Serum levels of LH and proges-terone were measured by radioimmunoassay and competitive protein binding techniques respectively in nonmated, sterile-mated (to a vasectomized male) and fertile-mated female beagles. There were no differences in the pattern of LH secretion in the non-pregnant and pregnant animals. The levels of LH were elevated for approximately 4 days, with the peak concentration (27 ± 5.2 ng/ml) occurring on the day prior t o the first day of standing estrus. At all other times, LH remained between 2-4 ng/ml. Ovulation was observed late on the first day of standing estrus or nearly 24 hr after the LH peak. By 10 days after the LH peak, the concentration of progesterone had increased to 20-30 ng/ml in the nonpregnant and pregnant dogs. In all nonpreg-nant animals, the pattern of progesterone secretion after day 10 was similar: decreasing from day 10 and subsequently waning to nondetectable levels (<0.5 ng/ml) by 80 days after the LH peak. On the other hand, in the pregnant dogs, progesterone levels generally decreased 10-15 days after the LH peak but then increased dramatically t o reach maximum levels (47 ± 3.1 ng/ml) 20-25 days after the LH peak. Progesterone concentration then gradually decreased to nondetectable levels by the day following parturition (day 65). These results suggest: a) the formation of functional corpora lutea is not dependent on mating, and they can remain functional beyond the normal time of parturition (80 vs 65 days), b) the patterns of LH and progesterone do not differ in overt pseudopregnant dogs (those that lactate) and covert pseudopregnant dogs (those that don't lactate) and c) some factor, perhaps from the placenta, may stimulate the corpora lutea of pregnancy 15-25 days after the LH peak.
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