Sugar-stimulated ion absorption is not different in seahare and vertebrate intestine.

We have reexamined the notion that sugars stimulate ion absorption differently in invertebrate and vertebrate intestine. In the seahare intestine, mucosal sugar presumably increases the rate of transcellular Na+ and Cl- absorption, whereas only transcellular Na+ absorption is increased in the vertebrate small intestine. Our data indicate that the seahare intestine responds to mucosal D-galactose like the vertebrate small intestine: namely, the apical membrane electrical potential difference depolarizes, the ratio of the mucosal to serosal membrane resistances decreases, and the short-circuit current (Isc) increases. Because mucosal substitution of tetramethylammonium for Na+ abolished the increased Isc, this stimulation resulted from an increase in rheogenic Na+ absorption. Unidirectional transepithelial Cl- fluxes indicate that mucosal D-galactose had no effect on the net Cl- flux under short-circuit conditions. Further, ion substitution experiments indicate that the apical membrane is K+ conductive rather than Cl- conductive as previously reported. These electrophysiological as well as parallel histological findings indicate that studies previously reported on the seahare intestine were in fact conducted on the esophagus.