PREDICTED CHANGES IN CONCENTRATIONS OF FREE AND BOUND ATP AND ADP DURING INTRACELLULAR CA2+ SIGNALING

High Ca2+ concentrations can develop near Ca2+ sources during intracel lular signaling and might lead to localized regulation of Ca2+-depende nt processes. By shifting the amount of Ca2+ and other cations associa ted with ATP, local high Ca2+ concentrations might also alter the subs trate available for membrane-associated and cytoplasmic enzymes. To st udy this, simultaneous equations were solved over a range of Ca2+ and Mg2+ concentrations to determine the general effects of Ca2+ on the co ncentrations of free and Ca2+- and Mg2+-bound forms of ATP. To obtain a more specific picture of the changes that might occur in smooth musc le cells, mathematical models of Ca2+ diffusion and regulation were us ed to predict the magnitude and time course of near-membrane Ca2+ tran sients and their effects on the free and bound forms of ATP near the m embrane. The results of this work indicate that changes in free Ca2+ c oncentration over the range of 50 nM-100 mu M would result in signific ant changes in free ATP concentration, MgATP concentration, and the Ca ATP-to-MgATP concentration ratio.