Aging and cholesterol uptake in the rabbit jejunum: role of the bile salt micelle and the unstirred water layer.

A previously validated in vitro technique was used to determine the effect of aging upon the rate of uptake of cholesterol into the jejunum of suckling, young, and older rabbits. Cholesterol uptake was greater in suckling than in older animals, over a wide range of durations of incubation and varying concentrations of cholesterol or bile acid. The rate of uptake of cholesterol in the young animals was intermediate between the values seen in the suckling and older rabbits. This greater uptake of cholesterol in the younger than in the older animals persisted when the effective resistance of the unstirred water layer (UWL) was varied by stirring the bulk phase. In contrast, the uptake of medium- and long-chain length fatty acids was greater in the young than in the older animals when UWL was low, but the converse was true when UWL was high at each rate of stirring of the bulk phase. The UWL was lower in the younger than in the older rabbits. Thus, the differences in the in vitro uptake of cholesterol into the jejunum of rabbits of varying age is due to the greater passive permeability properties and greater functional membrane surface area of the jejunum of young animals, and the lower effective resistance of the overlying unstirred water layer.