title: Activity-independent neural influences on cat soleus motor unit phenotypes.
abstract: The physiological and phenotypic properties of motor units in the cat soleus muscle were studied after 4 months of inactivity induced by spinal cord isolation (SI). The soleus of some SI cats were stimulated for 30 min/day during an isometric (SI-I), shortening (SI-S), or lengthening (SI-L) phase of a simulated step cycle. Mean maximum tetanic tensions were approximately 15, 26, 32, and 51% of the control in the SI, SI-S, SI-L, and SI-I groups. Mean time-to-peak tension was approximately 50% shorter than the control in all SI groups. One motor unit was glycogen-depleted in each muscle via repetitive stimulation. Eighteen physiologically slow and 9 fast motor units from the spinal cord-isolated groups consisted of fibers that contained only slow myosin heavy chain (MHC) and sarco(endo)plasmic reticulum calcium-adenotriphosphatase (SERCA) isoforms. Two motor units (physiologically fast) consisted primarily of fibers that contained both fast and slow MHC and SERCA. These data reflect a dissociation between isometric speed-related properties and MHC and SERCA isoforms following inactivity. The predominance of fibers containing both fast and slow MHC and SERCA isoforms in 2 motor units demonstrates a strong motoneuronal influence on the muscle-fiber phenotype even when the motoneurons are silent.
authors.affiliation: Brain Research Institute, University of California at Los Angeles, 1320 Gonda Neuroscience and Genetics Building, Box 951761, Los Angeles, California 90095-1761, USA.
authors.forename: Hui
authors.initials: H
authors.lastname: Zhong
authors.forename: Roland R
authors.initials: RR
authors.lastname: Roy
authors.forename: John A
authors.initials: JA
authors.lastname: Hodgson
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Talmadge
authors.forename: Elena J
authors.initials: EJ
authors.lastname: Grossman
authors.forename: V Reggie
authors.initials: VR
authors.lastname: Edgerton
mesh_terms: D000818:Animals; D000252:Calcium-Transporting ATPases / Q000032:analysis; D002415:Cats; D004558:Electric Stimulation; D007537:Isometric Contraction / Q000502:physiology; D009046:Motor Neurons / Q000502:physiology*; D018656:Muscle Fibers, Fast-Twitch / Q000737:chemistry / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000737:chemistry / Q000502:physiology*; D018482:Muscle, Skeletal / Q000166:cytology* / Q000294:innervation* / Q000502:physiology; D018995:Myosin Heavy Chains / Q000032:analysis; D010641:Phenotype; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases
chemical_list: D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D018995:Myosin Heavy Chains; D000252:Calcium-Transporting ATPases