The effect of lithium on the transport of sodium, potassium and chloride by the colon of normal and sodium-depleted rats.

1. The effect of Li, given systemically or placed in the gut lumen, on the transport of Na, K and C1 and on the transepithelial electrical potential difference (p.d.) was studied in vivo in the distal colon of normal and Na-depleted rats. 2. The specific effect of Li appeared to be on the Na transport system with K and C1 transport affected only indirectly. Active Na absorption was impaired and p.d. reduced when either Li was in the lumen or given systemically. In addition with Li in the lumen, a considerable rise in the plasma-to-lumen Na flux was observed, the flux increasing progressively with rising intraluminal Li concentration. The effects were greater in Na-depleted rats. 3. The greater part of Li absorption from the colon of the rat takes place by exchange for Na, the secretion of which is much enhanced while the p.d. is reduced. This contrasts with human colon in which potassium is the ion exchanged for Li with the p.d. increased.