylation reactions have bee 1 measured in mast cells and increased mei hylation hours in immunologically stin ulated telli. Phos,pholipid methylation pre-edes caIonm exchange across the membrane and is jnd~!~ndeftt of the prer ence of extrau:flular c.alcium’. Inhibitors of the methqltcmsfemses such as S-isoburyryl-3_cii~a-ader!osine prevent both calcium exchange across the membrane anid histamQe secretion. Thus, the original observaPion that phosphatidyl serine potent&es i&amine secretion and increases membrane permeiabilirlj to calcium ~ollo~~g receptor ~~,~~krng, may now be interpreted in terms of phosphatidyi serine being a lbmitirlg substrate in the pbosphoiipid metabolism necessary for calcium chatnnet fo~ation. Phosphatidyl inositol tlurnover in mast cells increases when the ce Usare stimulated by cross-linking of receptors but the increased turnover is bolh dependent on c&ium and ~stirnnlated by phosphatidyl serme, hence there is no eGdencc that phc!sphatidyl inositol turnover is anything more than an epiphenomenon of mast c&l activati&. C+iCAMP It has been proposed that inactivation of
calcium channels may limit, in part, secretory activity in mast cells. The question of the mechanism of this ~activ~tion therefore arises. Cyclic AMP inhibits histamine secretion, and there is evidence which suggests that a fall in intracellular cyclic AMP a~rn~n~s the activation of secretion. AS the levels of cyclic AMP return towards their basal value. inactivation of calcium channels is occurring. It has been demonstrated that cyclic AMP prevents the stimufated calcium ~munolog~lIy exchange across the mast cell membrane and it is, therefore, tempting to suggest that a fall in intracellular cyclic AMP level occurs during calcium channel formation and the induction of histamine secretion, whilst a return to basal level of this cyclic nucleotlde inactivates calcium channels and limits secretion. Mast &Is possess a protein lkinase which is activated by cyclic AMPand may be responsible for the phosphorylation of membrane protein (see above) during inactivation of calcium channels. The events discussed in this review relate to very early stages in the secretion of histamine and its control (Fig. 3). It would be naive to pretend that they are more than a fraction of the whole picture. Little is known about the events occurring after the entry of calcium other than that they
IOOI-If!04 Theoharidcs. T. C. and Douglas, W. W. (1978) sr&w7?202.1143-1145 Manin. f. W. and Lagunoff. D. 204.63 I-633
(I 979)
Science
Ishizaka. T.. Hirata. F.. Ishizake. K. and Axehod. 1. (1980) Proc. Nod. Acnd. Sci U.S.A. 77. 190.3-1906 Cockroft. S. and Gomperts. &a&em. J. 178.691-687
3
D.
(19%))
Lagunoff. D. and Chi. E. Y. (1976) 1. Ceil Blat. 71.182-195
John Foreman gntduoted with o B.Sc. and then Ph.D. in pharmaco@y &m University Wlege Lwtdoti bejweproueding to Uttivetxity College Hospital &fedical school to &t&t tk MB., BS. degrees. A&? hota appoitttmert~ in Pete&wugh he spent 18 months as Visiting Instructor of Medicine ‘01 Johtu Hopkins lJniver&y. He is now tt lecturer in pha?mawlogy at University G&ge Lonikwt.
460
Receptor-secretion coupling in imast cells
of IgE
John Foreman The cross-linking of membrane receptors in m4st cells and barophi: leucocytes inidates a series of bt?&emical
changes, involving methpIations in membrane
lipids. The restdt
of
r&c events ir the increase of membrane permeability to calcium which leads to the secretion oj hbtamine.
mast cell is a secretory cell which liberates its histamine-containing granules in responw to a number of specific stimuli. It has proved to be a pwerful and interesting model in the s!udy of secretion, and the release of hijstamine from mast cells following an immunological stimulus to the membrane is a basic pathological mechanism of allergic diseac :.
The
The membrane fithmhs Mast cells anld tt&ir circulating analogues, basophil leucocytes. possess membrane receptors which bind specifitally the so-called homocytotropic antibody, now known to be IgE in most species’. There are about I to 5 X 1(r IgE receptors per cell and the equi!ibrium dissociation constant for the binding ‘ofIgE to its cellular receptor is about lO_” M. The forward rate constant of the binding reaction is estimated to be of the order of 1CPbf-’ s-l and the reverse reaction is relatively slow with a raue constant of lo-’ smi. The bincling of IgE antibody to its c&ular receptrzr does not in itself bring about activation of the cell. Indeed, mast cells have varying degrees of saturation of their IgE receptors. depending on the circulating IgE! level. The signal to the mast cell or basophil leucocyte to secrete histaminecontaining granules is the union of the &-bound IgE antibody with antigen present in the surrounding milieu. The antigen must. of course. be that to which the IgE has specifically-directed binding sites (Fig, I). It has long been appreciated that monovalent antigens can combine with only one of the two antigen biding sites that each IgE molecule possesses (Fig. Ia) and this does not Ctig about activation of the cell for secretion. To be active, an antigen must be divalent or multivalent, an observation which can lx interpreted in terms of bridging of two antigen biding sites either witbin a sbgle IgE molectde (Fig. I b) or between adjacent IgE mole&es (Fig. Ic). Other evidence suggests that it is indeed s EbnicrlNtiHo&lld BimlXd&l pm ,980
and can cross-link adjacent receptors for lgE on the cell membrane. Antireceptor antibody stimulates histamine secretion (Fig. lg). Whilst the evidence for the cross-linking receptors in the cell membrane being the stimulus to secretion is compelling. an alternative hypothesis should be mentioned. It is based on the observation that certain peptides derived from the lgE antibody molecule are capable of eliciting h’tstaminc secretion, and the hypothesis states that binding of antibody to the receep tor does not itself activate secretion, but conformational change in the antibody brought about. say. by antigen binding, can expose these peptide fragments which induce release. Although the hypothesis does not easily explain the results of experiments such as those in Fig. la and lg. there is no solid evidence against it. The IgE receptors. IO0 to 500 x I@’of them per mast cell appear to be monovalent with respect to IgE binding, but. so far, no biochemical activity has been associated with the receptor. even in its purified form. It is a glycoprotein of molecular weight about 80,000 and there is some dispute about whether it consists of two subunits. Some elegant experiments employing photobleaching have demonstrated that the receptor is freely mobile in the lateral plane of the membrane’.
the bridging of adjacent IgE molecules which activates the secretory processof the mast cell or basophil leucocyte. The evidence may be summarized as follows: (1) Antibody (anti-IgE) directed against the Fc portion of the IgE molecule cross-links adjacent IgE molecules and induces histamine secretion (Fig. Id). (2) Concanavalin A binds to the carbohydrate associated with the R region of the IgE molecule. thereby cross-linking adjacent molecules and inducing histamine secretion (Fig. le). (3) Chemically dimetised IgE molecules added to the extracellular fluid bind to unoccupied IgE receptors on the cell surface. inducing cross-linking of IgE receptors. and histamine secretion occurs (Fig. If). (4) The IgE receptor. obtainled from a rat basophil leukaemia. has been isolated and purified. It is a glycoprotein with a molecular weight of about 80,000 and an antibody has been prepared against it. The antireceptor antibody (an IgG) is divalent
cakIum It has been known for some time that histamine secretion from mast cellsstimulated by the cross-linking of antibody on the membrane requires the presence of extracellular calcium, and it was suggested
fig. 1. Diagrammuic repnsrrumionofantigen-lgE&mtingon dmmawcell membme and other mechanism of cm&inking JgE receptonin Ihe cell membrane. Those reoctiotukaahg to ceIlactiwion andsecnkm we indicatcdby +.
TIPS - December I MO bl
wcta?ion
of antigen from
I@: OF of I#:
from 11s cellular receptor Furrhcsrmclre. rhcrc i\ no app:ircnt reJl\trihutlt)n. ‘capping’ (Jr en&c!
W&of
rcieptclrx Jurinp tnz
time when in.Jcti\aricrn is occurring recent experiment<
calcium into the mii\t cell tnduco phosphoplation
S,kmc
shg)u that the cntn
of
>pcittr
of a membrane
prrllcin.
and it is interesting to speculate that thl\ ma) be rclatrd to the formanon
and inacrt-
vation of calcium charm&.
A role for phospholipid in histamine XXrerion
was identified
before
the current
awareness of the phenomena of membrane activation lb4
Imkl
Fi#. 2. (a} TLme-counr o.fthedecay of mast ceil response to calcium /DUOM mg an anttgen-IgE stun&s HLvamme scretion ir measured as a @wtion of increawtginterval between stunt&ton in the abwnce of calrtum and the addittim ofralcium. (b) Time-course ofthe change of “Co uptake a&r antigen stunulat~on CeILr &WI the rame pool were eitherchallenged with antigen in the presence of Va: t = 0, or ceils were chal&nged wtth antigen In the rmsence ofnonlbelled
in theabsence of
calciwn
and ?a
nw addpdat
t min ajier stimulation.
The coonnc~lrepresenn
Va
uptake
stimulation.
>lrch as phosphatid>l
and before the inrerpretatlon
receptor
function
mosaic
model for Immunologically-tntg~r~J
retion from m&cell\ potrntiatcd
stimulus
and
histamine
sozretion.
The
evidence for this hypothesis came from experiments with the calcium ionophore A23 187* which transports calcium across the mast cell membrane from extracellular to intracellular compartments and thereby induces histamine secretion. Experiments with the ionophore demonstrated a major role for extracellular calcium ions in the initiation of histamine secretion, although it has been suspected for some time that release of calcium from intracellular stores may induce secretion in certain circumstances. and recent experipriricipal
secretion. but it is known that injection of calcium into mast cells using a micropipette will induce the exocytosis of granules’. A further piece of sumed to initiate histamine
evidence
consistent
with
:he hypothesis
that a rise in intracellular calcium concentration triggers histamine secretion comes from experiments employing phospholipid
of
ill terms elf rhc fluidmembrane\ hi\taminc WC-
has been shlrsn to I-W
%pccific;~ll) b!
scrine or its I!%.)-analogue.
by analogy with muscle that a rise in intracellular calcium ion concentration might be the link between membrane
inosit,4
turnover.
pho\phariJll rhe lipid &elf
not being capat:le of stimulating 5ecrefi0n Furthermore. this acuon of phusphariJ!l serine ua\ clear& related to rhe role of Calcium in histamine secretion anJ it has hen shobn that phosphatidtl
serinr
increases
the membrane permrabilit! to calcium in cells stimulated b> a cross-linhinp stimulus. The potentiar:ng effect of phosphatidvl serine in immu~~ologicall~ mediated Wcrc‘-
of
non is assoclatcJ with the binding of vesi-
these vesicles with mast cells induces his-
cles of rhe phospholipiJ 10 the mast cell?;. II
tamine secretion presumably
is interesting that a ai number N-substiruteit Jer;vati\rs of phosphatlJ>l
vesicles loaded
calcium ment’.
into
with calcium.
the
Fusion
by liberating
intracellular
compart-
serine. which are not metabolizrci.
are not
active on m;lst cell5’ IlMCtiV8tiOIl
These observations
is reasonable to expect that any cell employing a rise of intracellular calcium
scheme
concentration
It
proposed
fir vep
wrll
b> Axelrod
into 3
and WI-
leagues (see Fig. 3). Cross-linking
of IgE
thisa.
sess a mechanism for sequestering calcium.
receptors on the mast cell initiates a sequence of membrane phospholipici
Even before the ionophore was available it had been suggested that the immunoloy:ical reaction on the mast cell membrane opened channels to allow calcium to p21ss from the extracellular mmpartment to the intracellular compartment. ?Jsing radiolabelled calcium, it has been shown that the resting permeability of the mast cell membrane to calcium is about 8 fmol cm-’ s-l and that this rises to a peak of 380 fmol cm-’ s-’ after an optimal immunological stimulus. It has recently been demonstrated that direct cross-linking of IgE receptors themselves, using an antirecep tor antibody, is sufficient to increase the membrane permeability to calcium and thereby induce histamine secretion4. There has been no measurement of the rise in free cytosolic calcium which is pre-
so that the activity does not continue in-
mctah>lism
ments with A23187
are in keeping wir h
definitely.
for activation
Mast
will also pos-
and enr>me
acrivation
which
cells appear to possess a resuhs rn !he formanon ~),f calcium chan-
mechanism for turning off the increase in membrane permeability to calcium which the immunological stimulus brings about. Fig. 2 shows that after immunological stimulation of the membrane. the mem-
nels. I‘he initial cve~rt is Jecarboxylation
brane permeability
[iv=. A second meth~ttrdnsferasc! c~XIWrts
to calcium and the sec-
ethanolamme nhich is then methklated b> a nagnesium-dependent meth! Irransfera.% l to produce an l-munomrth>I
retion of histamine decline with a half-time
the
Since secretion is targel? complete 30 s after the stimulus, the deca) of membrane calcium permeabilit) cannot be the only means of terminating or limiting secretion and it is assumed that the action of calcium is rapidly terminated by pumping into mitochondria and other organelles or out of the cell. It is worth noting that the inactivation of membrane permeability to calcium is not the result of dis-
ethanola&e
of about
1 min.
of
phosphatiJ>-I serine to Corn1 phasphatiJ!l
methtl
derivatitr into
of
Jenv;r-
ph<)sphdtid>i
phosphatid!I
choline
which serves as a substrate for a membrane phospholipax It. The I!,sophosphariJ)
I choline produced is considered t<) be responsible for the opening t?f a ca!ciUnl channel. High concentrations *>flysophosphatidyl
choline
release histamine
frclm
mast cells bv a mechaniJm independent calcium and-in which the cells are Ipd. &is
effea
of but
on calcium channels is con-
Coltium chonnsl
require a source of ATP. It Seems fairly clear that microtubulrs are not involved in the mechanism of histamine secretion”, but actin has been demonstrated in mast cells and indirect evidence for a role for microfilament contraction in secretion has been presented.
Read@ IW I Metzger. W. (lY77)
it\ Re~vptours und Recognirinn series A. Vol. 4 (Cuatrwasas, P. and Greaves. M. E. eds), pp. 75-l If?. Chapman Halt, London
2 Foreman, J. C., Mungar. J. L. and Gump&s, B. D. (1973) fValure (t.oRdon) 254.249-251 Fatty acid
3 Johansen. T. (19811) Eur. 1. Phurmuco/. 62, 329-334
ln0dd
4 Ishizaka. T.. Foreman. J. C.. Sterk. A. R. and lshizaka. K. (197Y)Pmc. ,Vcul.Acad.Sci. U.S.A. 76.5858-586: S Kannv. T.. Cochrane. D. E. and Dougtas, W. W. (1973) ciin. f. PhvsioI. Phurmocol. 51 I Fig. 3. A dd.gtanmakswtmutyofri’mmbtatt~t~niv ronsia%re~ftu beiwolvcdin the acfiwtion ofmosf ce& to secr~%dtcir htatmine. Rewptor cro.sr&tking initiates o seqtuwe ofpharphalipid methyhatkmsand the activation ofo ~pho+ae which rogerher bring about the opening of aL+wn channeZs.C$rlic AMP inot shown) IS believed to be nsrnnsibk f21 Luuti~min~ the &vmel. twssib& by o protein kitwe media&d phosphoryltttion. II
s&red to k a more selective and specific effect. Tbt, methylation reactions have bee 1 measured in mast cells and increased mei hylation hours in immunologically stin ulated telli. Phos,pholipid methylation pre-edes caIonm exchange across the membrane and is jnd~!~ndeftt of the prer ence of extrau:flular c.alcium’. Inhibitors of the methqltcmsfemses such as S-isoburyryl-3_cii~a-ader!osine prevent both calcium exchange across the membrane anid histamQe secretion. Thus, the original observaPion that phosphatidyl serine potent&es i&amine secretion and increases membrane permeiabilirlj to calcium ~ollo~~g receptor ~~,~~krng, may now be interpreted in terms of phosphatidyi serine being a lbmitirlg substrate in the pbosphoiipid metabolism necessary for calcium chatnnet fo~ation. Phosphatidyl inositol tlurnover in mast cells increases when the ce Usare stimulated by cross-linking of receptors but the increased turnover is bolh dependent on c&ium and ~stirnnlated by phosphatidyl serme, hence there is no eGdencc that phc!sphatidyl inositol turnover is anything more than an epiphenomenon of mast c&l activati&. C+iCAMP It has been proposed that inactivation of
calcium channels may limit, in part, secretory activity in mast cells. The question of the mechanism of this ~activ~tion therefore arises. Cyclic AMP inhibits histamine secretion, and there is evidence which suggests that a fall in intracellular cyclic AMP a~rn~n~s the activation of secretion. AS the levels of cyclic AMP return towards their basal value. inactivation of calcium channels is occurring. It has been demonstrated that cyclic AMP prevents the stimufated calcium ~munolog~lIy exchange across the mast cell membrane and it is, therefore, tempting to suggest that a fall in intracellular cyclic AMP level occurs during calcium channel formation and the induction of histamine secretion, whilst a return to basal level of this cyclic nucleotlde inactivates calcium channels and limits secretion. Mast &Is possess a protein lkinase which is activated by cyclic AMPand may be responsible for the phosphorylation of membrane protein (see above) during inactivation of calcium channels. The events discussed in this review relate to very early stages in the secretion of histamine and its control (Fig. 3). It would be naive to pretend that they are more than a fraction of the whole picture. Little is known about the events occurring after the entry of calcium other than that they
IOOI-If!04 Theoharidcs. T. C. and Douglas, W. W. (1978) sr&w7?202.1143-1145 Manin. f. W. and Lagunoff. D. 204.63 I-633
(I 979)
Science
Ishizaka. T.. Hirata. F.. Ishizake. K. and Axehod. 1. (1980) Proc. Nod. Acnd. Sci U.S.A. 77. 190.3-1906 Cockroft. S. and Gomperts. &a&em. J. 178.691-687
3
D.
(19%))
Lagunoff. D. and Chi. E. Y. (1976) 1. Ceil Blat. 71.182-195
John Foreman gntduoted with o B.Sc. and then Ph.D. in pharmaco@y &m University Wlege Lwtdoti bejweproueding to Uttivetxity College Hospital &fedical school to &t&t tk MB., BS. degrees. A&? hota appoitttmert~ in Pete&wugh he spent 18 months as Visiting Instructor of Medicine ‘01 Johtu Hopkins lJniver&y. He is now tt lecturer in pha?mawlogy at University G&ge Lonikwt.