Corpus Luteum Function in the Alpaca12

Corpus luteum function was studied in pregnant and nonpregnant alpacas. In the first of two experiments, nonpregnant females in which ovulation was induced by sterile mating or single intramuscular injections of 750 IU of HCG and unmated controls were laparotomized at 9-day intervals beginning on day 3, 6, or 9 after treatment or heat detection (day 0=day of treatment). In a second trial, 33 pregnant and 31 nonpregnant females were killed 3, 8, 13, 18, or 23 days after mating or HCG injection, respectively. The results showed that the corpus luteum of the alpaca undergoes rapid development after ovulation, reaches its maximum size and secretory activity at days 8 to 9, and, in the absence of pregnancy, declines sharply in both size and secretory activity by day 12. The animals then return to estrus. Regression of corpora lutea was complete in most cases by day 18. In pregnant animals, the size and activity attained by day 8 remained almost unchanged, except for a transient decline in weight and progesterone concentration on day 13. Control, unmated females had large follicles throughout the experimental period. Average progesterone concentrations in luteal tissue of pregnant females on days 3, 8, 13, 18, and 23 were: 7.2, 20.0, 11.1, 22.7, and 22.6 μg/g, respectively; corresponding values in nonpregnant females were: 5.4, 23.4, 0.6, 0.3 and 0.4 μg/g. Progesterone levels in peripheral blood on the same days were 67, 222, 143, 287, and 268 ng/100 ml in pregnant females and 54, 441, 9, 23, and 11 ng/100 ml in nonpregnant females. The high blood levels observed in nonpregnant animals on day 8 may represent an HCG effect. 20αhydroxypregn-4-en-3-one levels in blood tended to be higher in pregnant than in nonpregnant females on day 8. Highly significant (p < .01) correlation coefficients were found between progesterone levels in blood and concentration and total content in luteal tissue (r = 0.83 and 0.84, respectively). Mean progesterone concentration in adrenals ranged from 1.0 to 6.3 μg/g; no differences existed between pregnant and nonpregnant females and the correlation coefficient with peripheral blood progesterone was —0.05.