Lithium absorption in tight and leaky segments of intestine

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Abstract
There is significant absorption of Li+ by human jejunum and ileum, but negligible absorption by human colon. Thus, a proximal-to-distal gradient of decreasing Li+ absorption and increasing junctional tightness exists in intestine as well as in renal tubule. For six leaky epithelia the relative permeabilities of K+, Na+, and Li+ by the junctional route are in the sequenceP K>P Na>P Li and all fall within a factor of 2.5. In contrast, for tight epitheliaP Li∼P Na≫P K in the amiloride-sensitive channel of the apical membrane, butP K≫P Li∼P Na in the basolateral membrane. The ability of several tight epithelia to sustain nonzero transepithelial Li+ absorption despite this basolateral barrier may be due to Na+/Li+ countertransport at the basolateral membrane, resulting in secondary active transport of Li+ across the epithelium.