Temporal patterns in plasma luteinizing hormone concentration in ovariectomized rats during different times of the day.

Rats were kept in a room with the lights on from 0500–1900 hr and bilaterally ovariectomized (OVX) 4–6 weeks prior to experimentation. Forty OVX animals were divided into 4 groups of 10 rats each and decapitated at 0800, 1100, 1500 or 1800 hr. Radioimmunoassay revealed no significant differences in mean serum LH concentrations between any of the groups. A cannula was placed in the right atrium of the heart in 30 additional OVX rats 4 hr prior to taking 10 blood samples at 5-min intervals. The same rats were bled 10 more times at 5-min intervals later the same day or on the following morning. Rats were either bled between 0800–0845 and 1500–1545, 1500–1545 and 0800–0845, 1100–1145 and 1800–1845 or 1800–1845 and 1100–1145 hr. Plasma LH concentrations in all 30 rats exhibited characteristic pulsatile patterns fluctuating within particular ranges of values for each individual rat during both collection periods. Eight other cannulated OVX rats were bled at 10-min intervals from 1200–1800 hr. Pulsatile patterns...