Effect of chronic oral furosemide administration on the 24-hour cycle of lithium clearance and electrolyte excretion in humans.

The effect of chronic furosemide treatment on the circadian cycle of lithium clearance (CLLi) and electrolyte excretion has been examined in 8 young, male volunteers, by performing two 24 h clearance experiments, before and after one week of treatment with furosemide 80 mg once daily. After 8 days on furosemide there was a significant decrease in creatinine clearance (-21%), plasma Na (-8.4 mM) and plasma K (-0.58 mM). At that time, however, there were no changes in 24 h-values of CLLi or Na excretion, although the magnitude of the circadian variation in CLLi and other renal parameters was increased. Both CLLi and CLNa were increased in the first 3 h following furosemide administration and thereafter they fell below the control level in the remaining hours of the experiment. From the absolute and fractional changes in CLLi it is suggested that compensatory Na conservation in response to chronic furosemide treatment occurs between doses, and that it involves decreased output from the proximal tubules combined with increased fractional Na reabsorption in the distal nephron.