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 becom ing increasingly evident that the transport system is susceptible to m odulation, both acutely and chronically, by various factors. In this s tudy, 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' a nd 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 high er than in the morning (P < 0.037). These changes were not accompanied by any significant change in the affinity of the transporter for exte rnal sodium, K-m. Changes in SLC V-max did not correlate with the corr esponding 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 b etter than that between mid-day and p.m. (r = 0.688; P = 0.028). The a uthors conclude that changes in SLC occur during the day, and this nee d be taken into account in the planning and execution of studies invol ving determination of the activity of this transport system.