Diurnal changes in serum metabolites and hormones in lean and obese Zucker rats.

A study of diurnal patterns of serum hormone levels was made in genetically lean and obese Zucker rats. Serum glucose and insulin were elevated in the obese through the 24-hour period; immunoreactive insulin being 12 times higher in the obese rat. A similar pattern of serum hormone levels in lean and obese rats was found for triiodothyronine, thyroid stimulating hormone and prolactin. At peak levels, prolactin was 2.5 times higher in lean than obese rats (P is less than .01). Thyroxine levels tended to be lower in the obese rats throughout the sampling periods when compared to lean rats. Growth hormone levels were severely depressed in the obese rat with the largest differences between lean and obese rats being observed during the light periods of the cycle. These differences in growth hormone secretion may be responsible for the impaired nitrogen retention in the obese rat. Serum levels of corticosterone were highest at the beginning of the dark cycle and decreased drastically thereafter in the lean rats. The obese rat did not exhibit a distinct pattern of corticosterone secretion and tended to be elevated throughout the periods tested.