Cellular origin of lactase decline in postweaned rats.

Intestinal lactase activity falls at weaning reaching low levels in the adult rat. Present work measures cellular migration rates and lactase activity along villi of intestines taken from rats of different ages to determine the cellular basis for this developmentally regulated fall in enzyme expression. An early fall in lactase activity taking place 15 to 23 days after birth was found to be associated with a shortening of the time available for lactase expression in brush-border membranes. A later fall in lactase activity occurring 23 to 46 days after birth was caused by an additional reduction in the rate at which lactase activity appeared in the brush-border membrane. These results are discussed in relation to previous work describing lactase biosynthesis in post-weaned rats.