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Abstract
By acting through its receptors (RXFP1, RXFP2), relaxin (RLN) exerts species-specific effects during pregnancy; possible luteotropic effects through stimulation of prolactin (PRL) release have been suggested. In the domestic dog (Canis lupus familiaris) serum PRL increases in pregnant bitches shortly after RLN appears in the circulation, and a possible functional relationship between the RLN and the PRL systems in regulating progesterone secretion has been implied. Therefore, here (Study 1) the luteal expression and localization of the RLN system was investigated by immunohistochemistry using custom-made antibodies and semi-quantitative PCR, at selected time points during gestation: pre-implantation (d. 8–12), post-implantation (d. 18–25), mid-gestation (d. 35–40) and at normal and antigestagen-induced luteolysis. Further, (Study 2) hypophyseal expression of the RLN system and its spatial association with PRL was assessed. Luteal expression of RLN, but not of its receptors, was time-dependent: it increased significantly following implantation towards mid-gestation and decreased at prepartum. Antigestagen treatment resulted in downregulation of RLN and RXFP2. Whereas RLN was localized in steroidogenic cells, RXFP1 and RXFP2 also stained strongly in macrophages and vascular endothelial cells. The RLN system was detected in the canine adenohypophysis and was co-localized with PRL in hypophyseal lactotrophs. The intraluteal RLN seems to be involved in regulating the canine corpus luteum (CL) in a time-dependent manner. The presence of RLN family members in the adenohypophysis implies their possible involvement in regulating the availability of PRL and other pituitary hormones.
Highlights
The peptide hormone relaxin (RLN) is involved in pregnancy-related remodeling of connective tissue [reviewed in [1, 2]]
We investigated the spatio-temporal expression of the RLN system at both mRNA and protein levels in the canine corpus luteum (CL) during the course of pregnancy
For the luteolytic group serum P4 was monitored at 6 h intervals starting from day 58 of pregnancy, and a decrease to at least 2-3ng/ml in three consecutive measurements was considered as the time of prepartum luteolysis
Summary
The peptide hormone relaxin (RLN) is involved in pregnancy-related remodeling of connective tissue [reviewed in [1, 2]]. Acting on the pubic ligament, pregnant uterus and uterine cervix, RLN participates in preparing the birth canal for parturition, thereby facilitating the course of labor. These effects of RLN are well-known and broadly described, several other actions of RLN are known to be species- and/or organ-specific. RLN was shown to be involved in decidualization and implantation in women [3, 4]
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