Expression of cyclooxygenase 2 (COX2, now known as PTGS2), prostaglandin E2 synthase (PTGES, PGES), and prostaglandin F2a synthase (PGFS), of the respective receptors PTGFR (FP), PTGER2 (EP2), and PTGER4 (EP4) and of the progesterone receptor (PGR, PR) was assessed by real-time PCR, immunohistochemistry (IHC), or in situ hybridization (ISH) in utero/placental tissue samples collected from three to five bitches on days 8-12 (pre-implantation), 18-25 (post-implantation), and 35-40 (mid-gestation) of pregnancy and during the prepartal luteolysis. Additionally, ten mid-pregnant bitches were treated with the antiprogestin aglepristone (10 mg/kg bw (2!/24 h)); ovariohysterectomy was 24 and 72 h after the second treatment. Plasma progesterone and 15-ketodihydro-PGF2a (PGFM) concentrations were determined by RIA. Expression of the PGR was highest before implantation and primarily located to the endometrium; expression in the placenta was restricted to the decidual cells. PTGS2 was constantly low expressed until mid-gestation; a strong upregulation occurred at prepartal luteolysis concomitant with an increase in PGFM. PGFS was upregulated after implantation and significantly elevated through early and mid-gestation. PTGES showed a gradual increase and a strong prepartal upregulation. PTGFR, PTGER2, and PTGER4 were downregulated after implantation; a gradual upregulation of PTGFR and PTGER2 occurred towards parturition. ISH and IHC co-localized PGFS, PTGFR, PTGES, and PTGS2 in the trophoblast and endometrium. The changes following application of aglepristone were in the same direction as those observed from mid-gestation to prepartal luteolysis. These data suggest that the prepartal increase of PGF2a results from a strong upregulation of PTGS2 in the fetal trophoblast with the withdrawal of progesterone having a signalling function and the decidual cells playing a key role in the underlying cell-to-cell crosstalk.

No abstract

Functional lifespan of the corpus luteum (CL) in non-pregnant dogs exceeds that of pregnant animals and may last for more than 80 days. Prolactin and LH act luteotropic, however, luteolytic mechanisms are poorly understood. Other than in life stock there is no uterine luteolysine and it was postulated that local paracrine/autocrine mechanisms might play a major role. In following this hypothesis the present investigations have clearly demonstrated that up-regulation of prostaglandin synthesis in the CL as indicated by the expression of cyclo-oxygenase II occurs with its formation and not regression, pointing towards a luteotropic rather than luteolytic action. Throughout dioestrus luteal and other cells of the CL express the oestrogen (ERalpha) and progesterone receptor (PR). While ERalpha expression was not cycle-related, PR concentrations were high in the early and late-luteal phase and a regulatory role of both steroids on CL-function is assumed. As in other species also in the dog the immune system seems to participate in the mechanisms regulating CL-function as an increased presence of lymphocytes within the CL could be detected at the beginning [CD4- CD8-, major histocompatibility complex (MHC)II-antigen expressing cells] and during the latter half of dioestrus (CD8- and MHCII-antigen expressing cells). Thus, leucocyte-derived cytokines may be important and the expression of the mRNA for interleukin (IL)8, IL10, IL12, tumour necrosis factor (TNF)alpha and transforming growth factor (TGF) beta1 was observed throughout dioestrus. Electron microscopy confirmed the slow process of luteolysis; first distinct signs of degeneration were seen on day 60, accompanied by some apoptotic events. From these data it is concluded that luteal regression as monitored by the gradual decrease of systemic progesterone concentrations in the dog is not an actively regulated but rather a permissive process. Immune-mediated events may play a key role. Changes in the vascular supply, as indicated by the expression of endoglin, seem to be of lower importance.

Canine reproductive physiology exhibits several unusual features. Among the most interesting of these are the lack of an acute luteolytic mechanism, coinciding with the apparent luteal independency of a uterine luteolysin in absence of pregnancy, contrasting with the acute prepartum luteolysis observed in pregnant animals. These features indicate the existence of mechanisms different from those in other species for regulating the extended luteal regression observed in non-pregnant dogs, and the actively regulated termination of luteal function observed prepartum as a prerequisite for parturition. Nevertheless, the supply of progesterone (P4) depends on corpora lutea (CL) as its primary source in both conditions, resulting in P4 levels that are similar in pregnant and non-pregnant bitches during almost the entire luteal life span prior to the prepartum luteolysis. Consequently, the duration of the prolonged luteal phase in non-pregnant bitches frequently exceeds that of pregnant ones, which is a peculiarity when compared with other domestic animal species. Both LH and prolactin (PRL) are endocrine luteotrophic factors in the dog, the latter being the predominant one. In spite of increased availability of these hormones, luteal regression/luteolysis still takes place. Recently, possible mechanisms regulating the expression and function of PRL receptor have been implicated in the local, i.e., intraluteal regulation of PRL bioavailability and thus its steroidogenic potential. Similar mechanisms may relate to the luteal LH receptor. Most recently, evidence has been provided for an autocrine/paracrine role of prostaglandin E2 (PGE2) as a luteotrophic factor in the canine CL acting at the level of steroidogenic acute regulatory (STAR)-protein mediated supply of steroidogenic substrate, without having a significant impact on the enzymatic activity of the respective steroidogenic enzymes, 3-hydroxysteroid-dehydrogenase (3HSD, HSD3B2) and cytochrome P450 side-chain cleavage enzyme (P450scc, CYP11A1). Together with the strongly time-dependent expression of prostaglandin transporter, luteal prostaglandins seem to be involved more in the process of luteal formation than in termination of CL function in the dog. The possible roles of other factors such as vasoactive compounds, growth factors or cytokines have not been extensively studied but should not be neglected. Abstract: Canine reproductive physiology exhibits several unusual features. Among the most interesting of these are the lack of an acute luteolytic mechanism, coinciding with the apparent luteal independency of a uterine luteolysin in absence of pregnancy, contrasting with the acute prepartum luteolysis observed in pregnant animals. These features indicate the existence of mechanisms different from those in other species for regulating the extended luteal regression observed in non-pregnant dogs, and the actively regulated termination of luteal function observed prepartum as a prerequisite for parturition. Nevertheless, the supply of progesterone (P4) de...

No abstract