In mammals, activin and inhibin are important regulators of FSH secretion. Previous studies have demonstrated that primary ovine pituitary cells express different activin receptor subtypes: activin receptor-like (ALK)2, ALK4, activin type II receptor A (ActRIIA), ActRIIB and Smad proteins in vitro. Here, we have carried out physiological studies to investigate the pattern of mRNA expression of the activin receptor subunits in the ewe pituitary throughout the oestrous cycle. The oestrous cycles of ewes were synchronized with progestagen sponges. The animals were killed 36 h (before the preovulatory surge, n=4), 48 h (during the preovulatory surge, n=4), 72 h (during the second surge of FSH, n=6) and 192 h (during the luteal phase, n=4) after sponge removal. Using Northern blots, we have shown that the levels of ALK2, ALK4 and ActRIIB mRNA were significantly higher before the preovulatory surge and during the secondary surge of FSH as compared with both during the preovulatory surge and the luteal phase, whereas the level of the ActRIIA mRNA was similar throughout the oestrous cycle. Using Western blots we have also demonstrated that the level of phospho-Smad2 did not vary during the reproductive cycle. Inhibin binding protein (InhBP/p120) and the transforming growth factor-beta type III receptor, betaglycan, have been identified as putative inhibin co-receptors. In this study, we cloned a fragment of both InhBP/p120 and betaglycan cDNAs in the ewe and showed by Northern blot that pituitary betaglycan and InhBP/p120 mRNA levels did not fluctuate across the oestrous cycle nor did they correlate with serum FSH levels.
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