Structure–activity relationships in rodent diaphragm muscle fibers vs. neuromuscular junctions

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Abstract

The diaphragm muscle (DIAm) is a highly active muscle of mixed fiber type composition. We hypothesized that consistent with greater activation history and proportion of fatigue-resistant fibers, neuromuscular transmission failure is lower in the mouse compared to the rat DIAm, and that neuromuscular junction (NMJ) morphology will match their different functional demands. Minute ventilation and duty cycle were higher in the mouse than in the rat. The proportion of fatigue-resistant fibers was similar in the rat and mouse; however the contribution of fatigue-resistant fibers to total DIAm mass was higher in the mouse. Neuromuscular transmission failure was less in mice than in rats. Motor end-plate area differed across fibers in rat but not in mouse DIAm, where NMJs displayed greater complexity overall. Thus, differences across species in activation history and susceptibility to neuromuscular transmission failure are reflected in the relative contribution of fatigue resistant muscle fibers to total DIAm mass, but not in type-dependent morphological differences at the NMJ.

Highlights

Diaphragm muscle in mice has a greater activation history than rats. ► Susceptibility to neuromuscular transmission failure is less in mice than rats. ► Fatigue-resistant fibers contribute to a greater fraction of muscle mass in mice. ► Rats, but not mice, show fiber type differences in neuromuscular junction morphology. ► Functional differences across species are not reflected in neuromuscular morphology.

Introduction

Within the diaphragm muscle (DIAm), the properties of a phrenic motor neuron and the muscle fibers it innervates are matched to comprise a motor unit (Enad et al., 1989, Fournier and Sieck, 1988, Sieck, 1988, Sieck et al., 1989a, Sieck et al., 1996). Diaphragm motor units are selectively recruited to accomplish different ventilatory and non-ventilatory behaviors such that fatigue-resistant motor units are recruited during sustained ventilatory behaviors while more fatigable units are recruited during the shorter-duration, higher-force non-ventilatory behaviors involved in airway clearance (e.g., coughing, sneezing) (Fournier and Sieck, 1988, Mantilla et al., 2010, Mantilla and Sieck, 2011, Sieck, 1991, Sieck and Fournier, 1989). Forces generated by the DIAm to accomplish quiet breathing (i.e., eupnea) represent ∼27% of maximal force (transdiaphragmatic pressure generated by bilateral supramaximal phrenic nerve stimulation) in rats (Mantilla et al., 2010), ∼17% in cats (Fournier and Sieck, 1988, Sieck and Fournier, 1989) and ∼10% in humans (Sieck, 1991). This scaling across species is consistent with the scaling of other ventilatory parameters such as tidal volume (VT), total lung capacity and ventilation (V˙E) (Lindstedt and Schaeffer, 2002, Stahl, 1967). Respiratory rate and inspiratory duty cycle also increase as animal size decreases. This scaling of ventilatory requirements generally matches the distribution of DIAm motor unit (muscle fiber) types across these species (Fournier and Sieck, 1988, Mantilla et al., 2010, Mantilla and Sieck, 2011, Sieck, 1991, Sieck, 1995, Sieck and Fournier, 1989). Accordingly, the proportion of type I and IIa fibers (comprising fatigue-resistant motor units) across species is also scaled to match the fractional motor unit recruitment that is necessary to accomplish sustained ventilatory behaviors (Mantilla and Sieck, 2011).

In the rat DIAm, neuromuscular junction (NMJ) morphology varies across motor unit (muscle fiber) types (Mantilla et al., 2004a, Prakash et al., 1996b, Rowley et al., 2007, Sieck and Prakash, 1997). For example, NMJs at type I and IIa fibers are smaller and less complex compared to NMJs at type IIx and/or IIb fibers. This suggests that NMJ morphology is matched to the functional demands of different motor unit types. Indeed, susceptibility to neuromuscular transmission failure varies across motor unit (muscle fiber) types (Ermilov et al., 2007, Johnson and Sieck, 1993, Kugelberg and Lindegren, 1979). In the rat DIAm, the safety factor for neuromuscular transmission is higher at type IIx and/or IIb fibers compared to type I or IIa fibers (Ermilov et al., 2007, Wood and Slater, 1997, Wood and Slater, 2001). However, with repetitive stimulation, quantal release at type IIx and/or IIb fibers declines far more rapidly than at type I or IIa fibers (Reid et al., 1999, Rowley et al., 2007) providing a basis for the demonstrated greater susceptibility for neuromuscular transmission failure at these fibers (Johnson and Sieck, 1993, Sieck and Prakash, 1995).

Given the scaling of ventilatory parameters that has been demonstrated across species, we hypothesized that because of the mouse DIAm is more active (prolonged inspiratory duty cycle and greater weight-adjusted ventilation) than the rat, the mouse DIAm will have: (1) greater relative proportion of fatigue-resistant motor units (type I or IIa fibers) and (2) reduced susceptibility to neuromuscular transmission failure. We also hypothesized that NMJ morphology in the mouse DIAm will be matched to the functional demands of different motor unit types.

Section snippets

Methods

Adult male Sprague-Dawley rats (body weight  300 g) and adult male C57BL/6J mice (body weight  25 g) were used in these experiments. All procedures were approved by the Institutional Animal Care and Use Committee.

Ventilatory parameters

In awake, unrestrained adult male Sprague-Dawley rats and C57BL/6J mice, whole body plethysmography was used to determine V˙E, VT, and duration of inspiration and expiratory phases as well as total respiratory cycle duration.

Discussion

The results of this study indicate that compared to the rat, the mouse had increased VT, V˙E (both normalized for body weight) and a prolonged inspiratory duty cycle during eupnea. These results suggest that the mouse DIAm has a greater activation history compared to the rat during eupneic ventilation. Consistent with these differences in ventilatory patterns, the mouse DIAm has a greater relative contribution of fatigue-resistant (type I or IIa) muscle fibers to total DIAm mass compared to the

Acknowledgments

Supported by NIH grants HL096750, AR051173, and a Career Development Award from Mayo Clinic.

References (67)

  • D.A. Riley et al.

    A regional histochemical and electromyographic analysis of the cat respiratory diaphragm

    Exp. Neurol.

    (1979)
  • K.L. Rowley et al.

    Respiratory muscle plasticity

    Respir. Physiol. Neurobiol.

    (2005)
  • G.C. Sieck

    Diaphragm muscle: structural and functional organization

    Clin. Chest Med.

    (1988)
  • G.C. Sieck

    Physiological effects of diaphragm muscle denervation and disuse

    Clin. Chest Med.

    (1994)
  • G.C. Sieck et al.

    Fiber type composition of muscle units in the cat diaphragm

    Neurosci. Lett.

    (1989)
  • G.C. Sieck et al.

    Volume measurements in confocal microscopy

    Methods Enzymol.

    (1999)
  • N. Voituron et al.

    Early abnormalities of post-sigh breathing in a mouse model of Rett syndrome

    Respir. Physiol. Neurobiol.

    (2010)
  • S.J. Wood et al.

    Safety factor at the neuromuscular junction

    Prog. Neurobiol.

    (2001)
  • T.K. Aldrich et al.

    Fatigue of isolated rat diaphragm: role of impaired neuromuscular transmission

    J. Appl. Physiol.

    (1986)
  • B.Q. Banker et al.

    Neuromuscular transmission and correlative morphology in young and old mice

    J. Physiol.

    (1983)
  • J. Christon et al.

    Effects of inspired gas on sleep-related apnea in the rat

    J. Appl. Physiol.

    (1996)
  • J.G. Enad et al.

    Oxidative capacity and capillary density of diaphragm motor units

    J. Appl. Physiol.

    (1989)
  • L.G. Ermilov et al.

    Safety factor for neuromuscular transmission at type-identified diaphragm fibers

    Muscle Nerve

    (2007)
  • L.G. Ermilov et al.

    Impairment of diaphragm muscle force and neuromuscular transmission after normothermic cardiopulmonary bypass: effect of low dose inhaled CO

    Am. J. Physiol. Regul. Integr. Comp. Physiol.

    (2010)
  • M. Fournier et al.

    Mechanical properties of muscle units in the cat diaphragm

    J. Neurophysiol.

    (1988)
  • P.C. Geiger et al.

    Mechanisms underlying increased force generation by rat diaphragm muscle fibers during development

    J. Appl. Physiol.

    (2001)
  • B.D. Johnson et al.

    Differential susceptibility of diaphragm muscle fibers to neuromuscular transmission failure

    J. Appl. Physiol.

    (1993)
  • J.H. Kuei et al.

    Relative contribution of neurotransmission failure to diaphragm fatigue

    J. Appl. Physiol.

    (1990)
  • E. Kugelberg et al.

    Transmission and contraction fatigue of rat motor units in relation to succinate dehydrogenase activity of motor unit fibres

    J. Physiol.

    (1979)
  • J.J. LaBella et al.

    Absence of myofibrillar creatine kinase and diaphragm isometric function during repetitive activation

    J. Appl. Physiol.

    (1998)
  • D.E. Leith

    Comparative mammalian respiratory mechanics

    Am. Rev. Respir. Dis.

    (1983)
  • S. Levine et al.

    Human diaphragm remodeling associated with chronic obstructive pulmonary disease: clinical implications

    Am. J. Respir. Crit. Care Med.

    (2003)
  • M.I. Lewis et al.

    Effect of nutritional deprivation on diaphragm contractility and muscle fiber size

    J. Appl. Physiol.

    (1986)
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