Neural regulation of mammalian fast and slow muscle myosins: an electrophoretic analysis.

Mammalian nerves to fast and slow muscles have the remarkable property of changing the speed of contraction of muscles following cross-reinnervation. The biochemical basis of speed transformation is the change in myosin in ATPase activity. This paper provides electrophoretic evidence for structural changes in myosin from cross-reinnervated muscles. A method is described for the separation of intact fast and slow muscle myosins by polyacrylamide gel electrophoresis. This method utilizes the fact that ATP and its analogs prevent the formation of myosin polymers in low ionic strength buffers. In this system, normal fast muscle myosin has a higher electrophoretic mobility than slow muscle myosin. Normal rat soleus myosin has a major slow and a minor fast component due to two populations of muscle fibers. The same muscle cross-reinnervated by a fast muscle nerve shows only the fast component, The normal, homogeneous fast extensor digitorum longus muscle has only the electrophoretically fast myosin, but following cross-reinnervation it shows both fast and slow components. These results suggest that mammalian motor nerves can induce or suppress the expression of genes that code for fast and slow skeletal muscle myosins.     

The effects of the interaction of myosin essential light chain isoforms with actin in skeletal muscles

In order to compare the ability of different isoforms of myosin essential light chain to interact with actin, the effect of the latter protein on the proteolytic susceptibility of myosin light chains (MLC-1S and MLC-1V - slow specific and same as ventricular isoform) from slow skeletal muscle was examined. Actin protects both slow muscle essential light chain isoforms from papain digestion, similarly as observed for fast skeletal muscle myosin (Nieznanska et al., 1998, Biochim. Biophys. Acta 1383: 71). The effect of actin decreases as ionic strength rises above physiological values for both fast and slow skeletal myosin, confirming the ionic character of the actin-essential light chain interaction. To better understand the role of this interaction, we examined the effect of synthetic peptides spanning the 10-amino-acid N-terminal sequences of myosin light chain 1 from fast skeletal muscle (MLC-1F) (MLCFpep: KKDVKKPAAA), MLC-1S (MLCSpep: KKDVPVKKPA) and MLC-1V (MLCVpep: KPEPKKDDAK) on the myofibrillar ATPase of fast and slow skeletal muscle. In the presence of MLCFpep, we observed an about 19% increase, and in the presence of MLCSpep about 36% increase, in the myofibrillar ATPase activity of fast muscle. On the other hand, in myofibrillar preparations from slow skeletal muscle, MLCSpep as well as MLCVpep caused a lowering of the ATPase activity by about 36%. The above results suggest that MLCSpep induces opposite effects on ATPase activity, depending on the type of myofibrils, but not through its specific N-terminal sequence - which differs from other MLC N-terminal peptides. Our observations lead to the conclusion that the action of different isoforms of long essential light chain is similar in slow and fast skeletal muscle. However the interaction of essential light chains with actin leads to different physiological effects probably depending on the isoforms of other myofibrillar proteins.     

Tension relaxation after stretch in resting mammalian muscle fibers: stretch activation at physiological temperatures.

Tension responses to ramp stretches of 1-3% Lo (fiber length) in amplitude were examined in resting muscle fibers of the rat at temperatures ranging from 10 degrees C to 36 degrees C. Experiments were done using bundles of approximately 10 intact fibers isolated from the extensor digitorum longus (a fast muscle) and the soleus (a slow muscle). At low temperatures (below approximately 20 degrees C), the tension response consisted of an initial rise to a peak during the ramp followed by a complex tension decay to a plateau level; the tension decay occurred at approximately constant sarcomere length. The tension decay after a standard stretch at approximately 3-4.Lo/s contained a fast, an intermediate, and a (small amplitude) slow component, which at 10 degrees C (sarcomere length approximately 2.5 microns) were approximately 2000.s-1, approximately 150.s-1, and approximately 25.s-1 for fast fibers and approximately 2000.s-1, approximately 70.s-1 and approximately 8.s-1 for slow fibers, respectively. The fast component may represent the decay of interfilamentary viscous resistance, and the intermediate component may be due to viscoelasticity in the gap (titin, connectin) filament. The two- to threefold fast-slow muscle difference in the rate of passive tension relaxation (in the intermediate and the slow components) compares with previously reported differences in the speed of their active contractions; this suggests that "passive viscoelasticity" is appropriately matched to contraction speed in different muscle fiber types. At approximately 35 degrees C, the fast and intermediate components of tension relaxation were followed by a delayed tension rise at approximately 10.s-1 (fast fibers) and 2.5.s-1 (slow fibers); the delayed tension rise was accompanied by sarcomere shortening. BDM (5-10 mM) reduced the active twitch and tetanic tension responses and the delayed tension rise at 35 degrees C; the results indicate stretch sensitive activation in mammalian sarcomeres at physiological temperatures.     

Histochemical localization of octopamine- and proctolin-sensitive adenylate cyclase activity in a locust skeletal muscle

A histochemical technique for the localization of adenylate cyclase activity has been applied to the extensor-tibiae muscle of the hindleg of the locust, Schistocerca gregaria to localise the sites of action of the modulatory compounds octopamine and proctolin. Octopamine-sensitive adenylate cyclase activity can be demonstrated in fast and intermediate type muscle fibres but not in the limited number of purely slow muscle fibres (3-6) in the fan region at the proximal end of the muscle. In contrast the latter fibres are the only ones in the muscle to exhibit proctolin-sensitive adenylate cyclase activity. In both cases the bulk of the reaction product is localised in the sarcoplasmic reticulum component of the dyads, with lesser amounts occurring beneath the sarcolemmal membrane, in the non-dyad sarcoplasmic reticulum and in the T-tubule system. The results are consistent with physiological data suggesting that proctolin, but not octopamine, mediates its effects on the myogenic rhythm of contraction and relaxation in this muscle by changing the levels of cyclic AMP in the small group of slow muscle fibres which act as the pacemaker for this rhythm.     

Zebrafish ftz-f1a (nuclear receptor 5a2) functions in skeletal muscle organization

Fushi-tarazu factor 1a (Ftz-F1a, Ff1a, Nr5a2) is a nuclear receptor with diverse functions in many tissues. Here, we report the function of ff1a in zebrafish muscle differentiation. In situ hybridization revealed that ff1a mRNA was present in the adaxial and migrating slow muscle precursors and was down-regulated when slow muscle cells matured. This expression was under the control of hedgehog genes, expanded when hedgehog was increased and missing in mutants defective in genes in the Hedgehog pathway like you-too (yot), sonic you (syu), and u-boot (ubo). Blocking ff1a activity by injecting a deleted form of ff1a or an antisense ff1a morpholino oligo into fish embryos caused thinner and disorganized fibers of both slow and fast properties. Transient expression of ff1a in syu, ubo, and yot embryos led to more fibril bundles, even when slow myoblasts were transfated into fast properties. We showed that ff1a and prox1 complemented each other in slow myofibril assembly, but they did not affect the expression of each other. These results demonstrate that ff1a functions in both slow and fast muscle morphogenesis in response to Hedgehog signaling, and this function parallels the activity of another slow muscle gene, prox1.     

An electrically coupled network of skeletal muscle in zebrafish distributes synaptic current

Fast and slow skeletal muscle types are readily distinguished in larval zebrafish on the basis of differences in location and orientation. Additionally, both muscle types are compact, rendering them amenable to in vivo patch clamp study of synaptic function. Slow muscle mediates rhythmic swimming, but it does so purely through synaptic drive, as these cells are unable to generate action potentials. Our patch clamp recordings from muscle pairs of zebrafish reveal a network of electrical coupling in slow muscle that allows sharing of synaptic current within and between segmental boundaries of the tail. The synaptic current exhibits slow kinetics (tau(decay) approximately 4 ms), which further facilitates passage through the low pass filter, a consequence of the electrically coupled network. In contrast to slow muscle, fast skeletal muscle generates action potentials to mediate the initial rapid component of the escape response. The combination of very weak electrical coupling and synaptic kinetics (tau(decay) <1 ms) too fast for the network low pass filter minimizes intercellular sharing of synaptic current in fast muscle. These differences between muscle types provide insights into the physiological role(s) of electrical coupling in skeletal muscle. First, intrasegmental coupling among slow muscle cells allows effective transfer of synaptic currents within tail segments, thereby minimizing differences in synaptic depolarization. Second, a fixed intersegmental delay in synaptic current transit, resulting from the low pass filter properties of the slow muscle network, helps coordinate the rostral-caudal wave of contraction.     

Myosin isozymes in normal and cross-reinnervated cat skeletal muscle fibers

Immunocytochemical characteristics of myosin have been demonstrated directly in normal and cross-reinnervated skeletal muscle fibers whose physiological properties have been defined. Fibers belonging to individual motor units were identified by the glycogen-depletion method, which permits correlation of cytochemical and physiological data on the same fibers. The normal flexor digitorum longus (FDL) of the cat is composed primarily of fast-twitch motor units having muscle fibers with high myosin ATPase activity. These fibers reacted with antibodies specific for the two light chains characteristic of fast myosin, but not with antibodies against slow myosin. Two categories of fast fibers, corresponding to two physiological motor unit types (FF and FR), differed in their immunochemical response, from which it can be concluded that their myosins are distinctive. The soleus (SOL) consists almost entirely of slow-twitch motor units having muscle fibers with low myosin ATPase activity. These fibers reacted with antibodies against slow myosin, but not with antibodies specific for fast myosin. When the FDL muscle was cross-reinnervated by the SOL nerve, twitch contraction times were slowed about twofold, and motor units resembled SOL units in a number of physiological properties. The corresponding muscle fibers had low ATPase activity, and they reacted with antibodies against slow myosin only. The myosin of individual cross-reinnervated FDL muscle units was therefore transformed, apparently completely, to a slow type. In contrast, cross-reinnervation of the SOL muscle by FDL motoneurons did not effect a complete converse transformation. Although cross-reinnervated SOL motor units had faster than normal twitch contraction times (about twofold), other physiological properties characteristic of type S motor units were unchanged. Despite the change in contraction times, cross-reinnervated SOL muscle fibers exhibited no change in ATPase activity. They also continued to react with antibodies against slow myosin, but in contrast to the normal SOL, they now showed a positive response to an antibody specific for one of the light chains of fast myosin. The myosins of both fast and slow muscles were thus converted by cross-reinnervation, but in the SOL, the newly synthesized myosin was not equivalent to that normally present in either the FDL or SOL. This suggests that, in the SOL, alteration of the nerve supply and the associated dynamic activity pattern are not sufficient to completely respecify the type of myosin expressed.     

Effect of cross-reinnervation on physiological parameters and on properties of myosin and sarcoplasmic reticulum of fast and slow muscles of the rabbit

Cross-reinnvervation of fast (extensor digitorum longus) and slow (soleus) twitch muscles of the rabbit showed essentially complete fast to slow and slow to fast conversion, respectively, 11-12 mo after surgery with respect to a number of physiological parameters including intrinsic shortening, velocity, and isometric twitch time to peak. There was pronounced bu incomplete biochemical conversion as judged by Ca2+ uptake by sarcoplasmic reticulum, myosin ATPase, alkali lability, and light chain complement. The question of trophic substances of neural origin is discussed in light of the fact that chronic stimulation for 15 wk of a fast muscle produces complete biochemical and physiological conversion to the slow type.     

Histochemical localization of octopamine- and proctolin-sensitive adenylate cyclase activity in a locust skeletal muscle

Summary A histochemical technique for the localization of adenylate cyclase activity has been applied to the extensortibiae muscle of the hindleg of the locust, Schistocerca gregaria to localise the sites of action of the modulatory compounds octopamine and proctolin. Octopamine-sensitive adenylate cyclase activity can be demonstrated in fast and intermediate type muscle fibres but not in the limited number of purely slow muscle fibres (3–6) in the fan region at the proximal end of the muscle. In contrast the latter fibres are the only ones in the muscle to exhibit proctolinsensitive adenylate cyclase activity. In both cases the bulk of the reaction product is localised in the sarcoplasmic reticulum component of the dyads, with lesser amounts occurring beneath the sarcolemmal membrane, in the non-dyad sarcoplasmic reticulum and in the T-tubule system. The results are consistent with physiological data suggesting that proctolin, but not octopamine, mediates its effects on the myogenic rhythm of contraction and relaxation in this muscle by changing the levels of cyclic AMP in the small group of slow muscle fibres which act as the pacemaker for this rhythm.     

Analysis of myosin light and heavy chain types in single human skeletal muscle fibers

In this study, myosin types in human skeletal muscle fibers were investigated with electrophoretic techniques. Single fibers were dissected out of lyophilized surgical biopsies and typed by staining for myofibrillar ATPase after preincubation in acid or alkaline buffers. After 14C-labelling of the fiber proteins in vitro by reductive methylation, the myosin light chain pattern was analysed on two-dimensional gels and the myosin heavy chains were investigated by one-dimensional peptide mapping. Surprisingly, human type I fibers, which contained only the slow heavy chain, were found to contain variable amounts of fast myosin light chains in addition to the two slow light chains LC1s and LC2s. The majority of the type I fibers in normal human muscle showed the pattern LC1s, LC2s and LC1f. Further evidence for the existence in human muscle of a hybrid myosin composed of a slow heavy chain with fast and slow light chains comes from the analysis of purified human myosin in the native state by pyrophosphate gel electrophoresis. With this method, a single band corresponding to slow myosin was obtained; this slow myosin had the light chain composition LC1s, LC2s and LC1f. Type IIA and IIB fibers, on the other hand, revealed identical light chain patterns consisting of only the fast light chains LC1f, LC2f and LC3f but were found to have different myosin havy chains. On the basis of the results presented, we suggest that the histochemical ATPase normally used for fibre typing is determined by the myosin heavy chain type (and not by the light chains). Thus, in normal human muscle a number of 'hybrid' myosins were found to occur, namely two extreme forms of fast myosins which have the same light chains but different heavy chains (IIA and IIB) and a continuum of slow forms consisting of the same heavy chain and slow light chains with a variable fast light chain composition. This is consistent with the different physiological roles these fibers are thought to have in muscle contraction.     

The abdominal muscle receptor organ in Astacus leptodactylus (Crustacea)

The structure of both the slow- and the fast-adapting abdominal muscle receptor organ of Astacus leptodactylus is described with particular reference to differences between the two systems. The receptors are composed of a thin muscle that extends from the front edge of one segment to the front edge of the following and a sensory cell connected with this muscle. In the zone where the sensory cells enter their respective muscle, muscle fibers are reduced (zone of relative muscle exclusion = ZRME) and partly replaced by connective tissue. The occurrence of dendritic processes of both the slow and the fast neurons is confined to this zone. The following differences between the two receptor types are established: (1) The fast receptor muscle reveals a smaller sarcomere length than the slow receptor muscle and a higher myosin/actin filament ratio. (2) Muscle fibers that pass the ZRME are always found at its periphery in the fast system, separated from dendritic processes by layers of connective tissue, while in the slow system muscle fibers frequently are intermingled with the sensory elements. (3) The ZRME of the slow receptor is 20-30% longer than that of the fast receptor. (4) The dendritic varicosities of the slow neuron, on an average, contain many more mitochondria than those of the fast neuron. (5) Dendritic processes (fine twigs as well as varicosities) are juxtaposed to the sarcolemma of the muscle fibers only in the slow system; in the fast system dendrites and muscle are spatially separated by connective tissue. It is assumed that these differences between the two receptor types are at least in part responsible for the different thresholds observed in physiological experiments.     

Biochemical properties of ordinary and dark muscle myosin from carp skeletal muscle

Two types of myosin isolated from ordinary (fast) and dark (slow) muscles of carp were examined by ATPase and in vitro motility assays. Vmax of the ATPase activity and sliding velocity of ordinary myosin showed 1.6 and 1.5 times higher activities than those of dark myosin, whereas those of mammalian fast myosin were much higher, 3 to 10 times, than those of slow myosin. Although ordinary myosin had almost identical activities to those of mammalian fast myosin, activities of dark myosin was twice of those of mammalian slow myosin. This high motile activity of dark myosin can account for the physiological role of dark muscle in cruising of fish. By comparing Km of the actin-activated ATPase activity, ordinary myosin was appeared to have higher affinity to F-actin than dark myosin, and this was confirmed by the binding assay of HMM or S-1 of carp myosin to F-actin. Investigation of myosin assembly by electron microscopy and the centrifugation assay revealed that ordinary myosin assembled much poorly than dark myosin or mammalian fast myosin. This phenomenon may reflect characteristic cellular function of fish skeletal muscle.     

Effects of adenosine 3':5'-monophosphate-dependent protein kinase on sarcoplasmic reticulum isolated from cardiac and slow and fast contracting skeletal muscles.

Effects of cyclic adenosine 3':5'-monophosphate (cyclic AMP)-dependent protein kinase were studied in sarcoplasmic reticulum prepared from cardiac and slow and fast (white) skeletal muscle. Cyclic AMP-dependent protein kinase failed to catalyze phosphorylation of fast skeletal muscle microsomes as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Cyclic AMP-dependent protein kinase was without effect on calcium uptake by these microsomes. Treatment of cardiac microsomes obtained from dog, cat, rabbit, and guinea pig with cyclic AMP-dependent protein kinase and ATP resulted in phosphorylation of a 22,000-dalton protein component in the amounts of 0.75, 0.25, 0.30, and 0.14 nmol of phosphorus/mg of microsomal protein, respectively. Calcium uptake by cardiac microsomes was stimulated 1.8- to 2.5-fold when microsomes were treated with cyclic AMP-dependent protein kinase. Protein kinases partially purified from bovine heart and rabbit skeletal muscle were both effective in mediating these effects on phosphorylation and calcium transport in dog cardiac sarcoplasmic reticulum. Slow skeletal muscle sarcoplasmic reticulum also contains a protein with a molecular weight of approximately 22,000 that can be phosphorylated by protein kinase. Phosphorylation of this component ranged from 0.005 to 0.016 nmol of phosphorous/mg of microsomal protein in dog biceps femoris. A statistically significant increase in calcium uptake by these membranes was produced by the protein kinase. Increases in protein kinase-catalyzed phosphorylation of a low molecular weight microsomal component and in calcium transport by sarcoplasmic reticulum of cardiac and slow skeletal muscle may be related to the relaxation-promoting effects of epinephrine seen in these types of muscle. Conversely, the absence of a relaxation-promoting effect of epinephrine in fast skeletal muscle may be associated with the lack of effect of cyclic AMP and protein kinase on calcium transport by the sarcoplasmic reticulum of this type of muscle.     

Regionalized adaptations and muscle fiber proliferation in stretch-induced enlargement

The relative contribution of increases in fiber area to stretch-induced muscle enlargement was evaluated in the slow tonic fibers of the anterior latissimus dorsi of adult Japanese quails. A weight corresponding to 10% of the bird's body mass was attached to one wing. Thirty days of stretch in 34 birds averaged 171.8 +/- 13.5% increase in muscle mass and 23.5 +/- 0.8% increase in muscle fiber length. The volume density of noncontractile tissue increased in middle and distal regions of stretch-enlarged muscles. Mean fiber cross-sectional area increased 56.7 +/- 12.3% in the midregion of stretched muscles. Further analysis indicated slow beta-fiber hypertrophy occurred in proximal, middle, and distal regions; however, fast alpha-type fiber hypertrophy was limited to middle regions of stretched muscles. Stretched muscles had a significant increase in the frequency of slow beta-fibers that were less than 500 microns 2 in all regions and fast alpha-type fibers in middle and distal regions. Total fiber number was determined after nitric acid digestion of connective tissue in 10 birds. Fiber number increased 51.8 +/- 19.4% in stretched muscle. These results are the first to clearly show that muscle fiber proliferation contributes substantially to adult skeletal muscle stretch-induced enlargement, although we do not know whether the responses of the slow tonic anterior latissimus dorsi might be similar or different from mammalian twitch muscle.     

Use of type-specific antimyosins to demonstrate the transformation of individual fibers in chronically stimulated rabbit fast muscles

Continuous stimulation of a rabbit fast muscle at 10 Hz changes its physiological and biochemical parameters to those of a slow muscle. These transformations include the replacement of myosin of one type by myosin of another type. Two hypotheses could explain the cellular basis of these changes. First, if fibers were permanently programmed to be fast or slow, but not both, a change from one muscle type to another would involve atrophy of one fiber type accompanied by de novo appearance of the other type. Alternatively, preexisting muscle fibers could be changing from the expression of one set of genes to the expression of another. Fluorescein-labeled antibodies against fast (AF) and slow (AS) muscle myosins of rabbits have been prepared by procedures originally applied to chicken muscle. In the unstimulated fast peroneus longus muscle, most fibers stained only with AF; a small percentage stained only with AS; and no fibers stained with both antibodies. In stimulated muscles, most fibers stained with both AF and AS; with increasing time of stimulation, there was a progressive decrease in staining intensity with AF and a progressive increase in staining intensity with AS within the same fibers. These results are consistent with a theory that individual preexisting muscle fibers can actually switch from the synthesis of fast myosin to the synthesis of slow myosin.     

cDNA clone and expression analysis of rodent fast and slow skeletal muscle troponin I mRNAs

We have characterized the structure and expression of rodent mRNAs encoding the fast and slow skeletal muscle isoforms of the contractile regulatory protein, troponin I (TnIfast and TnIslow). TnIfast and TnIslow cDNA clones were isolated from mouse and rat muscle cDNA clone libraries and were used as isoform-specific probes in Northern blot and in situ hybridization studies. These studies showed that the TnIfast and TnIslow mRNAs are expressed in skeletal muscle, but not cardiac muscle or other tissues, and that they are differentially expressed in individual muscle fibers. Fiber typing on the basis of in situ hybridization analysis of TnI isoform mRNA content showed an excellent correlation with fiber type as assessed by myosin ATPase histochemistry. These results directly demonstrate that the differential expression of skeletal muscle TnI isoforms in the various classes of vertebrate striated muscle cells is based on gene regulatory mechanisms which control the abundances of specific TnI mRNAs in individual muscle cells. Both TnIfast and TnIslow mRNAs are expressed, at comparable levels, in differentiated cultures of rat L6 and mouse C2 muscle cell lines. Thus, although neuronal input has been shown to be an important factor in determining fast versus slow isoform-specific expression in skeletal muscle, both TnIfast and TnIslow genes can be expressed in muscle cells in the absence of nerve. Comparison of the deduced rodent TnI amino acid sequences with previously determined rabbit protein sequences showed that residues with potential fast/slow isoform-specific function are present in several discrete clusters, two of which are located near previously identified actin and troponin C binding sites.     

Gating of I(sK) channels expressed in Xenopus oocytes.

The channel underlying the slow component of the voltage-dependent delayed outward rectifier K+ current, I(Ks), in heart is composed of the minK and KvLQT1 proteins. Expression of the minK protein in Xenopus oocytes results in I(Ks)-like currents, I(sK), due to coassembly with the endogenous XKvLQT1. The kinetics and voltage-dependent characteristics of I(sK) suggest a distinct mechanism for voltage-dependent gating. Currents recorded at 40 mV from holding potentials between -60 and -120 mV showed an unusual "cross-over," with the currents obtained from more depolarized holding potentials activating more slowly and deviating from the Cole-Moore prediction. Analysis of the current traces revealed two components with fast and slow kinetics that were not affected by the holding potential. Rather, the relative contribution of the fast component decreased with depolarized holding potentials. Deactivation and reactivation, after a short period of repolarization (100 ms), was markedly faster than the fast component of activation. These gating properties suggest a physiological mechanism by which cardiac I(Ks) may suppress premature action potentials.     

Vibration-induced activation of muscle afferents modulates bioassayable growth hormone release.

The effects of tendon vibration on bioassayable growth hormone (BGH) secretion from the pituitary gland were investigated in anesthetized adult male rats. The tendons from predominantly fast-twitch ankle extensor muscles (gastrocnemius and plantaris) or a predominantly slow-twitch ankle extensor (soleus) were vibrated by using a paradigm that selectively activates group Ia afferent fibers from muscle spindles. The lower hindlimb was secured with the muscles near physiological length, and the tendons were vibrated for 15 min at 150 Hz and a displacement of 1 mm. Control rats were prepared similarly, but the tendons were not vibrated. Compared with control, vibration of the tendons of the fast ankle extensors markedly increased (160%), whereas vibration of the slow soleus decreased (68%), BGH secretion. Complete denervation of the hindlimb had no independent effects on the normal resting levels of BGH, but it prevented the effects of tendon vibration on BGH secretion. The results are consistent with previous findings showing modulation of BGH release in response to in vivo activation or in situ electrical stimulation of muscle afferents (Bigbee AJ, Gosselink KL, Grindeland RE, Roy RR, Zhong H, and Edgerton VR. J Appl Physiol 89: 2174-2178, 2000; Gosselink KL, Grindeland RE, Roy RR, Zhong H, Bigbee AJ, and Edgerton VR. J Appl Physiol 88: 142-148, 2000; Gosselink KL, Grindeland RE, Roy RR, Zhong H, Bigbee AJ, Grossman EJ, and Edgerton VR. J Appl Physiol 84: 1425-1430, 1998). These data provide evidence that this previously described muscle afferent-pituitary axis is neurally mediated via group Ia afferents from peripheral skeletal muscle. Furthermore, these data show that activation of this group Ia afferent pathway from fast muscles enhances, whereas the same sensory afferent input from a slow muscle depresses, BGH release.     

Physico-chemical properties of rat and dog cardiac alpha-actinin

alpha-Actinin exists in several polymorphic forms which appear to be characteristic of the muscle type from which it is isolated. In order to determine the possible physiological role of this structural protein in cardiac muscle, we describe and compare here the physico-chemical properties of cardiac alpha-actinin from two different mammalian species, rat (fast contracting muscle) and dog (slow contracting muscle). Purification of cardiac alpha-actinin was achieved by chromatography on DEAE-cellulose and hydroxyapatite columns. The alpha-actinins isolated were different in their electrophoretic mobility (SDS-polyacrylamide gel electrophoresis), molecular size and alpha-helical content. However, their shape as revealed by electron microscopy and their activating effect on Mg2+-ATPase activity of actomyosin appear to be similar. These studies suggest that the rat and dog cardiac alpha-actinin are structurally different but functionally similar proteins.