Activation of atrial muscarinic K+ channels by low concentrations ofβγ subunits of rat brain G protein

Effects of G protein subunits from rat brain on cardiac K⁺ channel was examined in single atrial cells of guinea-pig, using patch clamp techniques. We found that 10 pM concentration of rat brain subunits preparation could activate the atrial muscarine receptor-gated K⁺ channel (I_K.ACh). Neither the detergent, CHAPS, used to suspend nor the boiled preparation activated I_K.ACh. Furthermore, preincubation of subunits preparation in Mg²⁺-free solution, which easily inactivated -GTP-S, did not affect -activation of I_K.ACh. We concluded, therefore, that subunits themselves can activate I_K.ACh.Supported by the grants from the Ministry of Education, Culture and Science of Japan and from the Calcium Signal Workshop on Cardiovascular Systems     

Anticipatory postural changes induced by active unloading and comparison with passive unloading in man

Normal human subjects, sitting in a chair, were required to maintain stable elbow flexion against loads of 0.5 kg or 1.0 kg. Unloading was affected either passively by the experimenter, or actively with the subject's own contralateral arm. Elbow angle, force exerted by the load, and electromyographic activity (EMG) of biceps and triceps muscles of both arms were recorded and averaged. Passive unloading was followed by a reduction of biceps EMG activity, starting 50–80 ms after weight lift, and by an upward deflection of the forearm. With active unloading, however, a reduction of the biceps EMG activity slightly preceded the onset of unloading (0–30 ms). This reduction of the actively unloaded arm occurred at about the same time as the activity of the contralateral unloading arm. In this experiment, the unloaded forearm maintained an almost stable position. Thus, the anticipatory adjustment of elbow posture, observed when unloading was performed by the subject, appears to optimize limb stability during the mechanical perturbation.     

Density heterogeneities of hepatitis C virus in human sera due to the binding of β-lipoproteins and immunoglobulins

Heterogeneities in the density of hepatitis C virus RNA-carrying material (HCV-RNA-CM) found in human sera (1.03–1.20 g/cm³) are attributed to the binding of low-density lipoproteins and/or of IgG. In some sera HCV-RNA-CM seems to be nearly totally bound to -lipoproteins and cannot be precipitated by anti-IgG (); in others more than 95% of HCV-RNA-CM is bound to IgG and cannot be precipitated by anti--lipoprotein. Furthermore, there are sera from which HCV-RNA-CM can be completely be precipitated by either anti--lipoprotein or anti-IgG (), pointing to a binding of the two serum proteins to the same HCV-RNA-CM. There are other sera from which HCV-RNA-CM can be partially precipitated by the one or the other antiserum, leaving behind fractions, which are bound to -lipoprotein or to IgG. HCV-RNA-CM cannot be precipitated from some sera either by anti--lipoprotein or by anti-IgG ().     

On the regulation of the expressed L-type calcium channel by cAMP-dependent phosphorylation

The Ca²⁺ channel subunits _1C-a and _1C-b were stably expressed in Chinese hamster ovary (CHO) and human embryonic kidney (HEK) 293 cells. The peak Ba²⁺ current (I _Ba) of these cells was not affected significantly by internal dialysis with 0.1 mM cAMP-dependent protein kinase inhibitor peptide (mPKI), 25 M cAMP-dependent protein kinase catalytic subunit (PKA), or a combination of 25 M PKA and 1 M okadaic acid. The activity of the _1C-b channel subunit expressed stably in HEK 293 cells was depressed by 1 M H 89 and was not increased by superfusion with 5 M forskolin plus 20 M isobutylmethylxanthine (IBMX). The _1C-a·₂·₂/ complex was transiently expressed in HEK 293 cells; it was inhibited by internal dialysis of the cells with 1 M H 89, but was not affected by internal dialysis with mPKI, PKA or microcystin. Internal dialysis of cells expressing the _1C-a·₂·₂/ channel with 10 M PKA did not induce facilitation after a 150-ms prepulse to +50 mV. The Ca²⁺ current (I _Ca) of cardiac myocytes increased threefold during internal dialysis with 5 M PKA or 25 M microcystin and during external superfusion with 0.1 M isoproterenol or 5 M forskolin plus 50 M IBMX. These results indicate that the L-type Ca²⁺ channel expressed is not modulated by cAMP-dependent phosphorylation to the same extent as in native cardiac myocytes.     

Immunohistochemical studies on S-100 cells in the anterior pituitary gland of Sprague Dawley rats and spontaneous dwarf rats

The S-100 cells in the pituitary glands of adult male Sprague Dawley rats (SDs) and spontaneous dwarf rats (SDRs) were immunohistochemically examined using anti-S-100 and anti-S-100 monoclonal antibodies. The immunoreactive cells against S-100 protein were divided into three subtypes on the basis of their immunore-activity against subunits of S-100 protein: S-100 dominant type (the -type cell), S-100 dominant type (the \-type cell) and immunoreactive against both S-100 and S-100 (the -type cell). In the SD, -type cells represented 26% of the total S-100 immunoreactive cells (S-100 cells) and were localized in the peripheral area of the ventral region of the pituitary gland. This type of cell was observed forming clusters, with more abundant cytoplasm than the -type cell. The proportion of -type cells was 53%. They were diffusely distributed throughout the gland, and their processes were thicker than those of the -type cell. In the SDR, the proportion of -type cells was 55%, and they were observed throughout the gland. In contrast, -type cells totalled 12% and were localized in small areas of the central and peripheral region of the gland. The proportion of -type cells was 21% in the SD and 33% in the SDR and they were observed forming small clusters in both animal groups. The proportion of -type cells compared with the total of S-100-immunoreactive cells was significantly higher (P < 0.05) in the SDR than in the SD, while the proportion of -type cells was markedly lower (P < 0.05).     

Genome Organization and Expression of the Penicillium stoloniferum Virus S

The complete sequences of two double-stranded RNAs (dsRNAs) (referred to S1 and S2) of Penicillium stoloniferum virus S (PsV-S) were established. The S1 dsRNA was 1,690bp in length, and it contained a unique open reading frame (ORF) of 539 amino acids (molecular weight of 62kDa, referred to P62). The S2 dsRNA was 1,523bp in length, and also it contained one ORF of 434 amino acids (molecular weight of 47kDa, referred to P47). Both S1 and S2 ORFs were identified only on the positive strand of each dsRNA segment. A sequence motif of (5-CUG-3) was found at the 3-termini of the positive strands of PsV-S1 and S2 dsRNAs. The predicted amino acid sequences of S1 dsRNA showed high sequence homology with the putative RNA-dependent RNA polymerases of RNA viruses. Near full-length and positive-sense single-stranded RNAs derived from the S1 and S2 dsRNAs were detected from the PsV-infected host cell. The expressed proteins of P62 and P47 showed a positive reaction against PsV-S antiserum in Western blot analysis. Phylogenetic analysis using the RDRP sequences and the capsid proteins of the various partitiviruses revealed that PsV-S is a definite member of the partitivirus, the family Partitiviridae, and especially clusters well along with D. destructiva virus 1 and 2.     

Ultrasonography gives directly but noninvasively elastic characteristic of human tendon in vivo

To obtain an insight into tendon elasticity during human movement, a real-time ultrasonography was applied to the contracting tibialis anterior muscle. The insertion point of fascicles onto the aponeurosis was clearly visualized, and its position relative to a fixed marker on the skin moved proximally (1) according to the increasing dorsiflexion force (F) with a fixed ankle joint. Notably, the 1 – F relationship in the tendon was found to be quadratic in nature (F = c1²; c=1.48 2.24, r=0.985 0.992, n=9) as has been reported in the isolated tendon, although the F – 1 curves were slightly underestimated in comparison with the stiffness constant estimated from tendon architecture. This underestimation might be caused by changes in the height of the foot arch with the application of force.     

Synoviocytes in chronic synovitis in situ and cytokine stimulated synovial cells in vitro neo-express α1, α3 and α5 chains of β1 integrins

The expression of the 1 integrins was examined immunohistochemically in synoviocytes from normal synovial membrane and from chronic synovitis of different aetiology and intensity. Normal synoviocytes were 61-positive but lacked 1 through 5. In mild inflammation type A synoviocytes neo-expressed 1, 3, and 5 chains. In severe inflammation both type A and B synoviocytes expressed 3, 4, 5, and 6 chains. The effects of inflammatory cytokines, as single agents or in combination, on the 1 integrin expression in cultured normal synoviocytes was determined by immunocytochemistry and flow cytometry. The 1 chain, while absent in unstimulated synoviocytes, was induced by interleukin-1 (IL-1), tumour necrosis factor- (TNF-), and interferon- (INF-). This effect was enhanced by combining IL-1 and TNF-. Expression of the 3 chain was up-regulated by IL-1 and, more intensely, by IFN-. Transforming growth factor (TGF-) inhibited the up-regulating effect of IL-1 and antagonized the effect of IFN- on 3 chain expression. Expression of the 5 chain was up-regulated significantly by co-stimulation through IL-1 together with TGF- or TNF-. Thus, the 1 integrin profile of cytokine activated synoviocytes in vitro resembled that of synoviocytes in synovitis in situ. These data suggest that IL-1, TNF-, IFN-, and TGF- are likely to be among the effectors regulating 1 integrin expression in synoviocytes in vivo.     

Acute Phase Responses and Cytokine Secretion in Chronic Fatigue Syndrome

This study addresses the hypothesis that clinical manifestations of chronic fatigue syndrome (CFS) are due in part to abnormal production of or sensitivity to cytokines such as interleukin-1 (IL-1) and IL-6 under basal conditions or in response to a particular physical stress: 15 min of exercise consisting of stepping up and down on a platform adjusted to the height of the patella. The study involved 10 CFS patients and 11 age-, sex-, and activity-matched controls: of these, 6 patients and 4 controls were tested in both the follicular and the luteal phases of the menstrual cycle, and the remainder were tested in only one phase, for a total of 31 experimental sessions. Prior to exercise, plasma concentrations of the acute phase reactant ₂-macroglobulin were 29% higher in CFS patients (P < 0.008) compared to controls. Secretion of IL-6 was generally higher for CFS patients (~38%), however, this difference was statistically significant only if all values over a 3-day period were analyzed by repeated-measures ANOVA (P = 0.035). IL-6 secretion correlated with plasma ₂-macroglobulin in control subjects at rest (R = 0.767, P = 0.001). Immediately after exercise, the CFS patients reported greater ratings of perceived exertion (P=0.027) compared to the healthy control subjects. Ratings of perceived exertion correlated with IL-1 secretion by cells from healthy control subjects (R = 0.603, P = 0.022), but not from CFS patients, and IL-1 secretion was not different between groups. Exercise induced a slight (<12%) but significant (P = 0.006) increase in IL-6 secretion, but the responses of the CFS patients were not different than controls. Furthermore, no significant exercise-induced changes in body temperature or plasma ₂-macroglobulin were observed. These data indicate that under basal conditions, CFS is associated with increased IL-6 secretion which is manifested by chronically elevated plasma ₂-macroglobulin concentrations. These modest differences suggest that cytokine dysregulation is not a singular or dominant factor in the pathogenesis of CFS.     

Effect of molsidomine on ex vivo platelet aggregation and plasma guanosine 3′∶5′-cyclic monophosphate levels in healthy volunteers

Summary To find out whether 3-morpholino-sydnonimine (SIN 1), the active metabolite of molsidomine, exerts its antiaggregatory effects not only in vitro but also in vivo, we tested ex vivo aggregation before and after intravenous application of molsidomine in healthy volunteers. We also measured plasma levels of guanosine 35-cyclic monophosphate (cyclic GMP) as SIN 1, the bioactive metabolite of molsidomine, becomes effective via activation of soluble guanylate cyclase. In eight out of ten subjects molsidomine had an inhibitory effect on platelet aggregation and a higher threshold concentration of platelet-activating factor was required after molsidomine application to induce irreversible aggregation. Despite the effect on platelets, plasma cyclic GMP levels did not increase. These results suggest that the nitric oxide-containing SIN 1 inhibits platelet aggregation not only in vitro but also in vivo and that this property can be a beneficial effect in antianginal therapy.Abbreviations Cyclic GMP guanosine 35-cyclic monophosphate - NO nitric oxide - PAF platelet-activating factor - PRP platelet-rich plasma - SIN 1 3-morpholino-sydnonimine     

Synergistic effects of tumour necrosis factor-α and interferon-γ on feline haemopoietic progenitors

Pathogenic mechanisms that underlie feline leukaemia virus subgroup-C (FeLV-C) induced erythroid aplasia are unknown. FeLV-C infection is associated with higher serum levels of interferon- (IFN-) and tumour necrosis factor- (TNF-), which may act synergistically to cause haemopoietic suppression. In the present studies, the synergistic effects of TNF- and IFN- on feline bone marrow progenitors in vitro were evaluated. Bone marrow mononuclear cells from specific-pathogen-free cats were exposed to TNF- (100 and 200 pg/ml) and IFN- (100 or 200 units/ml), alone or in combination, for 2 h before plating for clonal assays of colony forming units. Our results show that TNF- and IFN- in combination caused marked suppression of feline colony forming units-erythroid (CFU-E), burst forming units-erythroid (BFU-E), and colony forming units-fibroblasts (CFU-F), whereas colony forming units-granulocyte/macrophage (CFU-GM) were minimally affected. The same concentrations of TNF- and IFN- alone had minimal effects on CFU-E, BFU-E and CFU-F. These results suggest that TNF- and IFN- may play a significant role in regulating haemopoiesis in cats and may be involved in the pathogenesis of erythroid aplasia in cats infected with feline leukaemia virus.     

On the existence of a lyotropic and osmotic type of collagen swelling produced by representative electrolytes and anelectrolytes

Summary The influence of NaI, NaCl, urea, glucose and sucrose on venous tissue volume were studied with regard to osmotic (electrostatic and dehydrating) and lyotropic effects. NaI leads to the highest volume increase and to the extreme irreversibility of this change. The tested anelectrolytes cause a decrease of tissue volume over the whole range of concentration with the exception that, beyond 1.0 osM, urea causes an increase. The large reversibility of this increase is pointed out. A strict discrimination between osmotic and lyotropic volume change is questioned.     

Epidermal growth factor,transforming growth factor-α, and epidermal growth factor receptor content in normal and carcinomatous gastric and colonic tissue

Summary Epidermal growth factor (EGF) and transforming growth factor- (TGF-) are polypeptides which bind to the EGF receptor (EGFr) and may play a role in cell growth and carcinogenesis. Our study investigated the content of EGF, TGF-, and EGFr in tumors of the stomach and the colon in comparison with the sourrounding mucosa. EGF was detected in half of the stomach specimens with concentrations between 1 and 9 ng/g weight irrespective of histology. In the colon no EGF was found in the tumor or normal mucosa. In the stomach normal mucosa contained higher TGF- concentrations (mean 22.4 ng/g) than the tumors (mean 11.8 ng/g), but the difference was not statistically significant because of a wide variation in mucosal values. By contrast, the colon mucosa displayed significantly higher TGF- concentrations than the tumor tissues (33 ng/g versus 12 ng/g; P < 0.01). EGFr content in the gastric mucosa was lower compared to gastric carcinoma (48 fmol/g versus 75 fmol/g) yet not significantly different. In contrast, colorectal tumor specimens disclosed significantly higher concentrations than the mucosal tissues (mean of 155 fmol/g versus 80 fmol/g; P < 0.01). In conclusion, TGF- should not be considered a tumorigenic but a physiological growth factor in the stomach and colon. An elevated EGFr content in colorectal tumors in comparison with the normal mucosa could lead to a growth advantage by an autostimulating mechanism.Abbreviations EGF epidermal growth factor - EGFr epidermal growth factor receptor - TGF- transforming growth factor - ROC receiver operating characteristic Dedicated to Prof. Dr. G. Paumgartner on the occasion of his 60th birthday     

Spatial auditory attention is modulated by tactile priming

Previous studies have shown that cross-modal processing affects perception at a variety of neuronal levels. In this study, event-related brain responses were recorded via whole-head magnetoencephalography (MEG). Spatial auditory attention was directed via tactile pre-cues (primes) to one of four locations in the peripersonal space (left and right hand versus face). Auditory stimuli were white noise bursts, convoluted with head-related transfer functions, which ensured spatial perception of the four locations. Tactile primes (200–300 ms prior to acoustic onset) were applied randomly to one of these locations. Attentional load was controlled by three different visual distraction tasks. The auditory P50m (about 50 ms after stimulus onset) showed a significant proximity effect (larger responses to face stimulation as well as a contralaterality effect between side of stimulation and hemisphere). The tactile primes essentially reduced both the P50m and N100m components. However, facial tactile pre-stimulation yielded an enhanced ipsilateral N100m. These results show that earlier responses are mainly governed by exogenous stimulus properties whereas cross-sensory interaction is spatially selective at a later (endogenous) processing stage.This research has been supported by the DaimlerChrysler AG, DARPAs AugCog program, and the German Research Foundation DFG (SFB 550/B1).     

Neuronal mechanisms for pitch analysis in the time domain

Summary Many units in the auditory midbrain nucleus (MLD) of the Guinea fowl are found to be tuned to amplitude modulated tones (AM). For a given response maximum the relationship of the period _m of the modulation frequency f_m and the period _c of the carrier frequency f_c may be given by an empirical equation: m · _m + n · _c = 1 · ₁, where m, n and 1 are small integers typical for a unit. ₁ is a time constant of 0.4 ms. The temporal pattern of the neuronal response support these findings. The averages of spike trains oscillate with periods multiple to ₁. These oscillations are elicited by stimulus onsets and zero crossings of f_m and may be coupled strongly to f_m depending on f_c. Variation of f_m or f_c shifts the mean delay of the phase coupled activity proportional to m · _m and n · _c, respectively. These effects may be explained with activity phase coupled to f_c which coincides at the level of the recorded units with oscillations coupled to f_m. This is expressed by the above given periodicity equation. Psychophysical results with AM-stimuli indicate that the mechanisms described and the periodicity equation are adequate for the explanation of the analysis of periodicity pitch in humans. Hence the period corresponding to pitch is defined by m · _m + n · _c = 1 · ₁, where n and 1 are integers and ₁ = 0.4 ms. Plots of _p as a function of _c reveal steps at 0.4 ms intervals indicating that the neuronal time constant is the same in both species.Supported by the DFG, SFB 45     

Lichtmikroskopische Untersuchungen über die Entwicklung vonTheileria annulata (Dschunkowsky und Luhs, 1904) inHyalomma anatolicum excavatum (Koch, 1844)

Summary Fully differentiated kinetes, average length 17.6m, appeared in the haemolymph of engorged nymphs usually 17 to 20 days after repletion. Kinetes were observed at first in the salivary glands on day 18 after repletion. The kinetes then transformed into fission bodies of about 10m in diameter, mainly in type III alveoli and less frequently in type II alveoli. The fission bodies grew up to a size of about 20m after several divisions of their nucleus. At this time the ticks moulted and no further development occurred until activation. Shortly before infestation the salivary glands began to proliferate, and rapid growth of the fission bodies was observed, especially in young ticks where development of infective particles (sporozoites) was concluded within two days. Development in feeding adult ticks apparently occurred in four major steps: (1) Division of primary fission bodies (sporonts) into numerous secondary fission bodies (primary sporoblasts), (2) division of secondary fission bodies into tertiary fission bodies (secondary sporoblasts), (3) production of particles (sporozoites) by tertiary fission bodies and release of particles into the saliva, and (4) degeneration of fission bodies and their host cell but further release of particles.The host cell was stimulated to giant growth, thus its diameter increased, on average, from 15 to 110 m. Heavy infections resulting from parasitaemias of >40% caused disease and mortality in the tick population. Development was much retarded by aging. In ticks starved for six months sporozoites did not develop before day five to seven of infestation. Sporozoites may not develop at all in six to nine month old female ticks during the infestation period. The significance of the described developmental stages ofT. annulata was discussed and a sexual generation postulated. The hypothetic development ofT. annulata in its tick vector was illustrated.

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Gefördert von der Deutschen Forschungsgemeinschaft (DFG)     

Sensitivity to dihydropyridines, ω-conotoxin and noradrenaline reveals multiple high-voltage-activated Ca2+ channels in rat insulinoma and human pancreatic β-cells

High-voltage-activated (HVA) Ba²⁺ currents of rat insulinoma (RINm5F) and human pancreatic -cells were tested for their sensitivity to dihydropyridines (DHPs), -conotoxin (-CgTx) and noradrenaline. In RINm5F cells, block of HVA currents by nimodipine, nitrendipine and nifedipine was voltage- and dose-dependent (apparent K _D<37 nM) and largely incomplete even at saturating doses of DHPs (mean 53%, at 10 M and 0 mV). Analysis of slow tail currents in Bay K 8644-treated cells indicated the existence of Bay K 8644-insensitive channels that turned on at slightly more positive voltages and deactivated more quickly than Bay K 8644-modified channels. DHP Ca²⁺ agonists and antagonists in human -cells had similar features to RINm5F cells except that DHP block was more pronounced (76%, at 10 M and 0 mV) and Bay K 8644 action was more effective, suggesting a higher density of L-type Ca²⁺ channels in these cells. In RINm5F cells, but not in human -cells, DHP-resistant currents were sensitive to -CgTx. The toxin depressed 10–20% of the DHP-resistant currents sparing a residual current (25–35%) with similar voltage-dependent characteristics and Ca²⁺/Ba²⁺ permeability. Noradrenaline (10 M) exhibited different actions on the various HVA current components: (1) it prolonged the activation kinetics of -CgTx-sensitive currents, (2) it depressed by about 20% the size of DHP-sensitive currents, and (3) it had little or no effects on the residual DHP- and -CgTx-resistant current although intracellularly applied guanosine 5-O-(3-thiotriphosphate) (GTP--S) prolonged its activation time course. The first action was clearly voltage-dependent and most evident in RINm5F cells that displayed neuronal-like processes. The second was observed more frequently, was voltage-independent and fully blocked by saturating doses of nifedipine (10 M). Both actions were prevented by intracellular perfusion with guanosine 5-O-(2-thiodiphosphate) (GDP--S). Our data suggest that beside a majority of L-type channels, RINm5F and human pancreatic -cells may express a variable fraction of DHP-insensitive channels that may be involved in the control of insulin secretion during -cell activity.     

Effect of muscular work on the motor function of the stomach and duodenum

Summary The effect of muscle work on the gastric and duodenal motor function was studied in dogs.It was observed that the static stress inhibited the hunger contractions of the stomach and duodenum; the inhibitory effect disappeared as the stress ceased. The static muscle work effect was more pronounced in hunger contractions of the stomach than in those of the duodenum. Under the influence of static stress, the periodic hunger contractions of the stomach varied more extensively than its acid and digestive contractions.Presented by Active Member Acad. Med. Sci. USSR V. N. Chernigovsky     

Endocrine activity of the “silent” adrenocortical adenoma is uncovered by response to corticotropin-releasing hormone

Summary The purpose of this study was to ascertain whether the pituitary-adrenal responses to human corticotropin-releasing hormone (hCRH) in non-functioning adrenocortical adenoma would uncover a functional activity in these adrenal nodules. Eleven patients with incidentally discovered silent adrenocortical adenoma and eleven controls were studied. The initial clinical and laboratory examination, including an overnight 1 mg dexamethasone suppression test, revealed no abnormalities in any of the subjects. IR-ACTH and serum steroids (F, S, P, 17OHP, 18OHB, and aldosterone) were normal in both controls and patients. After pulse IV injection of 100 g hCRH, the cortisol response was significantly exaggerated (P=0.01). Stimulated plasma ACTH levels were, however, significantly lower in patients than in controls (P=0.01), indicating counter-feedback regulation of cortisol. The peak cortisol/peak ACTH ratio (F_max/ACTH_max) in the patients was significantly elevated (26.8±4.37 nmol/ng vs. 14.6±2.16 nmol/ ng,P=0.02). Two further patients with incidentally discovered pre-Cushing's adrenocortical adenoma displayed an even higher ratio (43.5 and 45.5 nmol/ng). In established Cushing's syndrome due to an autonomous adrenocortical adenoma, suppression of ACTH and of the ACTH response to hCRH occurs with a very high basal cortisol/ basal ACTH ratio. Our findings suggest some functional activity even in clinically silent adrenocortical adenoma. Response to hCRH uncovers a continuous spectrum between adrenocortical adenoma, pre-Cushing's, and Cushing's syndrome.Abbreviations ACA adrenocortical adenoma - ACTH adrenocorticotropin - DHEAS Dehydroepiandrosterone sulfate - F cortisol - hCRH human corticotropin releasing hormone - P progesterone - S 11-deoxycortisol - 17OHP 17-hydroxyprogesterone - 18OHB 18-hydroxy-corticosterone     

Fibre types inLimulus telson muscles: morphology and histochemistry

Summary Using a variety of techniques, we have demonstrated the presence of at least two fibre types inLimulus median telson levator muscle. By light and electron microscopy, large (21 56 m² mean cross-sectional area) fibres have A-bands of 4.1 m, one-half I bands of 2.15 m and Z lines 0.5 m in width. Few mitochondria are found in these fibres, which comprise 54% of those present in a given microscope field and which occupy 82% of the total cross-sectional area. Small fibres (484 m² mean cross-sectional area) have A bands of 6.3 m, one-half I bands of 3.1 m and Z lines between 0.5 and 1.0 m in width and are rich in mitochondria. Although small fibres comprise nearly one-half (46%) of the fibres in a field, they occupy only 18% of the total cross-sectional area.Histochemical staining for alkaline-stable myofibrillar ATPase activity and mitochondrial reduced -nicotinamide adenine nucleotide (-NADH) tetrazolium reductase activity confirms the presence of two fibre types. The large fibres react positively for the myofibrillar ATPase activity and negatively for the mitochondrial enzyme activity. The reverse is seen with the small fibres. Some fibres of intermediate size, having intermediate staining characteristics, were also observed. Native gel electrophoresis of both myofibrillar and purified myosin preparations supports the observed differences in myofibrillar ATPase activity in that two myosin isozymes are resolved on pyrophosphate gels. Although the thick filaments isolated from unstimulated small fibres are longer (>6.0 m) than those isolated from unstimulated large fibres (4.26 m), all have a similar appearance with respect to the arrangement of myosin heads on their surfaces, and similar diameters. The implications of the observed heterogeneity of fibre types is discussed with reference to previously reported phenomena inLimulus telson muscle, including changes in length of thick filaments on fibre stimulation and the shape of the length-tension curve obtained from fibre bundles.