Changes in serum concentration of luteinizing hormone in the female rat approaching puberty

Concentrations of LH in the serum were estimated in rats bled either once or twice during a 15-day period preceding first ovulation. In rats bled once (between 09.00 and 17.00 h) serum concentrations showed little change between 15 and 9 days before first ovulation and averaged 16 micrograms/l (days -15 to -9). A shift in LH level, to a mean of 31 micrograms/l, was seen on day -8, whereafter LH concentrations increased gradually. Basal LH values of less than 10 micrograms/l were only found until day -4. The finding that LH values increased with age was confirmed by data from rats bled twice with an interval of greater than or equal to 3 days between bleedings. Furthermore, both in rats bled twice at 11.00 h and in rats bled twice at 15.00 h LH concentrations were significantly higher in the second sample. Both morning (11.00 h) and afternoon (15.00 h) LH concentrations in rats bled once also indicated a rise in LH concentrations with age but it became apparent that only morning values showed a shift in LH concentration (from approximately 15 to approximately 42 micrograms/l) from day -9 to day -8. In contrast, mean afternoon values showed a gradual increase from day -15 on. From day -8 on the number of rats with LH values greater than or equal to 50 micrograms/l increased, and from day -5 on they were more frequent at 15.00 than at 11.00 h. Thus a clear increase in LH secretion, most likely of a pulsatile nature, was found in the female rat approaching puberty. The correlation in time and possible functional relationship with late-prepubertal follicular growth is discussed.