Submission Number: JOH-4-4-29

Abstract Number: 92

DEPLETION OF OXYTOCIN FROM OVINE CORPUS LUTEUM DOES NOT EXTEND CYCLE LENGTH.

John A McCracken* ¹, JA Amico ², MK Newbury ¹ and Robert A Milvae* ¹

Department of Animal Science, University of Connecticut, Storrs, CT, USA ¹
Departments of Medicine and Pharmaceutical Sciences, University of Pittsburgh, Pittsburgh, PA, USA ²

Abstract:
Diminished amplitude of PGFM (PGF₂) pulses in ovariectomized [J Reprod Fert (1993) 98: 341347] or lutectomized [Anim Reprod Sci (1999) 57:167175] hormone treated sheep was ascribed to absence of supplemental releases of luteal oxytocin (OT). As a corpus luteum (CL) was not present, it was not established whether such diminished pulses of PGF₂ would have caused luteolysis. The objectives of the present report were to: (1) Establish subluteolytic regimens of systemic infusions of PGF₂ to prematurely deplete luteal OT as measured after lutectomy in sheep on cycle day 11. (2) Use such regimens to determine if premature depletion of luteal OT affected plasma progesterone (P) levels and cycle length in intact sheep. Six systemic infusions of PGF₂, each lasting 30 min, were given at 8 hr intervals to sheep on days 10 and 11 of the cycle (2g, 10g, and 20g/min; n=3 at each dose + controls). None of these doses adversely affected progesterone levels but luteal OT conc. at the end of the infusions was depleted by 84.4%, 93.9% and 92.7% at the 2g, 10g, and 20g doses resp. compared to controls. In a subsequent experiment, the 10g (n=4) and 20g (n=3) dose regimens were given to intact sheep on cycle days 10 and 11 and plasma P conc. and cycle length compared to controls (n=4). CL regression began in all groups on cycle day 14 as shown by a steep decline in plasma P. The time of P decline was not different among groups although there was a tendency for the decline in P to occur over a shorter period in the treatment groups. Also, mean cycle length in the three groups was not different (17.3 days, 17.0 days and 16.7 days for controls, 10g and 20g doses resp). It appears that luteolysis in sheep can occur in a timely fashion after depleting most of the OT in the CL. (Supported by USDA NRI Grant # 98-35203-6635).

Keywords: Oxytocin, Corpus Luteum, Cycle Length

This abstract is being presented at: 8:00 AM in session:
Corpus Luteum