J Comp Physiol A (2004) 190: 321–329 DOI 10.1007/s00359-004-0501-z
O R I GI N A L P A P E R
J. C. McDonagh Æ R. J. Callister Æ M. L. Favron D. G. Stuart
Resistance to disuse atrophy in a turtle hindlimb muscle
Received: 13 August 2003 / Revised: 16 December 2003 / Accepted: 21 January 2004 / Published online: 17 February 2004 Springer-Verlag 2004
Abstract The purpose of this study was to characterize the changes in a turtle hindlimb muscle (external gastrocnemius) after exposure to three conditions of disuse: immobilization, tenotomy, and spinalization. Histochemical analysis and measurement of muscle fiber cross-sectional area and weighted cross-sectional area were used to assess the potential conversion of muscle fiber types and changes in fiber size. It was found that unlike its counterpart in mammalian endotherms, the external gastrocnemius muscle of the adult turtle, Trachemys scripta elegans, was remarkably resistant to each model of reduced muscle function. It is suggested that such resistance to disuse is due to intrinsic mechanisms that enable heterothermic mammals and ectothermic vertebrates to tolerate an unfavorable climate and food and water shortages by using hypometabolic states. Keywords Histochemistry Æ Immobilization Æ Spinalization Æ Tenotomy Æ Turtle Abbreviations CSA cross-sectional area (of muscle fiber) Æ EG external gastrocnemius (muscle) Æ Foth fast, other fiber Æ Fg fast, glycolytic turtle muscle fiber Æ FOG fast, oxidative-glycolytic fiber Æ SC spinal cord Æ SO slow, oxidative fiber Æ wCSA weighted cross-sectional area
Introduction The degree to which atrophy and other pathological changes occur in the skeletal muscle of mammalian endotherms1 under normal metabolic and temperature conditions depends on several factors. They include the disuse protocol, the duration of disuse, the particular muscle under study, the length of the muscle during the period of disuse, and the species studied. Experimental approaches have included joint immobilization by pinning or application of a plaster cast, tenotomy, spinalization, bedrest, and hindlimb suspension. The extent of atrophy is usually inversely proportional to the size of the endotherm: i.e., mouse>rat>guinea pig>cat>dog> human2 . Few studies have examined the response of reptilian skeletal muscle to disuse (for citations, see Arifa et al. 1993), except during hypometabolic states (e.g., snake, Andersson and Johansson 2001; turtle, Jackson 2002). The purpose of the present study was to determine whether the adaptive changes observed in the skeletal muscle of endotherms after extended periods of disuse at normal body temperature also occur in a hindlimb muscle of the turtle, Trachemys (formerly Pseudemys) 1
J. C. McDonagh Program in Physical Therapy, Arizona School of Health Sciences, A.T. Still University of Health Sciences, Mesa, AZ 85206, USA R. J. Callister School of Biomedical Sciences, Faculty of Health, University of Newcastle, 2308 Callaghan, NSW, Australia M. L. Favron Department of Rehabilitation Services, Flagstaff Medical Center, 1200 N. Beaver St., Flagstaff, AZ 86001, USA D. G. Stuart (&) Department of Physiology, The University of Arizona College of Medicine, Tucson, AZ 85724-5051, USA E-mail:
[email protected] Tel.: +1-520-6267103 Fax: +1-520-6262383
There is no universally accepted terminology for homeothermic mammals that maintain a constant body temperature (termed ‘‘endotherms’’ here) versus mammals that have the capacity to become hypometabolic via hibernation or estivation (‘‘heterotherms’’). In contrast, ‘‘ectotherm’’ is universally accepted as appropriate terminology for a non-mammalian vertebrate that can also convert to a hypometabolic state. For an extensive review of this terminology issue, see Schmidt-Nielsen (1997, p 218) 2 For a chronological review of disuse literature on endotherms, including humans, that is relevant to this article see: Vrbova´ 1963; Fischbach and Robbins 1969; Shafiq et al. 1969; Booth and Kelso 1973; Maier et al. 1976; MacDougall et al. 1977, 1980; Mayer et al. 1981; Witzmann et al. 1982; Baker 1983; Alaimo et al. 1984; Hnik et al. 1985; Alford et al. 1987; Hikida et al. 1989; Roy et al. 1987, 1991; Robinson et al. 1991; Goldspink et al. 1992; Gordon and Pattullo 1993; Gordon 1995; Nordstrom et al. 1995; Ferrando et al. 1996; McComas 1996; Edgerton et al. 1996, 2002; Semmler et al. 2000; Adams et al. 2003; Otis et al. 2004
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scripta elegans. This species was selected for study based on our previous work using it as a model for design and functional features of the segmental motor system (e.g., Callister et al. 1989, 1995, 2003; Laidlaw et al. 1995; McDonagh et al. 1998, 2002; Hornby et al. 2002a, 2002b, 2002c). We investigated the effects on muscle histochemistry and morphology after three reduced-activity protocols: immobilization, tenotomy, and spinalization. In endotherms, immobilization reduces the activity in a muscle when it is placed in a shortened, non-functional position. Isometric contractions are still possible, however. Nonetheless, immobilization is reported to reduce EMG activity, promote muscle wasting (up to 25%), and stimulate the expression of fast myosin heavy chains. The latter is consistent with slow-to-fast fiber-type conversion (MacDougall et al. 1980; Hnik et al. 1985; Nordstrom et al. 1995). Our second disuse paradigm, tenotomy, eliminates almost all muscle tension during a contraction. Because the tendon is no longer attached to the skeleton, the muscle may contract to extremely short lengths. In endotherms, tenotomy also results in passive muscle shortening, thereby resulting in reduced sensory input to motoneurons from muscle spindle afferents (McComas 1996). Our third protocol, complete spinal cord (SC) transection, results in loss of supraspinal input to motoneurons and interneurons caudal to the lesion. In endotherms, SC transection still allows varying degrees of segmental reflex activation of muscle but atrophy nonetheless prevails (Vrbova´ 1963; Alaimo et al. 1984; McComas 1996). In the present study, the test muscle was found to be remarkably resistant to all three of the above protocols of reduced muscle function. The Discussion focuses on the possibility that intrinsic mechanisms enable heterotherms and ectotherms to tolerate an unfavorable climate and food and water shortages by using hypometabolic states.
2003), the latter by whole-muscle mechanical and fatigue properties. Anesthesia Animals in the three experimental groups were anesthetized with a 1:1 mixture of Tiletamine HCl and Zolazepam HCl (17 mg kg)1, i.m.). After the induction of anesthesia, the tenotomy and spinalization groups were intubated and maintained in a state of deep anesthesia throughout the surgical procedure with 3–5% halothane and a nitrous oxide/oxygen mixture (1 l min)1) delivered by spontaneous respiration. All surgical procedures were carried out under aseptic conditions. After recovery from anesthesia, all animals (including the three controls) were maintained for 4–6 weeks at room temperature (23–24C) in shallow-water (
