The development of white adipose tissue. Effect of litter size on the lipoprotein lipase activity of four adipose-tissue depots, serum immunoreactive insulin and tissue cellularity during the first year of life in male and female rats.

(1.) Male and female rats reared in litters of four gained body weight more rapidly than animals reared in litters of 16. The differences were more marked in males than females and became less marked in both sexes with advancing age. (2.) The relative weights of the perigenital, perirenal, subcutaneous and intramuscular white-adipose-tissue sites in the animals from small litters indicated their relative obesity compared with animals from large litters. A sex-related difference in the distribution of adipose tissue between the four sites was seen in animals reared in litters of both four and 16. (3.) Although at 30 days of age all the animals had more numerous and larger fat-cells in their white-adipose-tissue depots than animals reared in large litters, the pattern of change thereafter was both site- and sex-specific. During the post-weaning period (30-300 days), although detailed differences were apparent between sites, a general pattern of increased cell size in males and increased cell numbers in females emerged as being the important determinants responsible for the differences in depot sizes seen when animals from litters of four and 16 were compared. (4.) Lipoprotein lipase activities, expressed as units/g fresh wt. of tissue, in the depots of animals reared in groups of four were unaltered compared with those reared in groups of sixteen during the post-weaning period (47-300 days of age), and enzyme activities expressed per depot merely reflected differences in tissue weights. (5.) Lipoprotein lipase activities per 10(6) cells were higher in males reared in fours compared with those reared in sixteens of equivalent age, but were unaltered for females. (6.) The persistent hyperinsulinaemia of animals reared in litters of four is discussed in relation to the observed differences in enzyme activity and white-adipose-tissue cellularity.