Refined estimation of kinetic parameters of the Na+/H+ antiport in human fibroblasts and platelets.

A technique is presented to estimate the initial rates of Na(+)-dependent alkalinization of acidified human fibroblasts and platelets and assess the kinetics of the Na+/H+ antiport in these cells. Cytosolic pH (pHi) exhibits an exponential recovery following cellular acidification. Thus, the length of the time interval selected to monitor changes in pHi (delta pHi) is critical to estimating the kinetics of the Na+/H+ antiport. We compared kinetic parameters of the Na+/H+ antiport, using computed and observed changes in delta pHi, for arbitrarily selected time intervals following Na(+)-dependent activation. In both cells, significant increases in both the [Na+] for half-maximal activation (K0.5) and maximal velocities (Vmax) were observed as delta pHi was decreased. We conclude that kinetic parameters derived from initial rate determinations enable a more accurate characterization of the Na+/H+ antiport.