Myogenic Heartbeat in the Primitive Crustacean Triops longicaudatus

9 Pages·1997·3.6 MB·English

by  H Yamagishi

Reference:Bin/. Bull. 193: 350-358.(December, 1497)
Myogenic Heartbeat in the Primitive Crustacean
Triops longicaudatus
HIROSHI YAMAGISHI*. HITOSHI ANDOt. AND TOSHIKI MAKIOKA
Institute ofBiologicalSciences, University'ofTsiikuha, Tsukiiba, Iharaki 305, Japan
Abstract. Pacemaker mechanisms in the heart of the 1973). The cardiac ganglion situated in the heart acts as
primitivecrustaceanTriopslongicaudatuswereexamined apacemaker.Neurogenicheartmusclehasnoendogenous
electrophysiologically. The heart is tubularand the heart rhythmical properties and is driven by periodic bursting
wall consists of a single layer of myocardial cells. No activityofthe cardiac ganglion viaexcitatory neuromus-
nervecellswerefoundintheheart,eitherwithmethylene cular junctions. However, some diversity in the pace-
blue vital staining or by light microscopy of serial sec- makermechanismofcrustacean heartshasbeenreported
tions. The heartbeatsrhythmicallyatafrequencyof 120 recently. Yamagishi (1996) and Yamagishi and Hirose
to240beats/min.andeachbeatisassociatedwithaslow (1997) have shown, in the isopodLigia exotica, that the
membranepotentialchange intheheartmuscle. Theam- heart ofembryos and earlyjuveniles exhibits myogenic
plitude ofthe slow potential varies widely and no spikes heartbeats; i.e.. the activity in the myocardiumarisesen-
appearon it. The heart muscle cells are electrically cou- dogenously. Later, as thejuveniles develop, the cardiac
pled with each otherand generate synchronous slow po- ganglion initiates its spontaneous bursting activity and
tentials. No localized portion of the heart exhibited a beginstoactasaprimarypacemaker,entrainingthemus-
frequency that always preceded the others. The muscle cle activity via neuromuscular excitatory junctional po-
activity could be phase-shifted by injection of a single tentials.Thistypeoflate-developingneurogenicheartbeat
briefcurrent pulse and could be entrained to a loweror is different from that in any otherknown crustacean.
higherfrequencybyrepeatedbriefcurrentpulsesinjected Theheartbeatofbranchiopods,oneofthelowerorders
into the muscle cell. The frequency of muscle activity within the Crustacea, is said to be myogenic (reviewed
could be changedby the injection ofDC current intothe by Krijgsman, 1952;Maynard, 1960).Thisideawaspro-
muscle cell, and the change in frequency was linearly posed on the basis of the following two observations.
relatedtothe intensity ofthecurrent. When the intensity First, no nerve cells were found in the heart ofDaphnia
ofhyperpolarizing DC current exceeded a certain value, (Ingle, cited by Prosser, 1942). Second, the heartbeat of
the muscle activity disappeared abruptly, and the heart Daphniuwassuppressedbyexternalapplicationofacetyl-
stopped beating completely. These results show clearly cholinetothebody, whichwasthoughttosuppressmyo-
that the heartbeat of Triops is myogenic. The heart is genic heartbeats (Baylor, 1942; Bekker and Krijgsman,
diffusely myogenic and should be regarded as a single 1951). However, the second observation is not now re-
muscle oscillator. gardedasevidenceofamyogenic heartbeat,becauseac-
Introduction celerationofthemyogenicheartbeatbyacetylcholinehas
The heartbeat of many crustaceans is neurogenic (re- bpeaecnemraekpeorrtmedec(he.agn.,isGmreoefnbberragn,ch1i9o6p5o)d.sTihsusst,illthuenccearrtdaiianc.
viewed by Krijgsman, 1952; Maynard. 1960; Prosser, To explore the diversity in the heartbeat pacemaker
mechanismsofcrustaceans,weexaminedtheelectrophys-
Received 10April 1997:accepted6October 1997. iology of the heart of the primitive crustacean Triops
*To whomcorrespondence shouldbe addressed. E-mail: yamagishi longicaudatus (Notostraca, Branchiopoda). The aim was
D(Se'nbttiioPslr.tetrsyse,unktTuboAak.dyadocr.ejMspse:dicDaelpaarntdmeDnetntoaflBUinoimvaetrseirtiya,lsYuSsciheinmcae,,BFuancuklytoykuo,f tsoultdsetsehromiwneclwehareltyhetrhatthethheeahretabretabteaits moyfoTgreinoipcs.iTshmeyroe--
Tokyo 113.Japan. genic, and that the heart should be regarded as a single
350
MYOGENIC HEARTBEAT IN TRIOPS 351
muscle oscillator. Some of these observations have ap-
peared in abstract form (Yamagishi etat., 1993).
Materials and Methods
Animals
Adult specimensofthe freshwatertadpole shrimp Tri- tomm
ops longicaudatus have been collected routinely in June
from rice fields in Saitama, Japan. They were kept in a
freshwateraquarium in the laboratory, at room tempera-
ture, for about 3 weeks. Over a hundred specimens, 15
to26mm inbody length, wereusedfortheexperiments.
Morphology
Morphological observationsofthe heart were made in
dissected animals by vital staining with methylene blue,
orby lightmicroscopyinserial sectionsofthe heart. For
sections, hearts that had been isolated together with the
dorsalbody wallwerefixedinaqueousBouin's solution.
Then, thepreparationsweredehydrated in agradedetha-
nol-H-butanolseries,embeddedinparaffin,andcuttrans-
verselyintoserialsections(5to7//m).Thesectionswere
stained for light microscopy with Mayer's hematoxylin
and eosin.
Preparation ofthe intactheart
The tubular heart of Triops is situatedjust under the
dorsal body wall in the thoracic region. After the legs
were removed, the body was opened by a longitudinal
incisionthroughtheventralbodywall.Thedigestiveand
genital organs in the thorax were carefully removed to 50pm
mexopvoesdebtyhethweoartt.ranTshveersheeacdutasntdhraobudghomtehne bwoedryeaatlseoithree-r (A)FiDgourrseal1.vieSwchoefmtahteicaddurlatwTirnigospso.fatph,eahpeapretndoafgTer;iobps,slboondgyicsaeugdmaetnuts;.
end ofthe thorax. Thus, the heart was isolated together ca,carapace;ce.compoundeye.(B)Heartexposedbyremovalofthe
with the dorsal body wall to which it was still attached. carapaceandthedorsalbodywall.h.heart. (C)Transversesectionof
Tinhetnh,eetxhepehreiamretntparlepcahraatmiboenrwaansdpwlaascefdi,xevdenttoratlhesibdoettuopm, dlthorenagwibnto,uddayicn.atlahlrmioumusegcnhltea;trhyemecta,hnoamrlya;oxc.bawrT,dhibeaoldhyecealrwlta.lla;ndca,sucrarroaupnadcie;ngh,tihsesaurets;Iamr.e
by pins through the dorsal body wall. The chamberwas
perfused with aerated physiological saline solution
bht(ehaHarsoroemtudegb,hoeonau1t9tt6hr6tee)hg,euilaoeannxridplceyhracifdomomertpnhotses.eifvteoUrilnalodlonewrhioonfugtrhTscer.soiemopTpcohsosenidhtiseitaomilnoionnl(seym,Amw/tpa)hhs:e ctosheonerrtdterhedeedciodbnroytdrosipanltlgahceheielnhaegercattarrthwoadimlegl.uh.-sCrcTeolshneitesrctaedalcanltt,ciaeobwnmueitocrfreatoteshalteeodcrhiteesdratoraitdnnecwealanisfghrFtroleMmy-
a7N.ad4Cd)l,e,d5.t7o5;ItnhKesCs1oa,mlie5n;ecCaaatseCvsla,,r,itoe2ut;srMocdogonCtcolexn:it,nr1a;(tTiaoTnnXds;TurpSiistgo-mHlaCO)l"*w(1Map.sH wrmaaeygrneoestccaiirclrlitoeasdpceoopuretecaaotnrdd2e0raatconhad2rt3wCerr.eecodridserp.laTyehdeoenxpaerciamtehnotdse
Electrophysiology
Results
recTohredemdewmibtrhanaecopnovteennttiiaolnaolfgtlhaesshecaarptilmluasrcylemiccerlolelweacs- Anatomyandhistologvofthe heart
trode filled with 3M KC1 (resistance; 10-30Mil). An- The heart of Triops, a long tubular organ, is situated
other microelectrode, for current injection, was also in- under the dorsal surface of the body (Fig. 1A, B). It
352 H. YAMAGISHI ETAL.
TTX arrests the beat ofdecapod crustacean hearts by
suppressingneuronalspikinginthecardiacganglion(e.g.,
Tazaki. 1971). However,thespontaneousslowpotentials
minentthewhietahrtTTmuXscaltecoonfceTnrtiroaptsiownesreupnototlaf(fTehcMted(Fbiyg.tr2eBa)t.-
S\nchronous activity ofthe heartmuscle cells
0.2S Muscleactivitywassimultaneouslyrecordedfromtwo
sitesintheheart.Figure3Ashowstwoexamplesinwhich
onerecordingsitewasneartheanteriorend(uppertraces),
and the other near the posterior end ofthe heart (lower
traces). The periodic membrane potential changes were
synchronizedamongthemusclecellswithinatimediffer-
ence of less than 50ms (Al, 16ms; A2, 12ms). Thus,
the whole heart beat almost simultaneously.
In the case shown in Figure 3A1. the slow potential
recorded from the anteriorregion preceded that recorded
Figure2. (A) Electricalactivityoftheheartmuscle(uppertrace) fromtheposteriorregion.ButinthecaseshowninFigure
andcontractionoftheheart(lowertrace)recordedsimultaneously.Intra- 3A2,theslowpotentialrecordedfromtheposteriorregion
cellular membrane potential was recorded from a muscle cell in the precededthatrecordedfromtheanteriorregion. Nolocal
oanfttehreiohreraretg.io(nB)ofTtThXehheaarst,noanedffceocnttorancttiheonmautsaclseiteacitnivtihtey.ceAnrtrraolwrheegaido:n regionofthehearthadactivitythatconsistentlypreceded
Perfusingsolutionwaschangedfromnormalsalinetosalinecontaining that ofthe rest ofthe organ.
TTX(10-*M).
extends from the first to the twelfth thoracic segments
and is suspended in the pericardia! cavity. The heart is
6.4 to 8.6mm in length, but less than 1 mm in width in
the preparations used. The heart tube bifurcates near its
anteriorend and opens at the head regionjust underthe
eyes; its posterior end is closed. The heart has 11 ostia
on each side, but no defined walled arteries are present.
Theheartwalliscomposedofasinglelayerofmyocardial
cells (Fig. 1C). Myofibrils are present in the outer half
ofthe myocardial cell. No nerve cells were found in the
heart.
0.2 s
Spontaneous membranepotentialchanges in the heart
muscle
Electrical activityoftheheartmuscleandcontractions
ofthe heart were recorded simultaneously. As shown in
Figure2A,eachheartbeatfollowedaperiodicslowdepo-
larizingpotentialofthe heart muscle.Thefrequencyand
amplitude of the slow potential were not stable, but
changed during measurement in the same preparation. 0.2s
Thefrequencyandamplitudeoftheslowpotentials,mea- Figure3. (A)Intracellularmuscleactivityrecordedsimultaneously
sured about 30min after isolation, were 188.2 32.9/ from two sites in theheart: one nearthe anteriorend (uppertraces),
min (mean SD, n = 64) and 17.5 6.3mV (mean andoneneartheposteriorend(lowertraces). Recordings I and2are
boSneDt,twheneesn=lot6wh4e)p,ostrleeonswtpieapclot.tievTnehtleiya.lmsNaoxwaissmpiu5km9e.m2peotmebntr3i.aa6nlsemVappotp(eenmateriaeandl sfhfeirramoorummtltadwaniantfeefsreoirucoesurnlttytoptfrrpreaopnosasmtrveaerttriiwsooeronl,sys.tehipe(naBtrro)aetcfIeonodrtudrrfsacrsewaelegplmruaeelrnaatrftsremdouomsffctrlthaheegemfeaisncrastttismv.e(iutypRhpeearecrerktco.tonrrTaidchneeegd)
SD, n = 56). andthethird(lowertrace)fragments.
MYOGENIC HEARTBEAT IN TR1OPS 353
gradually to the control level (Fig. 4A, middle and right
portions). Afterthe stimulation (Fig. 4B), the frequency
ofthe slow potential increased conspicuously (270/min)
in the anterior region, but was almost unchanged in the
posterior region (Bl). Subsequently, the frequency and
amplitude ofthe slow potential in both regions changed
and the rhythm became irregular (B, 2 and 3); then the
rhythm became synchronized as in the control condi-
tion (B4).
B
AAAAAAAAA/vAMAAA Electrical coupling aiming the heart muscle cells
1 VWWWVAMy Twoelectrodes, oneforcurrent injection andtheother
forrecording, were inserted into musclecellsatdifferent
sites of the heart. In the experiment of Figure 5A, the
A
20 1 MW
mV
,
1s \M/\MA AA/Wl Vwvw\llOmV
Figure4. (A)Intracellularmuscleactivityrecordedsimultaneously 1s
fromtwosites:neartheanteriorend(uppertrace)andneartheposterior
end (lower trace) of the heart. Arrow head: A mechanical stimulus
(touching) withasmallglassrodwasappliedtothecentralregionof B
theheart. (B)Recordswithafastersweep.Thenumberofeachrecord (mV)
correspondstothenumberin(A). 30
20
We nextexaminedmuscle activity incut fragmentsof 10
theheart. Figure3B showsanexampleinwhichtheheart 00)1
oj
was cut transversely into four separated fragments, and 15 5
muscleactivitywasrecordedsimultaneouslyfromtwoof
them. Activity similar to that found in the intact heart
wasrecorded from the twofragments, each with itsown
rhythm. Thus, a fragment isolated from any portion of
the heart exhibited spontaneous rhythmic activity.
Amechanical stimulusappliedtotheheartbytouching
orpressingtheheartwallwithasmallglassrodproduced 0.3 0.6 0.9 1.2 1.5
(mm)
a transient systolic arrest within a localized region near Distance
twhaesstairmreusltaetdedbsyitet.heWhleocnalthaeppcleinctartailonregoifonaomfecthheanhiecaarlt IntFriacgeulrleula5r.muEslcelcetriaccatlivictoyuprleicnogrdeadmosnimgultthaenehoeuasrltymfursocmletwcoellssi.tes(Ai)n
stimulus, the rhythmical activities in the anteriorand the the heart. Distancebetween the twoelectrodes wasabout600/zm. A
posteriorregions ofthe heart gradually became synchro- hyperpolarizingcurrentpulse.500msinduration,wasinjectedintothe
nized. Wethereforeexaminedthetimecourseforachiev- heartmusclethroughoneofthetwoelectrodes,usingabridgecircuit
minegntsyinnchFriognuirzeat4ioisneoxfemtphelamruys.clBeefaocrteiviatpipelsi.caTthieonexopfertih-e it(nhuepAp2eel.rec(ttBrra)octeRosen)li.actIipnootntesenhnstiiitpay,loianfnatdnhetahcreruerdsritesentdtanhwceaearstb,e1tb0we0etenwnAeetnihnetAhceluraramenpndltiat2nu0dd0etnohAef
stimulus (Fig. 4A, left portion), the slow potentials re- recordingelectrodes. Ahyperpolarizingcurrentpulse(100ms.80nA)
corded from the anterior (upper trace) and posterior was injected intothe heart muscle through the currentelectrode, and
(lower trace) regions were synchronized at a frequency the resultant electrotonic potential was recorded at various distances
ostfim1u7l7a/tmioinn,wtihtehmausticmleedcieflflermeenmcberoafneabsouitn 3b5otmhs.reUgipoonns ewflraeoscmtprltoohdteet.ceduMroerneannatlevolagelacutrerisotdh(em.iScTEhsMcea)laemopafglai5itnumsdteeaesoafucrheeadmciehsntetalsnecceitnrfotrthooemnistcahepmoectuerhnreteainrattl
depolarized by more than lOmV, and then recovered wereplotted.
354 H. YAMAGISHI ETAL.
twoelectrodes were about 600/jm apart. When a hyper-
polarizing current pulse (500ms, 100nA) was injected,
the membrane of the muscle cell hyperpolarized (Al).
Theamplitudeofthehyperpolarizationincreasedwiththe
intensity ofthe current injected (A2. 200nA).
The spread of electrotonic potentials was examined
withheartpreparationsthathadstoppedbeatingafterper- B +30
fusion for several hours. The electrotonic potential pro-
duced by injecting a hyperpolarizing current pulse
(100ms,80nA)wasrecordedwhilethedistancebetween +20
the current and recording electrodes, along the ventral
longitudinal midlineoftheheart, was varied. Theampli-
tude of the hyperpolarization decreased with increasing +10
distance betweenthe twoelectrodes. A semi-logarithmic Stimulationphase(%)
tplaontceofbtehtewaemepnlitthuedecuorfrehnytperapnodlatrhiezatrieocnoradgianignsteltehcetrdoidse- 0C)O 1 1 .1 H1*, Jw 10I0
showstheamplitudedecreasinglinearlywiththedistance CO
(Fig. 5B). The length constant (\) calculated from the -10
relationship was 0.55 0.08mm (mean SD, n = 5). *:
-20
Effects ofcurrentinjection on the muscle activity
A brief depolarizing or hyperpolarizing current pulse -30L
(50ms,50nA)wasinjectedintotheheartmuscleatvari- Figure6. Effectsonthemuscleactivityofinjectingabriefdepolar-
ous phases ofthe rhythmical activity. The inter-peak in- izingcurrentpulse.(A)Eachrecordshowstheintracellularactivityof
terval was defined as the time between the peaks oftwo the heart muscle (upper trace) and a monitorofthe stimulus (lower
ts(hsletoiwmpueplaoaktteinootnfiatplshh.easTsehl)oewwappshoatsdeeenftiionafledtphaerseactpehpdeliiniegndtetcrhuverarlpeunbltestepwuealensnde etpbrrhnaiadcesef)eo.sdfeT(ptshohtleeiamrhruieelazacritotnir,godniacnnupgdrhraeestlnhetee)ctpocrufuolrdstreeheen(wti5an0estlemerasc-l,tpwre5aoady0kesinnAnite)neasrrevwaraltt.sehdeSaenppeepoalstriteeexrdttihofeartoravendanterdefi.rioinuoiAs-r
theonsetofthepulse;itwastakenasapercentageofthe tionsofthe stimulation phaseand the inter-peakinterval. Stimulation
precedinginter-peakinterval(controlinter-peakinterval). phaseswere27%(1)and64%(2).(B)Relationshipbetweenphaseshift
The resultant phase shift ofsubsequent beat cycles was andstimulationphase(phase-responsecurve).Fiftybriefdepolarizing
mapepalsiuerdesdtiamtutlhues;peitawkaosfttahkeetnhiarsdaspleorwcepontteangteiaolfftrhoemcotnh-e porulpsehsasweeraedvaapnpcleie(dat)vwaarsiopulsotstteidmualgaatiinosntpehaacshes.stEimauclhatpihoansephdaeslea.y(+)
trol inter-peak interval.
As shown in Figure 6A, a depolarizing pulse applied
at27%producedaphasedelay (Al),andapulseapplied injectedintotheheart muscle atvariousfrequencies. For
at64% producedaphase advance in the subsequentbeat example (Fig. 8), muscle activity at a control frequency
cycles (A2). Figure 6B shows the relationship between of 150/mincould be entrainedtoafrequency 17% lower
thephase shiftandthestimulationphase(phase-response (Al),or 13%higher(A2).byapplicationofrepeatedbrief
curve). Thecurve wasbiphasic at a stimulation phase of depolarizing pulses. Similarly, muscle activity could be
approximately40%:astimulusappliedatanearlierphase entrainedto afrequency 19% lower(Bl), or 13% higher
pslarhtooedrwunpcheaidnseaFippgrhuoardseuec7edAde,laaayphh(ay+spee).rapaodnlvdaarnaiczseitni(gm-upl).uulIssneacpoapnpltpirleaisdet,dataasat h(By2pW)e,erpotelhxaaarnmiiztnihneegdcpeoufnlftserecostl.sobfyinajpepcltiicnagt,ifoonro1f0sr,edpeepaotleadribzriinegf
36% produced aphase advance, andone applied at 85% orhyperpolarizing DC current ofvarious intensities into
produced a phase delay in the subsequent beat cycles. the heart muscle. Figure 9 shows an example. The fre-
The phase response curve was biphasic at a stimulation quency ofslow potentials increased slightly (A, +2.1%)
phaseofapproximately50%: stimuli appliedatanearlier when depolarizing DC current (10nA) was applied. The
phaseproducedaphaseadvance(-),andstimuli applied effectincreased(B, +6.8%;C, +9.6%)withtheintensity
atWaelatneerxpthaesxeampirnoedductehdeaefpfhecatsseodneltahye(m+u)sc(Flieg.ac7tBi)v.ity ohfypearpppolliaerdizciunrgrentDC(B,c2ur0rnenAt: C(,D,304n0A)n.A)Bydecconrteraassetd,
ofinjectingrepeatedbriefcurrentpulses.Abriefdepolar- (-13.0%) the frequency ofthe slow potentials, and the
izingorhyperpolarizingcurrentpulse(50ms,50nA)was decrease was larger (E, -17.1%) as the intensity ofthe
MYOGENIC HEARTBEAT IN TRIOPS 355
B
+20 -
+10 -
(f>
356 H. YAMAGISHI ETAL.
cardiac ganglion (Anderson and Cooke, 1971). In the
myogenic heart of embryos and early juveniles of the
isopodLigia,themuscleactivityisnotaffectedbyappli-
cationofTTX,exceptfortheTTX-sensitivespikeonthe
slowpotential (Yamagishi, 1996; Yamagishi and Hirose,
1997). The ineffectiveness ofTTX on the Triops heart
supportstheideathattherhythmicityofthismyocardium
is generatedendogenously.
The electrical activity of the Triops heart consists of
onlyslowpotentials,andtheamplitudeoftheslowpoten-
tialchangesgradually duringmeasurement. But no spike
potentials everappeared, even aftertermination ofahy-
perpolarizing DC current injected into the heart muscle
(e.g.. Fig. 8). The heart muscle of Triops, generating
membrane potential oscillation, may lack the ability to
generate all-or-none membrane potentials.
Characteristics ofthe heartas a single muscle
oscillator
No localized regions of the Triops heart consistently
generated potentials that preceded those ofotherregions
10 s (Fig.3A).Moreover,anycutfragmentoftheheartexhib-
Figure9. Effectson myocardial activityofinjecting DCcurrent. ited spontaneous muscle activity (Fig. 3B).These results
Therecordingelectrodewasinsertedneartheanteriorendoftheheart, suggestthatthe heart ofTriopshas diffuse myogenicity,
andthecurrentelectrodeneartheposteriorend.Ineachrecord,intracel- andthatindividualmusclecellsareinherentlyoscillatory.
al(up-lp)alriDeaCcdti(cvluiortywreeonrftttwrhaeacseh)eaapraptrlemiuesdshcoflwoenr.(1u0Dpepsp.eorIlntatrreainczseii)tnaygnod(f+mto)hneoirtcuohrryrpoeefnrttpho(elnaAcru)irzrwieannsgt Wmehcehnanitchealcesnttirmaullurse,gitohneoafnttehreiohreaarntdwpaosstaerrrieosrterdegbiyonsa
+10(A), +20(B), +30(C), -40(D), -50(E).and-60(F).
vital stainingwithmethylene blue orby microscopic ob- +20-
servation of serial sections of the heart. Ingle (cited by
Prosser, 1942) alsousedvariousnerve-stainingmethods,
but failed to find nerve cells in the heart of Daphnia. +10
Furthermore, although electron microscopic methods
have been used to study the hearts ofseveral branchio- .60 -40 -20
pods Daphnia (Stein etai. 1966). Lepidurus (Tj0nne- +20 (nA)
land et ai, 1980), Brancliinecta, Anemia, Branchipus.
and Streptocephahis (0kland et al. 1982) no neural -10
elements have everbeen reported. It is unlikely that the
heart ofbranchiopods has acardiac ganglion.
-20
Myogenicactivity
-30
TheheartbeatofTriopsisassociatedwithperiodicslow
depolarizingpotentialsinthemyocardium(Fig.2A).This
activitywasnotaffectedbyapplicationofTTX(Fig.2B).
In the neurogenic heart ofcrustaceans, the heart muscle -100
Thgaahsnegnlhoieoaenrntd(orogefevntihoeewusesedrahbnyyitmhMamlaiscyintsaytrodap,nsdb1ie9sa6td0ir:nigvPerniofsbstyehrea,ccaa1rr9d7d3ii)aa.cc cmhuasFnciglgeeuraienctti1hv0ei.tsylaoRnwedlpaottthieeonntisinhatleinpfsirbteeyqtuowefentechnyeoctfuhertrheecnhmtaunaspgcpelleioefadc.tfiErvaeiqctyuhewnpaecsrycpeolnfottattgheeed
ganglion is removed (reviewed by Krijgsman, 1952) or against the intensity (nA) ofthe depolarizing (+) orhyperpolarizing
if TTX is applied, suppressing neuronal spiking in the (-)currentapplied.FromthesamepreparationasinFig.9.
MYOGENIC HEARTBEAT IN TRIOPS 357
These responses to external inputs are characteristic of
endogenous oscillators in general (e.g.. Pavlidis, 1973).
Moreover, these effects on muscle activity could be in-
duced by current injection at any site in the heart (Fig.
11). Thus,theheartofTriopscanberegardedasasingle
muscle oscillator.
Diversity in tliepacemakermechanism ofcrustacean
hearts
B
Manyinvestigationsontheheartbeatpacemakermech-
anismofcrustaceans,mainlyofdecapods,haveledtothe
generalizationthatcrustaceanheartsareneurogenic,with
the cardiac ganglion acting as the dominant pacemaker
(reviewedby Krijgsman, 1952; Maynard, 1960; Prosser,
1973). Recently, however, Yamagishi and Hirose (1997)
showed that the cardiac pacemaker in the isopod Ligia
exoticaistransferredfromtheheartmuscletothecardiac
Figure 11. Effects on myocardial activity ofcurrent injection at ganglion during juvenile development. Now we have
two sites ofthe heart. Intracellular activity ofthe heart muscle was shownthattheheartbeatofthebranchiopodTriopslongi-
erencdor(duepdpesrimturalcteasn)e,ouasnldyoantetwneoarsittehseipnosttheerihoearrte:ndon(elonweearrttrhaeceasn)t.er(iAo)r caudatus is myogenic, and that the heart muscle acts as
HyperpolarizingDCcurrent(lOOnA)wasinjectedfor5sthroughthe a diffuse pacemaker. Clearly, the heartbeat pacemaker
electrode inserted at the anterior region, using a bridge circuit. (B) mechanismincrustaceansismorediversethanpreviously
HyperpolarizingDCcurrent(lOOnA)wasinjectedfor5sthroughthe thought.
electrodeinsertedattheposteriorregion,usingabridgecircuit.
Acknowledgments
We thank Dr. K. Sekiguchi for his kind support
beatindependently withtheirownrhythms;subsequently throughout this study. We also thank Dr. K. Oami for
their rhythms were gradually resynchronized (Fig. 4). his helpful discussion, and Dr. Darryl Macer for his
Thissuggeststhattheheartmusclecellssynchronizetheir reading ofthe manuscript. This workwas supported in
activities through interaction. The high sensitivity ofthe partbyaGrant-in-Aid(No.0864057)fromtheMinistry
Triopshearttomechanical stimulisuggestssomestretch- ofEducation. Science, Sports and Culture ofJapan to
mediatedcontributiontothe synchronization ofthe mus- H.Y.
clecells. However,spontaneousactivity wasrapidlysyn-
chronized(Fig. 3A).and the heart musclecells appearto Literature Cited
beWeleecttrhiecraelfloyrecocuopnlceldud(eFigt.hat5).the heart muscle cells are Andheerasrotno,ftMh.e,laobnsdterI..HMo.inCuonoikxe.umc1r9i7i1.cuntsN.eu./r.aEl.\pa.ctBiivoalt.io5n5:o4f4t9h-e
synchronized through electrical interactions, as in the 468.
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