Changes in Expression of Inhibin/Activin α, βA and βB Subunit Messenger Ribonucleic Acids Following Increases in Size and during Different Stages of Differentiation or Atresia of Non-Ovulatory Follicles in Cows1

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Abstract
Northern and slot blot analyses were used to examine the sizes, variability, and changes in amounts of alpha, beta A and beta B inhibin/activin subunit mRNAs following increases in size and during different stages of differentiation or atresia of bovine non-ovulatory follicles. In Study I, a major 1.3-kb band and minor 2.2-, 3.3- and 6.4-kb bands of alpha mRNA; a major 6.4-kb band and minor 2.6 and 4.0-kb bands of beta A; and major 3.5- and 4.6-kb bands of beta B mRNA were detected in total and/or poly(A)+ RNA isolated from pools of small (1-5 mm), medium (6-10 mm), or large (> 10 mm) follicles. In Study II, 18 of 19 follicles had a 1.3-kb alpha mRNA, and amounts of 1.3 kb alpha mRNA increased with follicle size. However, 0 of 19, 9 of 19, and 3 of 19 follicles had 2.2-, 3.3-, and 6.4-kb alpha mRNA bands. In Study III, the ratio of estradiol (E) to progesterone (P) in follicular fluid (FF) was used to separate follicles into the following three stages of differentiation and atresia: estrogen-active (EA; E > P in FF), atretic (P:E = 1-100), and highly atretic (P:E > 100). Total amounts of alpha and beta A mRNAs were 2.4- and 5.8-fold greater in EA follicles compared with small follicles. Alpha and beta A mRNAs and concentration of total inhibin immunoactivity were 4.4-, 9.8- and 1.8-fold lower in highly atretic follicles compared with EA follicles. Changes in amounts of follicular alpha and beta A mRNAs were highly positively correlated with intrafollicular concentrations of E, ratio of E to P, and total inhibin immunoreactivity, but negatively correlated with P. Ratio of amounts of alpha to beta A mRNAs was 3.9- and 6.5-fold greater in small and highly atretic follicles compared with EA follicles. Amounts of beta B subunit mRNAs were unchanged during follicular development and unrelated to changes in follicular concentrations of E, P, or total inhibin immunoactivity. On the basis of these results, we concluded that 1) multiple forms of each inhibin/activin subunit mRNA are expressed in follicular tissue; 2) alpha and beta A subunit mRNAs and inhibin transiently increase; 3) alpha and beta mRNAs are not co-regulated similarly; and 4) beta B mRNA is constitutively expressed during differentiation and atresia of non-ovulatory follicles.

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