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 alkal-inization of acidified human fibroblasts and platelets and assess the kinetics of the Na⁺/ H⁺ antiport in these cells. Cytosolic pH (pH_i) exhibits an exponential recovery following cellular acidification. Thus, the length of the time interval selected to monitor changes in pH_i (ΔpH_i) 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 ΔpH_i, for arbitrarily selected time intervals following Na⁺-dependent activation. In both cells, significant increases in both the [Na⁺] for half-maximal activation (K _0.5) and maximal velocities (V _max) were observed as ΔpH_i was decreased. We conclude that kinetic parameters derived from initial rate determinations enable a more accurate characterization of the Na⁺/H⁺ antiport.