125I-labelled hCG binding characteristics in theca interna and other tissues from Romney ewes and from Booroola x Romney ewes with and without a major gene influencing their ovulation rate

Abstract

Specific receptors for 125I-labelled hCG in ovarian follicle wall were located in the theca interna. No specific binding of 125I-labelled hCG was found in theca externa and/or stromal tissue. The kinetics of 125I-labelled hCG binding to theca interna following second order kinetics with calculated association rate constants (ka .+-. s.d.) of 1.57 .+-. 0.16 .times. 106 and 0.57 .+-. 0.02 .times. 106 litres mol-1 sec-1 at 37.degree. C and 22.degree. C respectively. Dissociation of specifically bound 125I-labelled hCG from theca interna was minimal at 37.degree. C and 22.degree. C. The binding of 125I-labelled hCG to theca interna could be displaced with the PMSG, FSH-P and sheep LH but other sheep pituitary hormones and LH-releasing hormone showed little or no cross-reaction. The calculated binding capacities (Bmax) and equilibrium dissociation constants (Kd) for 125I-labelled hCG binding to theca interna did not differ between Romney ewes and Booroola .times. Romney ewes with and without the fecundity (F) gene on Day 10 of the oestrous cycle, during anoestrus or at 36 h after an injection of cloprostenol on Day 10 of the oestrous cycle. When the data for Day 10 and anoestrus were pooled, the median (range) Bmax and Kd values in non-atretic follicles (.gtoreq. 3 mm diameter) were 12.0 (5.1-23.5) fmol/mg protein and 0.10 (0.05-0.16) nM respectively. At 36 h after cloprostenol injection the respective median (range) Bmax and Kd values in non-atretic follicles (.gtoreq. 3 mm diam.) increased to 46.9 (28.4-70.3) fmol/mg protein and 0.23 (0.13-0.65) nM respectively. In corpora lutea the hCG binding characteristics were similar in all the above breeds/genotypes. On Day 10 of the cycle, the mean Bmax but not the mean Kd value was significantly higher (P < 0.01) than the corresponding value at 36 h after cloprostenol injection. In granulosa cells, from follicles of .gtoreq. 5 mm diameter of Rommey and Booroola .times. Romney (++) ewes and from follicles of .ltoreq. 3 mm diameter of Booroola .times. Romney (F+) ewes, the hCG binding characteristics were similar. In granulosa cells from smaller sized follicles from the above breeds/genotypes, no specific hCG binding was noted.