Characterization of the Erythrocyte Sodium-Lithium Countertransporter: Limitations and Assumptions of Traditional and Kinetic Methodologies

The present work examined the key elements featuring in the various methods used to characterize the erythrocyte sodium-lithium countertransport. Effects of medium composition on lithium efflux were investigated in 20 subjects. Mean lithium efflux (mmol Li/l RBC.h) into a 150 mm sodium medium was significantly higher than efflux into a revised sodium-rich medium (149 mm) containing 1 mm Mg (0.335 ± 0.100 vs. 0.298 ± 0.085 respectively; P < 0.03). Mean lithium efflux into sodium-free media where sodium had been entirely replaced by magnesium, was significantly lower than efflux into a choline-based medium containing only 1 mm magnesium (0.088 ± 0.027 vs. 0.109 ± 0.034 respectively; P= 0.03). Sodium-lithium countertransport activity and the transporter's kinetic profile were measured simultaneously in 35 subjects using traditional choline-based and kinetic methodologies. There was a significant correlation between countertransport activity and maximal rate of turnover (V _max) (r= 0.62; P < 0.001); V _max values were consistently greater than their corresponding countertransport activities (P < 0.001). On subdividing the subject group into tertiles based on the Michaelis-Menten constant (k _m ) values (mm), <75, 75 − 150 and >150, the slopes of the regression lines for each group diminished progressively (0.64, 0.49 and 0.23 respectively), correlations within each group remained significant (P < 0.001, P < 0.001 and P < 0.02). No significant correlation was found between k _m values and countertransport activity (r= 0.035; P=ns). Increasing the number of points representing sodium concentrations within the range 0–150 mm, improved the confidence in the emerging estimates of V _max and k _m obtained by linear transformation. Comparison of kinetic data derived using four different analytical methods (two linear transformations, a nonlinear regression and a statistical method), showed no significant differences between the estimates yielded for either V _max (P= 0.88, ns) or k _m (P= 0.92, ns). This study has highlighted the critical roles of assay conditions and derivation techniques used when measuring sodium-lithium countertransport, emphasizing the need for standardization of the methodology. Received: 10 December 1996/Revised: 2 October 1997