Histochemical, Enzymatic, And Contractile Properties Of Skeletal Muscle .


THE JOURNALOF EXPERIMENTALZOOLOGY 214: 293 -302 (1980)Histochemical, Enzymatic, and ContractileProperties of Skeletal Muscle Fibers inthe Lizard Dipsosaurus dorsalisTODD T. GLEESON, ROBERT W. PUTNAM, AND ALBERT F. BENNETTdepart men of Physiology (T.T.G.),Department of Developmental andCell Biology (R.W.P., A.FB.), University of California, Zrvine, California 9271 7ABSTRACTLizard skeletal muscle fiber types were investigated in the iliofibularis (IF) muscle of the desert iguana (Dipsosaurus dorsalis). Three fiber typeswere identified based on histochemical staining for myosin ATPase (mATPase),succinic dehydrogenase (SDH), and aglycerophosphate dehydrogenase (aGPDH)activity. The pale region of the IF contains exclusively fast-twitch-glycolytic (FG)fibers, which stain dark for mATPase and aGPDH, light for SDH. The red region of(FOG) fibers, which stain dark forthe IF contains fast-twitch-oxidative-glycolyticall three enzymes, and tonic fibers, which stain light for mATPase, dark for SDH,and moderate for aGPDH. Enzymatic activities of myofibrillar ATPase, citratesynthase, and &PDH confirm these histochemical interpretations. Lizard FG andFOG fibers possess twitch contraction times and resistance to fatigue comparableto analogous fibers in mammals, but are one-half as oxidative and several times asglycolytic as analogous fibers in rats. Lizard tonic fibers demonstrate the acetylcholine sensitivity common to other vertebrate tonic fibers.Mammalian locomotory muscles are composed of three types of muscle fibers. Thesefibers can be classified according to their twitchcontraction times and their relative activitiesof oxidative and glycolytic enzymes. By thesecriteria most mammalian skeletal muscle fibers can be categorized as being fast-twitchglycolytic (FG), fast-twitch-oxidative-glycolytic (FOG), or a slow-twitch-oxidative (SO)fiber (Peter et al., '72). A fourth type of mammalian skeletal muscle fiber if also recognized.Limited in their distribution, mammalian tonicfibers are found in certain extraocular andmiddle ear muscles (Hess, '70). Tonic fibers differ from twitch fibers in their morphology andfunction, most notably in that tonic fibers contract slowly, developing tension in secondsrather than milliseconds, as is typical of twitchfibers. Tonic fibers are not known to exist inmammalian locomotory muscles.Mammalian twitch fiber types can often bevisually discerned. Pale muscles, or regions ofmuscles, are predominantly FG fibers, whilepink or red regions are highly oxidative andpossess primarily FOG and SO fibers (Ariano e tal., '73; Gonyea and Galvas, '79). Pale and redregions have also been reported in the skeletalmuscles of other vertebrates (Fish: Johnston etal., '75; frogs: Ogata and Mori, '64; snakes:0022-104X/80/2143-0293902.00 1980ALAN R. LISS. INC.Gans et al., '78; birds: Kiessling, '771, althoughcorrelations with specific fiber types have notalways been made.The skeletal muscles of iguanid lizards aresuperficially pale in appearance, but they frequently possess pink or red regions which aremedially located or occur near joints. Ultrastructural (Proske and Vaughan, '68; Fino1 andOgura, '72) and histochemical studies (Ogataand Mori, '64; John, '66, '70) have shown thatlizard muscles possess both twitch and tonicfibers, although their distribution and theirsimilarity to mammalian twitch and tonic fibers are not clear. A detailed review of reptilianmuscle ultrastructure and physiology has beencompiled by Guthe (in press).We have characterized the twitch and tonicfibers in the skeletal muscles of the iguanidlizard Dipsosaurus dorsalis using standardhistochemical, biochemical, and physiologicaltechniques. This comprehensive approach toreptilian fiber typing allows comparison withthe classification scheme and characteristics ofmammalian muscle fibers, and will provide aframework for investigation of other species ofCorrespondence should be addressed to Dr. Todd T. Gleeson, Department of Physiology, University of California, Irvine, California92717.

294T.T. GLEESON ET AL.reptiles. In this paper, we report the characteristics of fibers which compose the iliofibularis muscle of the hindlimb. We classifythese fibers and compare them to analogousfibers in other vertebrates. In a companionpaper (Putnam et al., 'go), we histochemicallysurvey 13 locomotory and postural musclesin Dipsosaurus and discuss the distributionand probable functions of the different fibertypes as they are defined here. A preliminaryreport of these data has appeared elsewhere(Putnarn et al., '80).MATERIALS AND METHODSA nimabDesert iguanas (Dipsosaurus dorsalis, 1%58 gm) were collected near Palm Springs,California during September 1979 (CaliforniaScientific Collecting Permit No. 514). Iguanasheld in the laboratory were provided with aphotothermal gradient and maintained on adiet of lettuce, dogfood, mealworms, and water.Muscles for comparison were taken fromfemale Wistar rats (Simonsen Labs) of 2 0 s300 gm.HistochemistryMuscles were removed from freshly decapitated animals and frozen onto cryostat chucksby plunging them into a 2-methyl-butaneliquid nitrogen bath. Frozen muscles were thenstored 1-12 days prior to sectioning. Cut sections (14 pm) were mounted on glass coverslipsand air-dried a t room temperature (25" C)0.5-4 hours prior to histochemical treatment.Myosin ATPase activity was assayed in amanner similar to that of Guth and Samaha('69, '701, but without alkaline or acid preincubation. Unfixed sections were incubated inATP incubation media (60 mM NaCl, 60 mMGlycine, 24 mM CaCI,, 23 mM NaOH, 3 mMATP, pH 9.4) a t 37" C for 10 minutes in a shaking water bath, then soaked in three 1-minutechanges of 1% CaCl, (25" C), and rinsed in four30-second changes of pH 9.0 distilled H,O. Sections were then soaked 3 minutes in 2% CoCl,(25"C), rinsed a s above, and soaked for anadditional 3 minutes in 1%(NH,),S (25" C).Stained sections were then rinsed in three 1minute changes of distilled H,O and air-dried.Extensive preliminary investigation showedthat alkaline and acidic preincubation wouldnot reliably distinguish different fiber types inlizards, as it does in mammals. Alkaline preincubation (pH 10.5, 10 minutes) resulted in astaining pattern identical to that when nopreincubation was used. Acid preincubation(pH 4.5, 3 minutes) inactivated some tonic fibers, all FG fibers, and some FOG fibers. OtherFOG fibers stained dark. Longer preincubationtime or greater acidity inactivated all but thedark FOG fibers.Succinic dehydrogenase (SDH) was assayedin a manner similar to Nachlas e t al. ('57) butwith NADH added. Unfixed sections were incubated 2 hours a t 37" C in SDH incubationmedium which contained succinic acid, 7 mM;NADH, 0.85 mM; nitro blue tetrazolium,1.2 mM; and Trizma buffer a t pH 7.4,200 mM.Sections were then rinsed %5 minutes underrunning deionized H,O, dehydrated 5 minutesin 500/0 acetone, and then air-dried.Activity of a-glycerophosphate dehydrogenase (aGPDH) was assayed according to themethod of Wattenberg and Leong ('60). Sections were incubated 2 hours a t 37" C in a nincubation medium which contained DL-&glycerophosphate, 13.9 mM; nitro blue tetrazolium, 1.2 mM; Menadione, 1.4 mM; andTrizma buffer a t pH 7.4,200 mM. Sections werethen rinsed and dehydrated as described forSDH assay.Serial sections stained for all three enzymeswere mounted on glass slides with Depex andphotographed a t 50 x magnification withKodachrome ASA 64 color slide film.Our ability to distinguish fast- and slowtwitch fibers with our techniques was confirmed by histochemically staining rat musclesin a manner identical to that used for lizards.Rat soleus, plantaris, and white vastus lateralis muscles were stained for mATPase, SDH,and aGPDH under identical conditions. Ourtreatment clearly identified the SO, FOG, andFG fibers known to exist in these muscles(Baldwin et al., '72; Ariano et al., '73).Enzymatic analysisMuscles were removed from freshly decapitated animals, trimmed of fat and connectivetissue, placed in foil envelopes, and frozen between blocks of dry ice. Frozen samples werestored a t -20" C until analysis. Both rat andlizard muscles were treated similarly unlessotherwise noted.Citrate synthase and a-glycerophosphatedehydrogenase activities were measured in frozen muscle samples (18120 mg) which werechilled to -70" C, weighed to the nearest milligram, and transferred to a smooth-surfaced,glass mortar and pestle prechilled to -70" C.Muscles were pulverized to a fine powder andtransferred to a 1-ml volume glass-glass tissuehomogenizer chilled in ice H,O. Tissues were

FIBER TYPE PROPERTIES IN LIZARD MUSCLE295homogepized on ice in 19 x (w/v) 2 mM EDTA addition of 1.0 ml 1Wo TCA. A 2-ml sample ofin 175 mM KC1, pH 7.4. Homogenates were this mixture was then analyzed for phosphatethen slowly frozen to -20" C, thawed three to according to Fiske and Subbarow ('25).five times to rupture subcellular compartAcidity and temperature of the three enments, and then centrifuged 3 minutes a t zymatic reaction mixtures were adjusted to ap1000 x g ( 5" C) to separate connective tissue proximate the intracellular conditions of bothand other debris. This supernatant was utilized the rat and Dipsosaurus muscle. Muscle pHfor enzymatic analysis. Preliminary experi- was assumed to be 0.6 pH units below restingments showed that endogenous activity was blood pH based on muscle-blood pH differencessubstantially reduced in 1000 x g super- reported by Reeves ('77). Rat enzymes werenatants, although enzymatic activity was not therefore assayed a t pH 6.9 and 37" C whilereduced relative to crude homogenates. Citrate Dipsosaurus enzymes were assayed a t pH 6.9synthase activity was assayed according to and 40" C. The preferred body temperature ofSrere ('69) in a 1-ml volume which contained this ectotherm is 40" C (DeWitt, '67). Assays0.5 mM oxaloacetate, 0.3 mM acetyl CoA, were performed in a thermostated recording0.1 mM 5-5'-dithiobis-(2-nitrobenzoate), Beckrnan Model 25 spectrophotometer. En70 mM Tris-HC1 buffer, and 5-10 p1 of the zyme activities are expressed as Ulgm protein(U pmole product formed/minute).1000 x g supernatant.Cytoplasmic a-glycerophosphate dehydrogContractile propertiesenase activity was assayed according to HolIsometric twitch and tetanic properties wereloszy and Oscai ('69) in a 1-ml reaction mixturewhich contained 0.18 mM NADH, 2.9 mM measured in iliofibulari muscles removed fromdihydroxyacetone phosphate, 71-75 mM decapitated animals. The iliofibularis muscleTris-HC1 buffer, and 1&50 pl of the 1000 x g of each limb was exposed, the distal tendon tiedsupernatant.with surgical silk ( S O ) , and the tendon cut. TheMyofibrillar ATPase activity was measured muscle was freed from the animal along withusing a technique modified from t h a t of the attached ilium of the pelvic girdle andBaldwin et al. ('77b). Muscle samples (1% placed in Ringer's solution (155 mM NaCl,80 mg) were weighed and pulverized a s de- 4mM KCl, 2 mM CaCI,, 2 mM phosphate bufscribed above. Muscles were homogenized in a fer, pH 7.2, 25" C).The iliofibularis (IF) is a cylindrically shapedsolution of 250 mM sucrose, 100 mM KCl, and5 mM EDTA adjusted to pH 6.5 a t 5" C muscle composed of parallel fibers running its(10 mllgm muscle). The homogenate was cen- entire length. It possesses a discrete red regiontrifuged a t 1100 x g for 10 minutes (5" C) and which is medially parallel to the femur. Tothe supernatant was discarded. The pellet was measure the contractile properties of the redresuspended and washed (10 mllgrn) twice in and white regions of the IF, the red region fi0.5% Triton-X solution (175 mM KCl, 2 mM bers of one muscle were dissected free, leavingEDTA, 0.5% Triton-X 100, pH 6.8 a t 5" C) fol- a n intact white region for study. The whitelowed by two washings (10 mllg) in 150 mM region of the contralateral IF was similarly reKC1 (pH 7.0, 5" C). Each wash was followed by moved and the remaining red region used forcentrifugation of 1100 x g for 10 minutes a t contractile studies. The order in which the red5" C. Following the final wash, the myofibrillar and white IF were studied was randomized.pellet was resuspended in 15 x (wlv) 150 mMThe iliofibularis (white or red) was attachedKC1 in 30 mM Tris (pH 7.4,5" C). Protein con- to a Grass FTO3C force transducer with ancentration of this suspension was then deter- inextensible chain tied to the distal tendon. Themined by the Biuret technique and the final ilium was tied to a large glass rod and theprotein concentration adjusted to 5 mglml with muscle then lowered into a 300 ml thermoKC1-Tris.stated (40 loC) bath of aerated Ringer's soluCa -activatedmyofibrillar ATPase activity tion. Force transducer output was displayed onwas assayed in a 2-ml reaction volume which a Textronix Model RM 564 dual-beam storagecontained 100 p1 myofibrillar protein solution, oscilloscopeand recorded on a Grass Model 79D200 p1 of MgS0,-CaC1, solution (10 mM polygraph.Contractile properties were measured afterMgSO,, 0.1 mM CaCl,, 20 mM Na azide in30 rnM Tris, pH 6.9, 40" C), 200 p1 ATP solu- 10 minutes of thermal equilibration. The retion (50 mM in 30 mM Tris, pH 7.0,25" C), and sponses to single stimuli (40-60 volts, 1 msec1.5 m130 mM Tris-HCI buffer. The ATPase re- duration) were recorded. The muscle was stimaction was stopped after 2 minutes with the ulated through two platinum-wire surface

296T.T. GLEESON ET ALelectrodes using a Grass SD9 stimulator. Contraction time (CT) was measured from the initiation of a mechanical response to the peak ofthe twitch on the oscilloscope. Half-relaxationtime (112 RT) was the time from peak mechanical response to the point during recovery whentension had fallen to one-half maximum twitchtension. Ten twitches or less were sufficient toobtain these data. Maximum tetanic tensionand fusion frequency were then obtained bystimulating the muscle a t high frequencies(4&60Hz) for short duration (1-3 seconds).The lowest frequency a t which the tensioncurve appeared smooth was defined as the fusion frequency; the tension generated a t thisfrequency was defined as the maximum tetanictension. After a brief rest, muscles were thentwitched a t a frequency of 1 pulselsecond (1msec pulse duration) for 5 minutes. The fatigueindex of the muscle was defined as the ratio ofthe final to initial twitch tension generatedduring this 5-minute stimulus regime.The sensitivities of the white and red regionsof the iliofibularis to acetylcholine were measured in muscles which were treated as abovebut were not used to determine fatigue index.One ml of an acetylcholine solution (3 mglml)was added to the 300-ml bath in the region oft h e muscle and t h e contractile responserecorded.Tensions a r e reported a s g m l c m r o s s sectional area. Cross-sectional area was calculated by dividing muscle weight by its length.All data are reported as mean SEM.RESULTSHistochemical characteristicsHistochemical staining of the Dipsosaurusiliofibularis for myosin ATPase (rnATPase) activity demonstrates two classes of muscle fibers. The majority of fibers stain darkly formATPase, indicating high enzyme activity. Asmall percentage of fibers stain very lightlyunder the same conditions. The mATPase-lightfibers are small in diameter and occur mediallyin the iliofibularis near the femur.Succinic dehydrogenase (SDH) activity isused as a n index of the oxidative capacity ofmuscle fibers. Histochemical staining for SDHactivity reveals a distinct region of high SDHactivity (Fig. 1). This oxidative pocket liesnearest to the femur and corresponds to thevisibly red region observed during dissection.Fibers which possess high SDH activity includenot only those with low rnATPase activity, butalso a subset of high mATPase fibers.Fig. 1. Cross section of an entire iliofibularis muscle ofDipsosaurus illustrating the distribution of fiber types.Clear fibers demonstrate high mATPase and aGPDH activities, low SDH. These fibers are termed M: fibers andcomprise the white iliofibularis. Stippled fibers (FOG) havehigh activities for all three enzymes. Dark fibers (tonic)havelow mATPase, high SDH, and uGPDH activities. FOG andtonic fibers compose the red iliofibularis.All fibers within the iliofibularis demonstrate substantial a-glycerophosphate deh drogenase(aGPDH) activity when stainedfor this enzyme. Fibers within the oxidativezonedescribed by SDH staining appear slightlyless dark when stained for &PDH than fibersin the white region, but the difference is notgreat.The staining pattern in the muscle demonstrates three fiber types in Dipsosaurusskeletal muscle (Fig. 1) - two fiber classeswhich are fast-twitch in their contractile properties and one fiber type which our contractiledata @resented below) indicate is a tonic fiber.The bulk of the iliofibularis, white or pale inappearance, is composed of fibers which staindarkly for mATPase and &PDH, lightly forSDH. We have labeled these fibers fastglycolytic (FG) fibers, adopting the classification system for mammalian fibers proposed byPeter et al. ('72). The majority of fibers within

FIBER TYPE PROPERTIES IN LIZARD MUSCLEthe red portion of the iliofibularis histochemically stain darkly for all three enzymes. Weclassify these fibers as fast-twitch-oxidativeglycolytic (FOG) fibers. The third fiber typepresent inDipsosaurus iliofibilaris muscles is atonic fiber, histochemically characterized ashaving low mATPase activity, moderate-tohigh SDH and aGPDH activities, and restricted to the medially located red region of themuscle.Fiber type abundance and distributionThe iliofibularis ofDipsosaurus is a cylindrical muscle composed of 900-1,000 fibers. Themuscle illustrated in Figure 1contains 979 fibers of which 493 are in the white region, 486 inthe red region. The white region comprises approximately 70% of the cross-sectional area andmass of the iliofibularis, and is composedlargely of FG fibers. The white region of muscles from seven lizards possessed 96100% FGfibers, the remainder being FOG fibers. The FGfibers of the white iliofibularis are large, withdiameters between 111 and 143 pm. The redregion is a mixed compartment, with FOG fibers representing 5977% of the total red region fiber population. Tonic fibers represented21-33% and FG fibers represented between 0and lWo of all red zone fibers (Fig. 2). Fiberdiameters of FOG and tonic fibers within thered region are similar, ranging between 54 and81 pm.Enzymatic analysisThe enzymatic activities of citrate synthase,a-glycerophosphate dehydrogenase, and myofibrillar ATPase in the red and white portionsof the lizard iliofibularis are compared in Figure 3. The red iliofibularis of Dipsosaurus hasapproximately seven times the citrate synthaseactivity relative to the white iliofibularis(207 ? 18versus 30 ? 3 Ulgm protein), reflecting the greater oxidative capacity in the redregion. In contrast, the white IF possesses approximately five times the a-glycerophosphatedehydrogenase activity relative to the red IF(530 ? 33 versus 98 5 11Ulgm protein). Theenzyme a-glycerophosphate dehydrogenase isused as an index of glycolytic capacity. Themyofibrillar ATPase activity of the white region (190 ? 15 U/gm protein) is approximatelytwice that of the red IF (98 ? 9 Ulgm protein).The differences between the red and white iliofibularis for all three enzymes are highly significant (P 0.0001, t-tests).The activities of these enzymes in Dipsosaurus iliofibularis are compared to those of rat297soleus, plantaris, and white vastus lateralismuscles in Figure 3. In rat, the soleus muscle is8&9W0 SO fibers, the plantaris 55% FOG fibers and 40% FG fibers, and the white vastus95-10W0 FG fibers (Baldwin et a]., '72; Arianoet al., '73). The white iliofibularis exhibits fiveto eight times t h e aGPDH activity of r a tglycolytic fibers (FG FOG). Citrate synthaseactivity in the lizard red IF is roughly half thatfound in the rat soleus, which is predominatelyoxidative. Myofibrillar ATPase activity in thelizard iliofibularis muscle approximates thatfound in the slow-twitch fibers of the r a t soleusmuscle.Physiological propertiesTwitch and tetanicproperties. The contractileproperties of the red and white regions ofDipsosaurus iliofibularis muscles were measured in muscles from lizards of both sexes witha mean body weight of 34 2 2 gm. Musclesranged from 1.4 to 2.7 cm in length with meancross-sectional areas of the red and white regions of 0.013 - 0.002 cm2 and 0.027 ?0.003 cm2, respectively. Surgical separation ofwhite from red regions prior to measuring contractile tension damages fibers on t h eperiphery of both fiber bundles; thus, activecross-sectional areas are somewhat less andtensions per cross-sectional area somewhatgreater than those actually reported here.The contractile properties of the red andwhite iliofibularis are summarized in Table 1.Both twitch and tetanic tensions generated inthe red IF are less than that in the white region,although the red I F has a twofold greatertetanic-twitch ratio than the white IF. At 40" C,both regions generate peak twitch tension inless than 27 msec; the red region, however,takes 50% longer to reach one-half relaxationthan the white region.The muscle fibers of the red region show substantial fatigue resistance relative to fibers inthe white region. After a standardized 5-minute stimulation regime, fibers in the red regionstill generated approximately 80% of their initial peak tension. In contrast, white region fibers fatigued readily and after 5 minutes ofstimulation generated only 30% of their initialtension.ACh sensitivity. The red and white regions offour iliofibulari muscles were tested for acetylcholine (ACh) sensitivity to detect the functional presence of tonic fibers. The white region, shown histochemically to contain 9%1 W o FG fibers, showed no contractile response

T.T. GLEESON ET ALFGFOG TonicWhite Iliof ibularis327/ 7Red Iliofi bular sh366/6Fig. 2. Distribution of fiber types in Dipsosaurus red andwhite iliofibularis. Numbers along horizontal axis representpercentages of the total population within each region composed of each fiber type. Ratios denote mean number of fiberscounted per animallsample size.Diososaurusto repeated acetylcholine applications. Application of acetycholine to the bath containingthe red iliofibularis resulted in a contractileresponse in each of four muscles. In three muscles, repeated ACh application caused a slowcontracture which generated tensions of 29130% of the twitch tension stimulated electrically just prior to ACh application. In contrast to the milliseconds required by the muscleto twitch and relax, ACh contraction a t 40" Crequired 1%15 seconds to generate peak tension, and another 20-35 seconds to reach onehalf relaxation. Acetylcholine sensitivity wasalso demonstrated in a forearm muscle ofDipsosaurus, the flexor palmaris superficialis(FPS). Histochemical evaluation of the red regions of both the FPS and iliofibularis demonstrate an abundance of FOG fibers in additionto tonic fibers (Fig. 1; Putnam et al., '80).We tested the possibility that FOG fibersrather than our presumed tonic fibers weresensitive to ACh by testing the ACh sensitivityof the peroneus muscle. The peroneus is a lowerhindlimb muscle whose white region containsFG and FOG fibers, but no tonic fibers (Putnamet al., in press). The twitch and tetanic contractile properties of the peroneus are summarized in Table 1. In four experiments, the FGand FOG fibers of the peroneus showed no sensitivity to acetylcholine application.White Rat(37Y 1OI dRed WhiteMyofibrillar ATPaseS O FOG F GFGCitrate SynthaseRed WhiteIF IFflRed WhiteSO FOG F GFGAa-GlycerophosphateDehydrogenoseS O FOG F GFGFig. 3. Myofibrillar ATPase, citrate synthase, and a-glycerophosphate dehydrogenase activities in muscles of Dipsosaurus and rat. Red and white iliofibularis a s defined in Figure 1. Rat fibers are derived from the following hindlimbmuscles: SO, soleus; FOG and FG mix, plantaris; FG, white vastus lateralis. Temperatures denote enzyme reaction and bodytemperatures.

299FIBER TYPE PROPERTIES IN LIZARD MUSCLEDISCUSSIONFiber types in DipsosaurusHistochemical analysis of Dipsosaurus iliofibularis muscle for myosin ATPase, SDH, and&DH has allowed us to characterize threedistinct fiber types according to their contractile speeds (Barany, '67) and by their oxidativeand lycolyticcapacities. n & r n a t i cand contractile data support our histochemical interpretation.Fast-glycolytic (FG) fibers of the white iliofibularis are large muscle fibers which staindarkly for mATPase and aGPDH. The enzymatic and contractile profile of this fiber type isbased on the properties of the white IF, which isnearly 10W0 FG fibers (Fig. 2).Fast-twitch-glycoltyic fibers possess fivetimes the glycolytic activity and twice thernATPase activity as fibers making up the redIF, indicating that these fibers are adapted tohigh rates of energy utilization. This view issupported by the high rate of tension generation and rapid fatigue of these fibers (Table l).The low oxidative activity of FG fibers suggestthat a large portion of the energy they utilizeduring vigorous contraction is generated viaanaerobic glycolysis. This fiber type representsthe majority of muscle fibers in Dipsosaurus(Putnarn et al., 'SO), and the enzymatic profileof this fiber closely reflects the large anaerobiccapacity that this animal utilizes during vigorous muscular activity (Bennett and Dawson,'72).Fast-twitch-oxidative-glycolytic (FOG) fi-bers are smaller in diameter than FG fibers andhave high histochemical activities of all threeenzymes. SDH and aGPDH staining intensitiesare quitedark, so one may infer that FOG fibersare metabolically the most active of all threefiber types. Enzymatic analysis of lizard FOGfibers was limited by our inability to locate apure FOG region in any of 13 Dipsosaurusskeletal muscles (Putnam et al., '80). The biochemistry of the FOG fiber is reflected in theenzymatic activity of the 3:l mix of FOG andtonic fibers in the red iliofibularis. The similarintense staining of FOG and tonic fibers forSDH in the red IF, coupled with the high enzymatic citrate synthase activity in that regionof the muscle (Fig. 31, suggest that lizard FOGand tonic fibers are several times more oxidative than lizard FG fibers.The contractile properties of FOG fibers arerepresented by the twitch characteristics of thered iliofibularis (Table 1).Tonic fibers do notrespond mechanically to the brief (1msec) electrical stimulus (Luff and Proske, '79) of the typeused in this study; thus, the twitch characteristics of the red I F are due to FOG activityalone. The key contractile properties of theFOG fiber are its rapid twitch-contraction-time(CT) and its high fatigue resistance. The CT ofFOG fibers is fast, supporting their high histochemical mATPase activity. When repeatedly stimulated to twitch, FOG fibers shownearly three times the resistance to fatigue relative to FG fibers. FOG fatigue resistance isreflective of its higher oxidative capacity.The tonic fibers of Dipsosaurus stain lightlyfor rnATPase, dark for SDH, and moderate forTABLE1. Contractilepmpertiesof two hindlimbmusclesin Dipsosaurus'IlliofibularisRed regionContractile characteristicTwitch CT (msec)Twitch 112 RT (msec)Tetanic tension (gm/cm2)(Po)Fusion freq (Hz)Fatigue indexAcetylcholine sensitivity'40" C, x z SEM (sample size).'White region different than red region: P 0.05.:'White region dirrerent than red reglon: P 0.005.% White regionPeroneusWhite region

300T.T. GLEESON ET AL.aGPDH. This histochemical pattern is similarto that of mammalian slow-twitch-oxidativefibers (Barnard et al., '71; Peter et al.,'72). Our'71; Frederick e t al., '78) might further differentiate this low mATPase fiber type. We havebeen intentionally conservative in our classification of lizard fiber types, and do not feel that aqualitative technique such as histochemistryshould be used alone to distinguish betweenvery similar fiber types.interpretation of this fiber type as tonic is basedon its contractile properties. Slow and prolonged contraction in response to acetylcholineapplication is a characteristic attributed toamphibian and mammalian tonic fibers (Kuffler and Vaughan-Williams, '53; Hess andComparisons to other vertebrate fiber typesPilar, '63; Lannergren and Smith, '66; EngelExamination of Dipsosaurus iliofibularisand Irwin, '67; Lehmann and Schmidt, '79). Thered region of the iliofibularis is acetylcholine- muscle identifies two types of fast-twitch fiberssensitive, as are other muscles which contain and one tonic fiber type. A similar fiber typefibers with the above histochemical charac- composition is found in other muscles of Dipteristics. Regions of muscles which do not con- sosaurus (Putnam et al., '80) and in other vertetain this type of fiber, on the other hand, do not brates. Two twitch and one tonic fiber typesrespond to ACh. The tetanic-twitch tension have been reported in anuran amphibiansratio of the red I F also suggests the presence of (Lannergren and Smith, '66; Engel andtonic fibers. Lizard tonic fibers have been Irwin, '67; Luff and Proske, '79) and in snakesshown to respond only to electrical stimulation (Talesara, '73; Talesara and Mala, '76). Twoof 10 Hz or greater (Proske and Vaughan, '68); twitch and two tonic fiber types were identifiedthus, tonic fibers may be recruited during in the lizards Tiliqua, Cnemidophorus, andtetanic tension determination but not during Iguana, based on ultrastructural or electrotwitch tension determination. This would re- physiological characteristics (Proske andsult in high tetanic-twitch tension ratios Vaughan, '68; Finol and Ogura, '72, '77). There(PJP,) for muscles containing tonic fibers. We is abundant additional evidence for the presbelieve the high PJP, ratio of the red iliofibu- ence of both twitch and tonic fibers in lizardslaris relative to the white region indicates the (reviewed by Hess, '70; Guthe, in press), alpresence of tonic fibers

Rat soleus, plantaris, and white vastus later- alis muscles were stained for mATPase, SDH, and aGPDH under identical conditions. Our treatment clearly identified the SO, FOG, and FG fibers known to exist in these muscles (Baldwin et al., '72; Ariano et al., '73). Enzymatic analysis Muscles were removed from freshly decapi-