Ca2+ buffering in the heart: Ca2+ binding to and activation of cardiac myofibrils

The measurement of cardiac Ca2+ transients using spectroscopic Ca2+ indicat ors is significantly affected by the buffering properties of the indicators . The aim of the present study was to construct a model of cardiac Ca2+ buf fering that satisfied the kinetic constraints imposed by the maximum attain able rates of cardiac contraction and relaxation on the Ca2+ dissociation r ate constants and which would account for the observed effects of F-19-NMR indicators on the cardiac Ca2+ transient in the Langendorff-perfused ferret heart. It is generally assumed that the Ca2+ dependency of myofibril activ ation in cardiac myocytes is mediated by a single Ca2+. binding site on tro ponin C. A model based on 1:1 Ca2+ binding to the myofilaments, however, wa s unable to reproduce our experimental data, but a model in which we assume d ATP-dependent co-operative Ca2+ binding to the myofilaments was able to r eproduce these data. This model was used to calculate the concentration and dissociation constant of the ATP-independent myofilament Ca2+ binding, giv ing 58 and 2.0 mu M respectively. In addition to reproducing our experiment al data on the concentration of free Ca2+ ions in the cytoplasm ([Ca2+](i)) , the resulting Ca2+ and ATP affinities given by fitting of the model also provided good predictions of the Ca2+ dependence of the myofibrillar ATPase activity measured under iii vitro conditions. Solutions to the model also indicate that the Ca2+ mobilized during each beat remains unchanged in the presence of the additional buffering load from Ca2+ indicators. The new mod el was used to estimate the extent of perturbation of the Ca2+ transient ca used by different concentrations of indicators. As little as 10 mu M of a C a2+ indicator with a dissociation constant of 200 nM will cause a 20 % redu ction in peak-systolic [Ca2+](i) and 30 mu M will cause approx. 50%, reduct ion in the peak-systolic [Ca2+](i) in a heart paced at 1.0 Hz.