Effects of ionic strength on calcium binding to rabbit skeletal myofibrils, thin filaments and myosin

Calcium binding by rabbit skeletal myosin, thin filaments and myofibrils was measured in solutions with and without 2 mM MgATP and with ionic strengths adjusted with KCl to 0.05, 0.10 and 0.14 M. Free Mg2+ was held constant at 1 mM, pH at 7.0 and temperature at 25 degrees C. In the presence of MgATP, the relation between free Ca2+ and myofibrillar bound calcium shifted to the left as ionic strength was decreased from 0.14 to 0.05 M. In the absence of MgAPT, myofibrillar calcium binding was enhanced over a wide range of free Ca2+ concentration, but calcium binding was no longer a function of ionic strength. Similarly, calcium binding by thin filaments and myosin was unaffected by changes in ionic strength from 0.05 to 0.14 M. In view of evidence that cross-bridge connections between thick and thin filaments increase as ionic strength decreases, our results suggest that these connections enhance myofibrillar calcium binding. These results thus confirm previous data of Bremel and Weber (Bremel, R. D. and Weber, A. (1972) Nature New Biol. 238, 97-101) who first showed that nucleotide-free cross-bridge connections enhance thin filament calcium binding.