title: Balanced changes in Ca buffering by SERCA and troponin contribute to Ca handling during β-adrenergic stimulation in cardiac myocytes.
abstract: OBJECTIVE
  During activation of cardiac myocytes, less than 1% of cytosolic Ca is free; the rest is bound to buffers, largely SERCA, and troponin C. Signalling by phosphorylation, as occurs during β-adrenergic stimulation, changes the Ca-binding affinity of these proteins and may affect the systolic Ca transient. Our aim was to determine the effects of β-adrenergic stimulation on Ca buffering and to differentiate between the roles of SERCA and troponin.
  
  
  RESULTS
  Ca buffering was studied in cardiac myocytes from mice: wild-type (WT), phospholamban-knockout (PLN-KO), and mice expressing slow skeletal troponin I (ssTnI) that is not protein kinase A phosphorylatable. WT cells showed no change in Ca buffering in response to the β-adrenoceptor agonist isoproterenol (ISO). However, ISO decreased Ca buffering in PLN-KO myocytes, presumably unmasking the role of troponin. This effect was confirmed in WT cells in which SERCA activity was blocked with the application of thapsigargin. In contrast, ISO increased Ca buffering in ssTnI cells, presumably revealing the effect of an increase in Ca binding to SERCA.
  
  
  CONCLUSIONS
  These data indicate the individual roles played by SERCA and troponin in Ca buffering during β-adrenergic stimulation and that these two buffers effectively counterbalance each other so that Ca buffering remains constant during β-adrenergic stimulation, a factor which may be physiologically important. This study also emphasizes the importance of taking into account Ca buffering, particularly in disease states where Ca binding to myofilaments or SERCA may be altered.
authors.affiliation: Unit of Cardiac Physiology, Manchester Academic Health Science Centre, Core Technology Facility, 46 Grafton St, Manchester M13 9NT, UK.
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Briston
authors.affiliation: Unit of Cardiac Physiology, Manchester Academic Health Science Centre, Core Technology Facility, 46 Grafton St, Manchester M13 9NT, UK.
authors.forename: Katharine M
authors.initials: KM
authors.lastname: Dibb
authors.affiliation: Department of Physiology and Biophysics, University of Illinois at Chicago, Chicago, IL, USA.
authors.forename: R John
authors.initials: RJ
authors.lastname: Solaro
authors.affiliation: Unit of Cardiac Physiology, Manchester Academic Health Science Centre, Core Technology Facility, 46 Grafton St, Manchester M13 9NT, UK.
authors.forename: David A
authors.initials: DA
authors.lastname: Eisner
authors.affiliation: Unit of Cardiac Physiology, Manchester Academic Health Science Centre, Core Technology Facility, 46 Grafton St, Manchester M13 9NT, UK andrew.w.trafford@manchester.ac.uk trafford@man.ac.uk.
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Trafford
mesh_terms: D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects*; D002135:Calcium-Binding Proteins / Q000172:deficiency / Q000235:genetics; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007545:Isoproterenol / Q000494:pharmacology*; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000201:enzymology; D010766:Phosphorylation; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013997:Time Factors; D019210:Troponin I / Q000235:genetics / Q000378:metabolism*; D000099211:Phospholamban
chemical_list: D000318:Adrenergic beta-Agonists; D002118:Calcium; D002135:Calcium-Binding Proteins; D004791:Enzyme Inhibitors; D007545:Isoproterenol; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019210:Troponin I; D000099211:Phospholamban
keywords: Buffering; Calcium; Phospholamban; SERCA; Troponin