**3.3 Pituitary responsiveness**

Administration of Ovalyse at ZT 5.5 resulted in a rapid and consistent increase in LH plasma levels irrespective of age (Table 2). Highest plasma LH levels were measured at 1 or 2 h after Ovalyse administration and decreased thereafter (defined as the 'early', induced LH surge). After ZT 8.5, LH levels increased again (defined as the 'late', endogenous LH surge).

LH levels were of comparable magnitude at 1 and 2 h after Ovalyse injection between 4 and 8.5 months old females. Also, the second, endogenous LH surge was comparable in magnitude (peak height and total LH levels) between groups and accompanied by a gradual increase in P levels comparable between ages.


Table 2. LH and P surge characteristics following Ovalyse® administration in young and middle-aged 4-day cyclic rats on proestrus. The surge was divided into a 'early' part (ZT<9; 'induced' surge) and a 'late' surge (ZT>9; 'endogenous' surge). Measured characteristics: peak height of the 'early' and 'late' LH surge (the highest concentration measured), and the total amount of LH or P released during the 'early' and 'late' and the entire sampling period (cumulative LH or P levels during the corresponding sampling periods). All data are expressed as group means SEM in ng/ml (concentrations).
