Assay of human erythrocyte sodium-dependent lithium efflux: the importance of timing of blood sampling

The activity of the human erythrocyte sodium–lithium countertransport (SLC) is stable over long periods in individuals. However, it is becoming increasingly evident that the transport system is susceptible to modulation, both acutely and chronically, by various factors. In this study, the authors observed temporal variation in SLC over a period of 10 h (08.00–18.00 hours) in healthy volunteers. SLC V _max was maximum (0.354 ± 0.051 mmol L⁻¹ cell⁻¹ h⁻¹; mean ± SE) at 'mid-day' and significantly higher than in the morning (0.291 ± 0.035 mmol L⁻¹ cell, h; P  < 0.010). Its value in the evening (0.316 ± 0.042 mmol L⁻¹ cell⁻¹ h⁻¹) was lower than at 'mid-day' ( P  < 0.045) but higher than in the morning ( P  < 0.037). These changes were not accompanied by any significant change in the affinity of the transporter for external sodium, K _m. Changes in SLC V _max did not correlate with the corresponding ones in either plasma cortisol or aldosterone. However, they correlated well with those in plasma renin activity, the correlation between mid-day and a.m. sets of values ( r  = 0.718; P  = 0.019) being better than that between mid-day and p.m. ( r  = 0.688; P  = 0.028). The authors conclude that changes in SLC occur during the day, and this need be taken into account in the planning and execution of studies involving determination of the activity of this transport system.