An ultrasonographic study of luteal function in breeds of sheep with different ovulation rates

Development and demise of luteal structures were monitored using daily tran srectal ultrasonography in 2 breeds of sheep differing in ovulation rates ( nonprolific Western white-faced cross-bred, n=12 and prolific pure-bred Fin n sheep, n=7), during 1 estrous cycle in the mid-breeding season. Jugular b lood samples were collected once a day for radioimmunoassay (RIA) of proges terone. The mean diameter of ovulatory follicles was higher in Western whit e-faced than in Finn ewes (6.4 +/- 0.2 and 5.3 +/- 0.2 mm, respectively; P< 0.001). The mean volume of luteal structures was higher (P<0.05) in Western white-faced compared with Finn sheep from Days 5 to 15 of the cycle (Day 0 =day of ovulation). This accounted for the higher (P<0.05) total luteal vol umes recorded in Western white-faced ewes on Day 7 and from Days 1 1 to 15, despite the higher ovulation rate in Finn ewes (2.7 +/- 0.3 and 1.7 +/- 0. 2, respectively, P<0.05). Mean serum progesterone concentrations were highe r (P<0.05) in Western white-faced than in Finn ewes from Days 4 to 14. Dail y total luteal volumes were positively correlated with daily serum progeste rone concentrations throughout the cycle in Finn sheep (r greater than or e qual to 0.40, P<0.02), and during luteal growth and regression (r>0.60, P l ess than or equal to 0.00001) but not during mid-cycle in white-faced ewes (r=0.16; P=0.22). During the growth of the corpora lutea (CL), luteal tissu e volume increased faster (P<0.05) than serum progesterone concentrations i n both breeds of sheep. During luteolysis, the decrease in luteal volumes p arallelled that in serum progesterone concentrations in Finn (P=0.11) but n ot in Western white-faced ewes, where luteal volumes decreased more slowly (P=0.02) in relation to progesterone secretion. Increased ovulation rate in prolific Finn ewes resulted in more but smaller CL, and lower serum proges terone levels compared with nonprolific Western white-faced ewes. We conclu de that breed-specific mechanisms exist to control the formation of luteal tissue and progesterone secretion in cyclic ewes differing in prolificacy. The mechanisms may involve ovulation of Graafian follicles at different siz es and inhibitory paracrine effects of CL on co-existing CL. (C) 1999 by El sevier Science Inc.