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

Abstract

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 F(2alpha) (PGF(2alpha)) 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-PGF(2alpha)-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 PGF(2alpha) (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 PGF(2alpha) 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 PGF(2alpha), 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.

Positive Association, in Local Release, of Luteal Oxytocin with Endothelin 1 and Prostaglandin F2alpha During Spontaneous Luteolysis in the Cow: A Possible Intermediatory Role for Luteolytic Cascade Within the Corpus Luteum1

“…That study measured the luteal OXT concentration using unextracted dialysate perfusion, whereas we used an extraction method followed by a 24-fold concentration of samples. Therefore, the discrepancy between our present study and the other study [37] is likely due to the different method of extraction of MDS samples. The present study clearly showed that OXT is actively released even after the onset of spontaneous luteolysis in the cow.…”

“…This result is consistent with our recent study in which we detected luteal OXT secretion using the same MDS during PGF 2a -induced luteolysis in the cow [19]. However, a previous study by other researchers using MDS reported that the luteal OXT content began to decline on Day 15 and was undetectable by Day 17 of the estrous cycle in the cow [37]. Moreover, luteal OXT was undetectable within 8 h after PGF 2a -induced luteolysis in the cow [37].…”

“…However, a previous study by other researchers using MDS reported that the luteal OXT content began to decline on Day 15 and was undetectable by Day 17 of the estrous cycle in the cow [37]. Moreover, luteal OXT was undetectable within 8 h after PGF 2a -induced luteolysis in the cow [37]. That study measured the luteal OXT concentration using unextracted dialysate perfusion, whereas we used an extraction method followed by a 24-fold concentration of samples.…”

Roles of Tumor Necrosis Factor-α of the Estrous Cycle in Cattle: An In Vivo Study1

“…Moreover, the blockade of uterine OT receptors with a specific OT antagonist from Day 15 to Day 22 of the cycle affected neither luteolysis nor the duration of the estrous cycle in heifers [51]. These findings raise serious questions as to whether OT of luteal origin plays a significant role in initiating luteal regression in cattle, as suggested previously [4,6,9,17,50,51]. Therefore, PGF 2␣ secretion by the endometrium during luteolysis in cattle may be regulated not only by OT but also by one or more other factors, including TNF␣ [4,9,10].…”

“…Altogether, these findings suggest that the luteal concentration of OT is down-regulated during TNF␣-and NOdependent luteolysis. In fact, the concentrations of OT in the blood [49] and in intact [47] and microdialyzed in vivo CL [17,50] are extremely low at the time of spontaneous luteolysis. Moreover, the blockade of uterine OT receptors with a specific OT antagonist from Day 15 to Day 22 of the cycle affected neither luteolysis nor the duration of the estrous cycle in heifers [51].…”