OBJECTIVETo investigate the mechanism of serum testosterone reduction, its relationship with metabolism, changes in the number and morphology of Leydig cells and endocrine function in aging male rats.

METHODSThe levels of serum total testosterone (tT), LH, FSH, HDL, LDL, TG, TC, Glu, INS, IRG and LP were determined in young (9 mo) and aging rats (12, 15, 18 and 21 mo), with 6 in each group. The morphological changes of Leydig cells were observed under the microscope. The concentrations of testosterone secreted from the cultured Leydig cells with the stimulation of hCG and Forskolin were assayed. The apoptosis rates of Leydig cells were detected by TUNEL. The visceral fat was isolated and weighed, and the Lee's index calculated. All the above indexes were recorded and compared among different age groups.

RESULTSThe aging rats showed a significant decrease in the levels of serum tT and TSI ([1.26 +/- 0.65] ng/ml and [0.07 +/- 0.65] ng/mIU) as compared with the young rats ([3.24 +/- 0.38] ng/ml and [0.21 +/- 0.01] ng/mIU) (P < 0.01). Obvious differences were found in the morphology of Leydig cells among different age groups. The T secretion of Leydig cells at 24, 48 and 72 h in aging rats was markedly decreased (P < 0.05) while their TUNEL positive rate remarkably increased in the aging rats (17.36% +/- 1.31%) compared with the young ones (7.02% +/- 1.05%) (P < 0.05). There were statistically significant differences between the young and aging rats in all the biochemical parameters including IRG, HDL, LDL, TG, TC and visceral fat content (P < 0.05), except the levels of serum Glu, INS and LP (P > 0.05).

CONCLUSIONThe serum T level and secreting capacity of Leydig cells are significantly lower in aging rats than in young ones, and the metabolic parameters undergo regular changes with the decreasing level of serum T. The reduction of testosterone in aging male rats may be associated with the decreased secreting capacity and number of Leydig cells and declined function of the pituitary.