title: Ca2+ buffering in the heart: Ca2+ binding to and activation of cardiac myofibrils.
abstract: The measurement of cardiac Ca(2+) transients using spectroscopic Ca(2+) indicators is significantly affected by the buffering properties of the indicators. The aim of the present study was to construct a model of cardiac Ca(2+) buffering that satisfied the kinetic constraints imposed by the maximum attainable rates of cardiac contraction and relaxation on the Ca(2+) dissociation rate constants and which would account for the observed effects of (19)F-NMR indicators on the cardiac Ca(2+) transient in the Langendorff-perfused ferret heart. It is generally assumed that the Ca(2+) dependency of myofibril activation in cardiac myocytes is mediated by a single Ca(2+)-binding site on troponin C. A model based on 1:1 Ca(2+) binding to the myofilaments, however, was unable to reproduce our experimental data, but a model in which we assumed ATP-dependent co-operative Ca(2+) binding to the myofilaments was able to reproduce these data. This model was used to calculate the concentration and dissociation constant of the ATP-independent myofilament Ca(2+) binding, giving 58 and 2.0 microM respectively. In addition to reproducing our experimental data on the concentration of free Ca(2+) ions in the cytoplasm ([Ca(2+)](i)), the resulting Ca(2+) and ATP affinities given by fitting of the model also provided good predictions of the Ca(2+) dependence of the myofibrillar ATPase activity measured under in vitro conditions. Solutions to the model also indicate that the Ca(2+) mobilized during each beat remains unchanged in the presence of the additional buffering load from Ca(2+) indicators. The new model was used to estimate the extent of perturbation of the Ca(2+) transient caused by different concentrations of indicators. As little as 10 microM of a Ca(2+) indicator with a dissociation constant of 200 nM will cause a 20% reduction in peak-systolic [Ca(2+)](i) and 30 microM will cause approx. 50% reduction in the peak-systolic [Ca(2+)](i) in a heart paced at 1.0 Hz.
authors.affiliation: Section of Cardiovascular Biology, Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, UK. g.a.smith@bioc.cam.ac.uk
authors.forename: G A
authors.initials: GA
authors.lastname: Smith
authors.forename: H B
authors.initials: HB
authors.lastname: Dixon
authors.forename: H L
authors.initials: HL
authors.lastname: Kirschenlohr
authors.forename: A A
authors.initials: AA
authors.lastname: Grace
authors.forename: J C
authors.initials: JC
authors.lastname: Metcalfe
authors.forename: J I
authors.initials: JI
authors.lastname: Vandenberg
mesh_terms: D000255:Adenosine Triphosphate; D000818:Animals; D002118:Calcium / Q000032:analysis / Q000502:physiology*; D005289:Ferrets; D006321:Heart / Q000502:physiology*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D009200:Myocardial Contraction / Q000502:physiology*; D014336:Troponin
chemical_list: D014336:Troponin; D000255:Adenosine Triphosphate; D002118:Calcium