1. High fractional reabsorption of lithium occurs when rats and dogs are given a low sodium diet. This has been suggested to be due to the low tubular fluid sodium to lithium concentration ratio which arises during sodium restriction allowing transport mechanisms along the distal nephron segment to accept significant quantities of lithium instead of sodium by simple competition.
2. Clearance experiments in conscious water-loaded Wistar rats maintained on either a low sodium (6 mmol/kg) or a normal sodium (120 mmol/kg) diet were performed. The rats were infused with a solution of 120 mmol/l glucose and 10 mmol/l NaCl at a rate of 6 ml/h.
3. The median fractional excretion of lithium was 19.4% in the group with a normal sodium intake, and 2.5% in the group given a low sodium diet. The urine to plasma concentration ratio of lithium was 2.5 in the normal sodium group and 0.4 in the sodium-restricted group. The urinary sodium to lithium concentration ratio was 13 in the control group and 63 in the sodium-restricted group.
4. It is concluded that the increased fractional reabsorption of lithium initiated by a low sodium intake is not likely to be due to simple competition at distal tubular nephron sites between lithium and sodium.