In vitro and in vivo studies on Leydig cell function in old rats.

Young adult (3 months old) and old (26-28 months old) male Wistar rats were studied. Testicular weight was 1.66 g (range: 1.33--2.86) in the younger group and 1.62 g (range: 1.04--1.90) in the older group. The total Leydig cell volume as measured by a quantitative histometric method was significantly larger in the old animals (-x = 0.153 ml vs. -x = 0.089 ml). The testicular HCG binding capacity was 2.54 ng HCG per 100 mg tissue (range: 1.88--3.77) in the younger animals and 1.83 ng HCG per 100 mg tissue (range: 0.80--3.02) in the older ones (P less than 0.01). Plasma testosterone was on an average 242 ng/100 ml (range: 72--1162) in the young adult rats and 91 ng/100 ml (range: 23--277) in the older rats. Plasma LH was only slightly (P less than 0.05) decreased and was 49 ng LH-RP-1/ml (range: 14--120) in the younger group and 40 ng LH-RP-1 ng/ml (range: 5--98) in the older. When the testicular tissue pieces were incubated with different doses of HCG or dibutyryl cAMP and testosterone production was measured, identical dose-response curves for old and young tissue were obtained. After in vitro incubation with a NADPH generating system, which under the conditions used was a much stronger stimulus for steroid biosynthesis than HCG or dibutyryl cAMP, the tissue of young rats produced about twice the amount of testosterone precursors (pregnenolone, progesterone, 17alpha-hydroxyprogesterone and androstenedione) than old tissue.