Effect of formate and oxalate on fluid reabsorption from the proximal convoluted tubule of the anaesthetized rat.

1. It has recently been suggested that both formate and oxalate may be involved in the reabsorption of chloride from the proximal convoluted tubule (PCT). Reabsorption is achieved via a mechanism coupling Na(+)‐H+ exchange to Cl(‐)‐anion exchange thus facilitating net NaCl transport. This possible mechanism was investigated further by observing the effect of luminal addition of formate and oxalate on proximal tubule transport in anaesthetized rats, previously prepared for in vivo microperfusion. 2. Addition of formate (0.25 mmol l‐1) to the luminal perfusate resulted in a significant stimulation of fluid reabsorptive rate. 3. Oxalate was found to have a concentration‐dependent effect on fluid reabsorption. Luminal oxalate concentrations below 0.05 mmol l‐1 stimulated fluid reabsorption, while an oxalate concentration of 0.5 mmol l‐1 inhibited fluid reabsorption. 4. The presence of formate (0.25 mmol l‐1) and oxalate (0.001 mmol l‐1) in the same luminal perfusate stimulated fluid reabsorption from the proximal tubule lumen, but the effects of formate and oxalate were not additive. 5. The removal of bicarbonate from the luminal perfusate and its replacement with phosphate had no significant effect on fluid reabsorptive rate. An alteration in perfusate pH was also ineffective. Addition of oxalate at high and low pH failed to stimulate fluid reabsorption compared with pH‐matched control perfusions. 6. The experimental data suggest the involvement of both formate and oxalate in the reabsorption of NaCl from the proximal tubule lumen, though their influence may not be exerted through the same mechanisms.