Some major transport mechanisms of insect absorptive epithelia.

1. After hormonal stimulation, fluid reabsorption (JV) in locust hindgut from the KCl-rich, low-Na primary urine is driven primarily by an unusual mucosal electrogenic Cl- pump (JCl). 2. Cyclic-AMP increases JCl and also mucosal K+ and basolateral Cl- conductances, so that KCl absorption exceeds that of Na+ in rectum but not ileum. 3. Mucosal entry of Na+ occurs by exchange for NH4+ (H+), by cotransport with some neutral amino acids, and through putative channels. 4. However, transport of proline, the predominant organic substrate, is largely Na-independent and drives a sizable component of Jv in rectal but not ileal segments. 5. There is evidence for hormonal control of Na+ reabsorption in ileum but not rectum.