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In Vivo Oxytocin Release from Microdialyzed Bovine Corpora Lutea During Spontaneous and Prostaglandin-Induced Regression

^a Departments of Animal Science and Anatomy, Physiological Sciences, and Radiology, North Carolina State University, Raleigh, North Carolina 27695

The release of luteal oxytocin during spontaneous and prostaglandin-induced luteolysis was investigated in cows. A continuous-flow microdialysis system was used in 11 cows to collect dialysates of the luteal extracellular space between Days 12 and 24 postestrus. Seven cows were untreated and were expected to exhibit spontaneous luteolysis during sampling, whereas 4 cows received prostaglandin F₂ (PGF₂) systemically between Days 13 and 15 to induce luteolysis during sampling. Oxytocin was detectable in the dialysate of all cows before Day 16 postestrus and occurred as 2 or 3 discrete pulses per 12-h sampling period. For non-PGF₂-treated cows, dialysate oxytocin content began to decline spontaneously on Day 15 postestrus and was undetectable by Day 17 postestrus. Oxytocin decay curves preceded onset of serum progesterone decline by at least 72 h and were not related temporally with onset of progesterone decline within cow. Exogenous PGF₂ (25 mg, i.m.) produced a 10-fold increase in dialysate oxytocin within 1 h (1.9 ± 0.3 pg/ml to 20.8 ± 3.0 pg/ml; P < 0.01). Dialysate oxytocin then declined to pretreatment concentrations within 2 h and was undetectable within 8 h posttreatment. A second PGF₂ injection given 20 h after the first did not result in a measurable increase in dialysate oxytocin, probably because luteolysis was underway. Although robust luteal oxytocin release was observed after treatment with a pharmacological dose of PGF₂, the lack of detectable oxytocin secretion during spontaneous luteolysis suggests that the contribution of luteal oxytocin in the cow may be less than that proposed for the ewe.

First decision: 19 November 1998.

¹ Correspondence and current address: Doug Shaw, Department of Veterinary Preventive Medicine, The Ohio State University, 1900 Coffey Rd., Columbus, OH 43210. FAX: 614 292 4142; shaw.184{at}osu.edu

² Current address: Jack Britt, University of Tennessee, 101 Morgan Hall, Knoxville, TN 37996.

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