Thallium(I) secretion across the isolated mucosa of rat descending colon

Unidirectional Tl+-fluxes across the isolated mucosa of rat descending colon were measured under short circuit (SCC) or voltage clamp conditions (VCC). Under SCC a serosal-to-mucosal net flux (Jnet = -0.22 nmol X cm-2 X h-1) was observed that agrees with the voltage-independent component measured under VCC. At 7 degrees C the secretory net flux was abolished. In controls both unidirectional fluxes were unsaturable between 0.1 and 500 mumol/l thallium (I). After furosemide or withdrawal of Cl- or Na+ Jnet was abolished. Ouabain decreased serosal-to-mucosal Tl+ flux and an energy-dependent absorptive net flux of Tl+ was observed, characterized by a rather low Km (10.2 mumol/l), Vmax (3.8 nmol X cm-2 X h-1) and apparent activation energies (delta E = 10.7 kcal X mol-1) typically for enzyme catalaysed reactions or narrow channel interactions. The data suggest that in the mucosa of rat descending colon Tl+ ions share, at least in part, the same transport systems at K+.