TESTOSTERONE SECRETION DURING EARLY-PREGNANCY IN MARES

We have characterized the testosterone secretion pattern during the fi rst 80 d of pregnancy in mares and determined the sources that contrib ute to circulating testosterone levels during this period. Ten untreat ed, pregnant mares (Group 1), 10 altrenogest-treated, pregnant mares ( Group 2), and 10 altrenogest-treated, pregnant mares in which the CL w as eliminated by administration of PGF-2 alpha on Day 16 (Group 3) wer e used in this study. Complete luteolysis occurred following PGF-2 alp ha administration in all mares in Group 3. Six of the 10 mares in Grou p 3 did not have an active CL until after Day 60 of pregnancy (Group 3 a) and were included in the analysis. The remaining four mares develop ed a new CL on Days 32, 40, 43 and 49 of pregnancy and were excluded f rom analysis. Mares without a functional CL (Group 3a) had significant ly lower testosterone concentrations than mares with a functional CL ( Groups 1 and 2), during the period before equine chorionic gonadotropi n (eCG) secretion. At the onset of eCG secretion, testosterone concent rations increased rapidly but the rate of increased decreased with tim e in mares with a functional CL (Groups 1 and 2). In mares without a f unctional CL (Group 3a), testosterone concentrations did not increase at the onset of eCG secretion but increased at a gradually increasing rate after Day 50. The lower testosterone concentration in mares witho ut a functional CL before eCG secretion suggests that the CL contribut es significantly to the circulating testosterone concentration during the period before eCG secretion. The close time relationship between t he onset of eCG secretion and the increase in testosterone secretion i n mares with a functional CL and the lack of a testosterone increase i n pregnant mares without a functional CL suggest that the increase in testosterone secretion after Day 35 of pregnancy is the result of eCG- stimulated, luteal testosterone synthesis.