Fig. 4.

The preovulatory LH surge is blunted in middle-aged hamsters. LH concentrations are shown relative to zeitgeber (light: dark cycle) time, with ZT0 = lights on and ZT14 = lights off. Concentrations of LH in the serum were measured in duplicate in a single RIA with reagents obtained from the National Institutes of Health (Bethesda, MD) as previously described. The antiserum was rLH-S-11, and the standard was rLH-RP3. The sensitivity was 0.01 ng/tube, and the intraassay coefficient of variation was 2.8% for the low pool and 8.4% for the high pool. The antisera were highly specific for the hormones measured, with low cross-reactivity with other hormones. *Significantly greater than all other groups, P < 0.05.