The primary function of the corpus luteum is secretion of the hormone progesterone, which is required for maintenance of normal pregnancy in mammals. The corpus luteum develops from residual follic...

https://www.physiology.org/doi/pdf/10.1152/physrev.2000.80.1.1

Mechanisms Controlling the Function and Life Span of the Corpus Luteum

In many nonprimate mammalian species, cyclical regression of the corpus luteum (luteolysis) is caused by the episodic pulsatile secretion of uterine PGF2α, which acts either locally on the corpus l...

Luteolysis: A Neuroendocrine-Mediated Event

Hormone receptor control of pulsatile secretion of PGF2α from the ovine uterus during luteolysis and its abrogation in early pregnancy

Oxytocin induces regression of corpora lutea when administered to several domestic species1–3 and a role for endogenous oxytocin in controlling the ovarian cycle is supported by the delay in luteal regression observed in sheep which have been immunized against oxytocin4. Although the posterior pituitary is the major source of oxytocin released at parturition and before suckling, recent evidence suggests that during the oestrous cycle the ovary may also secrete oxytocin. Circulating oxytocin and progesterone levels change in parallel during the oestrous cycle and after ovariectomy5–7, and the corpus luteum8 (but not other ovarian tissues [A.P.F.F. and E.L.S., unpublished]) contains high concentrations of oxytocin. We now extend this evidence by showing that in sheep, the concentration of oxytocin in ovarian venous blood exceeds that in arterial blood, and we find that ovarian secretion of oxytocin is stimulated by a prostaglandin F2α analogue.

Ovarian secretion of oxytocin is stimulated by prostaglandin

Summary. Jugular venous concentrations of oxytocin and progesterone changed in parallel during the oestrous cycle in the ewe, falling at luteal regression and rising with formation of the new corpus luteum. These fluctuations in the circulating concentration of oxytocin were not caused by changes in its metabolic clearance rate. On Days 6\p=n-\9of the cycle circulating oxytocin concentrations exhibited a diurnal rhythm, peaking at 09 :00 h; this rhythm was absent on Days 11\p=n-\14.Although there was no evidence for increased production of oxytocin at or preceding luteal regression in samples taken daily, more frequent sampling revealed that two thirds of detected surges of uterine secretion of prostaglandin (PG) F-2\g=a\were accompanied by raised levels of oxytocin. This oxytocin was not of pituitary origin. Luteal regression induced with cloprostenol on Day 8 after oestrus caused a decrease in circulating progesterone level followed after 24 h by a fall in oxytocin. Measurements of oxytocin in the ovary and other organs before and after treatment with cloprostenol identified the corpora lutea as a major potential source of oxytocin, and suggested that 98% of luteal oxytocin was available for secretion in response to prostaglandin stimulation. The data are consistent with a role for ovarian secretion of oxytocin in response to uterine release of PGF-2\g=a\in the control of luteal regression.

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Evidence for a systemic role for ovarian oxytocin in luteal regression in sheep

Temporal relationship between plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F and neurophysin I/II around luteolysis in sheep

Summary. Three cows and 2 sheep were passively immunized against prostaglandin (PG) F on Day 16 and Days 13\p=n-\15of the oestrous cycle respectively. The PGF antiplasma was raised in ovariectomized ewes against a PGF-2\g=a\\p=n-\bovineserum albumin complex and showed 100%, 12\m=.\5%,0\m=.\3%,<0\m=.\05%and <0\m=.\01% cross-reactivity with PGF-2\g=a\,PGE-2, PGA-2, PGB-2 and arachidonic acid, respectively. Control animals were given an equivalent amount of ovariectomized ewe plasma. In all passively immunized animals there was evidence of a persistent corpus luteum as indicated by plasma progesterone concentrations and the failure of the animals to return to oestrus until at least 29 days after treatment. These data are consistent with previous proposals that PGF-2\g=a\ is the uterine luteolytic factor in sheep and cattle.

/pdf/prolongation-of-the-oestrous-cycle-in-cows-and-ewes-after-2jw7cqlos2.pdf

Prolongation of the oestrous cycle in cows and ewes after passive immunization with PGF antibodies.

Proceedings: Effect of oestradiol-17beta, progesterone and prostaglandin F-2alpha antiplasma on luteal function in the ewe.

Plasma concentrations of the prostaglandin F metabolite 13,14-dihydro-15-keto-prostaglandin F (PGFM), the oxytocin-associated neurophysin (OT-N) and progesterone were monitored by radioimmunoassay (RIA) in five ewes sampled from the jugular vein at hourly intervals between 0700-1900 h from Days 12-16 of the estrous cycle. These hormones were also determined in plasma samples collected at similar times from five intact and five ovariectomized ewes given twice daily injections of medroxyprogesterone acetate (MPA) over Days 10 to 20 after the last observed estrus. In both the control and intact MPA-treated ewes, coincident surges of OT-N and PGFM were observed in jugular plasma during the time of luteal regression. No significant differences were noted in the number and amplitude of OT-N and number of PGFM peaks between the control and intact MPA-treated animals, although in the latter the amplitude of the PGFM peaks was significantly reduced (P less than 0.01). No marked surges in the plasma concentrations of PGFM or OT-N were observed in the ovariectomized ewes given exogenous MPA. This latter finding is consistent with previous proposals, suggesting that the ovaries are a major source of oxytocin in the ewe. In addition, the observation that exogenous progestogens in the intact ewes did not influence the number and peak height of the OT-N surges, indicates that a fall in progesterone levels during the normal cycle is not obligatory for oxytocin release although it may facilitate the release of uterine PGF2 alpha.

/pdf/effect-of-exogenous-progestogens-on-plasma-concentrations-of-3y2go6np68.pdf

Effect of exogenous progestogens on plasma concentrations of the oxytocin-associated neurophysin and 13,14-dihydro-15-keto-prostaglandin F in intact and ovariectomized ewes over the time of expected luteal regression.

Lactation was successfully induced in 14 of 15 non-pregnant cows treated for 7 days with twice daily subcutaneous injections of 15 mg oestradiol-17β and 37.5 mg progesterone. The successfully induced cows were first machine-milked about 8 days after the end of treatment and, although they milked for as long as their naturally calving identical twins, produced significantly less milk, fat and lactose. Augmenting the basic regimen with Opticortenol or thyrotrophin-releasing hormone produced no advantage. Plasma and milk hormone analyses were conducted and used to study the onset of lactogenesis and possible meat and milk residues. Ten of the 15 treated cows showed frequent oestrous activity and 4 became lame. Pregnancy rates after an 8-week insemination period were 69% and 93% for treated and control cows, respectively. It is concluded that the technique is unlikely to gain wide acceptance in the form used here.

L.T. McGowan

Papers

Temporal relationship between plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F and neurophysin I/II around luteolysis in sheep

Prolongation of the oestrous cycle in cows and ewes after passive immunization with PGF antibodies.

Proceedings: Effect of oestradiol-17beta, progesterone and prostaglandin F-2alpha antiplasma on luteal function in the ewe.

Effect of exogenous progestogens on plasma concentrations of the oxytocin-associated neurophysin and 13,14-dihydro-15-keto-prostaglandin F in intact and ovariectomized ewes over the time of expected luteal regression.

Induction of lactation in dry dairy dattle.