Emergence of Na+-glucose cotransport in an epithelial secretory cell line sensitive to cholera toxin.

Epithelial properties and effects of cholera toxin (CT) and glucose were investigated in human rectal tumor cell line HRT-18. Addition of 10(-3) M dibutyryl adenosine 3',5'-cyclic monophosphate (DbcAMP), 10(-8) M vasoactive intestinal peptide, 10(-5) M epinephrine, and 10(-5) M forskolin to the serosal side and of 3.5 micrograms/ml CT to the mucosal side and of 2 micrograms/ml A23187 to both the serosal and mucosal sides raised short-circuit current (Isc). This rise was reversed by serosal addition of 5 x 10(-5) M bumetanide or 10(-4) M ouabain. In filters treated with CT, Isc and net chloride flux (JClnet) increased after 60 min from 0.05 +/- 0.008 and -0.04 in the Ringer to 0.32 +/- 0.05 and -0.33 mueq.h-1.cm-2, respectively. Addition of 10(-2) M glucose further raised Isc by stimulating net sodium flux (JNanet) (0.70 +/- 0.08 and + 0.58 mueq.h-1.cm-2, respectively). This additional augmentation of Isc was reversed by 0.5 mM phlorizin and was mimicked by 3-O-methyl-D-glucose. When the filters were stimulated by cAMP for 15 min, Isc was also enhanced by addition of glucose. In untreated filters, Isc, JNanet, and JClnet did not differ significantly before and after addition of glucose. It is concluded that HRT-18 cells in basal state do not display absorptive properties but secretory properties stimulated by CT. However they exhibit Na+-glucose cotransport once stimulated by either CT or cAMP.