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Daily rhythmicity of serum testosterone concentration in the mature male laboratory rat was examined under various lighting schedules. In rats living in a standard light cycle (12-h light, 12-h dark; lights on at 0600 h), a trimodal rhythm was predominant, with elevations near 0200, 1200, and 1800 h. This pattern was reasonably stable in seven different studies, despite differences in experimental design, method of blood collection, anesthesia, and whether individual rats were sampled once or repeatedly, and was found both in groups of animals and in individuals, including a study using 40-day-old rats. In constant illumination, the pattern was disrupted, but in constant darkness the trimodal pattern was maintained, indicating that the rhythm is endogenous. In a reversed light cycle (12-h dark, 12-h light; lights on at 1800 h), the "midday" elevation was reversed; in an altered light cycle (12-h dark, 12-h light; lights on at 2300 h), the time of the "midday" elevation was shifted. Serum testosterone concentration was higher during the light phase than the dark phase, and was higher in constant light than in constant darkness. A seasonal shift in the daily rhythmicity of serum testosterone concentration is suggested. The trimodal rhythmicity contrasts with the circadian rhythmicity of other hormones. Its functional role in the life of the animal is unknown.

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The rate of internalization and degradation of radioiodinated hCG, ovine LH (oLH), human LH (hLH), and mouse epidermal growth factor (mEGF) was studied in suspensions of ovine luteal cells. Hormone bound to the plasma membrane was quantified by treating the cells with acidic buffer (pH 3.0), and the radioactivity removed from the receptor was measured. The radioactivity remaining in the cell pellet was considered to have been internalized. The extent of degradation of the radioactive hormones was determined by subjecting the radioactivity that was released into the medium during the incubation periods to precipitation with 20% trichloroacetic acid. The non-precipitable radioactivity was considered to have been degraded. The required time for loss of half of the radioactivity that had been bound to the cell membrane at time zero was 22.8 +/- 2.3 h for hCG, 23.3 +/- 1.1 h for asialo-hCG, 15.1 +/- 1.4 h for hLH, 0.4 +/- 0.2 h for oLH, and 0.3 +/- 0.1 for mEGF. The quantities of radioactivity that had been internalized (intracellular plus degraded) were greater at earlier times (15-120 min) for oLH or mEGF than for hCG, asialo-hCG, or hLH. These data suggest that ligands bound to different receptors on ovine luteal cells are internalized and degraded at different rates by the same cell and that ligands that bind to the same receptor (hCG, hLH, and oLH) can also be internalized and degraded at very different rates.