16 

 absorption is species dependant. The duodenum, however, has 

 been generally accepted as the primary site for Cu absorption 

 in most species (O'Dell, 1990) . 



The mechanism of Cu absorption is not clear, but 

 absorption is known to be regulated at the intestinal mucosa. 

 Passage of Cu across mucosal membrane and transport across 

 cells are concentration-dependent and saturable and uptake by 

 mucosal cells is not energy dependent (Crampton et al . , 1965) . 

 Since MT has a stronger affinity for Cu than for Zn, the 

 protein greatly influences Cu absorption in intestinal cells 

 (Cousins, 1985) . In adequate or high dietary Cu, when the 

 animals demand of Cu is low, Cu enters the enterocyte and 

 binds to MT preventing any additional uptake. With low 

 dietary Cu, the MT bound Cu present in the intestine would 

 have been released through the basolateral membrane to the 

 portal vein and consequently taken to the liver. Sheep, 

 however, are more sensitive to Cu toxicosis because of the 

 lack of intestinal MT synthesis (Saylor et al . , 1980). 

 Furthermore, Turner et al . (1987) working with everted sacks 

 of sheep jejunum, suggested that Cu uptake from lumen to cells 

 was a process neither saturable nor energy-dependant but whose 

 kinetics reflected that of simple diffusion. Another 

 important factor in Cu homeostasis in sheep is that sheep 

 cannot excrete high amounts of Cu in bile acids (Gooneratne et 

 al. , 1989) . 



