Developmental changes in thymidine kinase, DNA, and fat cellularity in Zucker rats.

Thymidine kinase activity and the pattern of DNA accretion in the genetically obese Zucker rat (fafa) were shown to develop in a manner fundamentally different from that of the lean rat. In normal lean Zucker rats, fat cell size and number, thymidine kinase activity, total DNA, and DNA in lipid-filled and nonlipid-filled tissue changed as previously reported for the normally growing lean Sprague-Dawley rat. In the epididymal depot of the developing obese rat, the progressive obesity is characterized by marked early enlargement of fat cell size, elevated thymidine kinase activity until 273 days of age, increased rate of total tissue DNA accretion until 182 days of age, and fat cell hyperplasia that becomes manifest after an apparent "peak" cell size is reached at 98 days of age.