On the mechanism of nickel absorption in the rat jejunum

Nickel is taken up from the lumen of the perfused rat jejunum (step 1 of Ni absorption) by a mechanism closely resembling that responsible for step 1 of Cd absorption. Both processes are depressed to a similar extent by Zn or the constituents of dried skimmed milk. They procede at a rate proportional to concentration up to 20 microM; at higher levels they approach apparent saturation. In contrast to the slow movement of Cd from mucosa into the body (step 2 of absorption), Ni is not appreciably retained in the mucosa. Retention of Ni is not increased by induction of metallothionein synthesis, in agreement with its low affinity for this protein. Several observations suggest that basolateral membranes may be passively permeable to Ni: step 2 follows first order kinetics; Ni is also taken up by the mucosa from the serosal side, in vivo and in vitro; and no inhibitory effect of Zn could be shown. The ability of the mucosa to discriminate between the toxic, non-essential metal Cd and the essential metal Ni may depend therefore on the efficiency of mucosal trapping rather than the presence of Ni carriers on brush border or basolateral cell membranes.