We have previously shown that the apical membrane of chicken small intestine has a Na⁺–D-mannose cotransport system (Cano et al. 2001). In the current study D-[¹⁴C]mannose transport has been studied in enterocytes isolated from Hubbard chickens, 4-6 weeks old. The animals were killed by decapitation. The results show that following incubation in the presence of Na⁺, part of the D-mannose taken up by the cells is either metabolized (phosphorylated or bound to membrane components) or remains in a readily diffusive form. The nature of the cytosolic radioactivity was determined by ethanol-water extraction of the cells. Thin layer chromatography of the extracts revealed a peak (R_f = 0.8), which accounts for 80 % of the total applied radioactivity and migrates with free D-mannose, and another peak (R_f = 0.1) that migrates with the phosphorylated mannose. The cytosolic mannose that remains as a readily diffusive form is released from the cells by either adding cold D-mannose or transferring the cells into an ice bath. Following 30 min mannose uptake in the presence of Na⁺, the addition of either phloretin or cytochalasin B (inhibitors of Glut2 transporter) slightly reduced the previously accumulated D-mannose. These two agents, however, increased the cell to external medium 3-oxymethyl-glucose concentration ratio. D-Mannose efflux rate from preloaded D-[¹⁴C]mannose enterocytes is Na⁺ independent and insensitive to phloretin and cytochalasin B. It is concluded that part of the mannose taken up by the enterocytes is metabolized and that enterocytes have two D-mannose transport systems: one is concentrative and Na⁺ dependent and the other is Na⁺ independent, passive and different from Glut2.

This work was supported by grants from the Spanish DGICYT PM99-0121.