title: Enhanced Ca2+ transport and muscle relaxation in skeletal muscle from sarcolipin-null mice.
abstract: Sarcolipin (SLN) inhibits sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pumps. To evaluate the physiological significance of SLN in skeletal muscle, we compared muscle contractility and SERCA activity between Sln-null and wild-type mice. SLN protein expression in wild-type mice was abundant in soleus and red gastrocnemius (RG), low in extensor digitorum longus (EDL), and absent from white gastrocnemius (WG). SERCA activity rates were increased in soleus and RG, but not in EDL or WG, from Sln-null muscles, compared with wild type. No differences were seen between wild-type and Sln-null EDL muscles in force-frequency curves or maximum rates of force development (+dF/dt). Maximum relaxation rates (-dF/dt) of EDL were higher in Sln-null than wild type across a range of submaximal stimulation frequencies, but not during a twitch or peak tetanic contraction. For soleus, no differences were seen between wild type and Sln-null in peak tetanic force or +dF/dt; however, force-frequency curves showed that peak force during a twitch and 10-Hz contraction was lower in Sln-null. Changes in the soleus force-frequency curve corresponded with faster rates of force relaxation at nearly all stimulation frequencies in Sln-null compared with wild type. Repeated tetanic stimulation of soleus caused increased (-dF/dt) in wild type, but not in Sln-null. No compensatory responses were detected in analysis of other Ca(2+) regulatory proteins using Western blotting and immunohistochemistry or myosin heavy chain expression using immunofluorescence. These results show that 1) SLN regulates Ca(2+)-ATPase activity thereby regulating contractile kinetics in at least some skeletal muscles, 2) the functional significance of SLN is graded to the endogenous SLN expression level, and 3) SLN inhibitory effects on SERCA function are relieved in response to repeated contractions thus enhancing relaxation rates.
authors.affiliation: Department of Kinesiology, University of Waterloo, Ontario, Canada. rtupling@uwaterloo.ca
authors.forename: A Russell
authors.initials: AR
authors.lastname: Tupling
authors.forename: Eric
authors.initials: E
authors.lastname: Bombardier
authors.forename: Subash C
authors.initials: SC
authors.lastname: Gupta
authors.forename: Dawar
authors.initials: D
authors.lastname: Hussain
authors.forename: Chris
authors.initials: C
authors.lastname: Vigna
authors.forename: Darin
authors.initials: D
authors.lastname: Bloemberg
authors.forename: Joe
authors.initials: J
authors.lastname: Quadrilatero
authors.forename: Maria G
authors.initials: MG
authors.lastname: Trivieri
authors.forename: Gopal J
authors.initials: GJ
authors.lastname: Babu
authors.forename: Peter H
authors.initials: PH
authors.lastname: Backx
authors.forename: Muthu
authors.initials: M
authors.lastname: Periasamy
authors.forename: David H
authors.initials: DH
authors.lastname: MacLennan
authors.forename: Anthony O
authors.initials: AO
authors.lastname: Gramolini
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000502:physiology*; D002118:Calcium / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000502:physiology; D051379:Mice; D018345:Mice, Knockout; D009119:Muscle Contraction / Q000502:physiology; D009124:Muscle Proteins / Q000235:genetics* / Q000378:metabolism*; D009126:Muscle Relaxation / Q000502:physiology*; D009154:Mutation; D009206:Myocardium / Q000378:metabolism; D011510:Proteolipids / Q000235:genetics* / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000378:metabolism
chemical_list: D009124:Muscle Proteins; D011510:Proteolipids; C077258:sarcolipin; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D002118:Calcium