Distribution and cycle phase dependency of gonadotropin receptors in musculature and blood vessels of the porcine broad ligament

Summary

Luteinizing hormone (LH) and human chorionic gonadotropin (hCG) share common receptors in the uterus of many species. The aim of the present work was to investigate the possibility of LH/hCG binding sites present in the musculature of the porcine broad ligament, and to study the topography of LH/hCG receptors in ligamental blood vessels. Uteri of cyclic pigs were collected during follicular (n=6) and midluteal (n=6) phases of the estrous cycle. The horns and broad ligaments were dissected for measurements of numbers and affinities of unoccupied LH/hCG binding sites in crude cell membrane fractions of the myometrium and the smooth muscle layers of the ligament. The tissue samples for contact autoradiography, on film using [¹²⁵I]hCG as ligand, were collected from different regions of the reproductive tract including: ovary, uterine horn, paraovarian vascular plexus, uterine vein, and uterine artery with supplying vessels and the outer arches of the broad ligament. The kidney and liver were used as negative control tissue. The specific binding of [¹²⁵I]-labelled hCG in blood vessels was quantified on the base of optical density of the film.

Scatchard analysis of data from broad ligamental and myometrial crude membrane preparations gave comparable estimates of the association constants and numbers of LH/hCG receptors. Mean concentrations (± SEM) of receptors (nmol/kg protein) were lower in the broad ligament than in the myometrium (p < 0.05) during both follicular and luteal phases (2.2 ± 0.5 vs. 1.0 ± 0.3 and 2.5 ± 0.5 vs 1.7 ± 0.2; respectively). The affinity of broad ligament LH/hCG binding sites was the same as that of myometrial receptors (K_a X 10¹⁰ 1/mol). Analysis of autoradiograms have shown that LH/hCG receptors are situated in the superficial smooth layer of the ligament as well as in blood vessels. Autoradiograms, using [¹²⁵I]hCG, revealed more intense binding (p < 0.05) in both the ovarian artery and vein during the follicular rather than the luteal phase of the estrous cycle. The outer arches of arteries and veins had more (p < 0.05) LH/hCG receptors (arbitrary units) during the luteal than the follicular phase (32.5 ± 1.9 and 36.5 ± 1.0 vs 23.5 ± 4.2 and 20.8 ± 2.3; respectively). The lowest amount of LH/hCG receptors was found in the medium arteries (5.2 ± 1.3) and vein branches (11.4 ± 3.6) of the broad ligament during the follicular as compared to the luteal phase of estrous cycle (36.6 ±1.1 and 28.4 ±5.1; p < 0.01 and p < 0.05; respectively). This is probably the first evidence demonstrating specific binding of [¹²⁵I]hCG by LH receptors in the female broad ligament and its blood vessels and their estrous cycle phase dependency.

Key words

LH/hCG - receptor - uterus - pig