High Ca²⁺ concentrations can develop near Ca²⁺ sources during intracellular signaling and might lead to localized regulation of Ca²⁺-dependent processes. By shifting the amount of Ca²⁺ and other cations associated with ATP, local high Ca²⁺ concentrations might also alter the substrate available for membrane-associated and cytoplasmic enzymes. To study this, simultaneous equations were solved over a range of Ca²⁺ and Mg²⁺ concentrations to determine the general effects of Ca²⁺ on the concentrations of free and Ca²⁺- and Mg²⁺-bound forms of ATP. To obtain a more specific picture of the changes that might occur in smooth muscle cells, mathematical models of Ca²⁺ diffusion and regulation were used to predict the magnitude and time course of near-membrane Ca²⁺ transients and their effects on the free and bound forms of ATP near the membrane. The results of this work indicate that changes in free Ca²⁺ concentration over the range of 50 nM-100 µM would result in significant changes in free ATP concentration, MgATP concentration, and the CaATP-to-MgATP concentration ratio.