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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

Summary. 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 followed second order kinetics with calculated association rate constants (k_a ± s.d.) of 1·57 ± 0·16 x 10⁶ and 0·57 ± 0·02 x 10⁶ litres mol^{– 1} sec^–1 at 37°Cand 22°C respectively. Dissociation of specifically bound ¹²⁵I-labelled hCG from theca interna was minimal at 37°C and 22°C. The binding of ¹²⁵I-labelled hCG to theca interna could be displaced with 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 (B_max) and equilibrium dissociation constants (K_d) for ¹²⁵I-labelled hCG binding to theca interna did not differ between Romney ewes and Booroola x 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) B_max and K_d values in non-atretic follicles ( 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) B_max and K_d values in non-atretic follicles 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 B_max but not the mean K_d value was significantly higher (P < 0·01) than the corresponding value at 36 h after cloprostenol injection. In granulosa cells, from follicles of 5 mm diameter of Romney and Booroola x Romney ( + + ) ewes and from follicles of mm diameter of Booroola x 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.

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