Production of inhibin A not B in rams: changes in plasma inhibin A during testis growth, and expression of inhibin/activin subunit mRNA and protein in adult testis

presence of a, bA and bB inhibin/activin subunit mRNA and protein in the testes of adult rams. Changes in plasma inhibin A concentrations occurred in parallel with the growth and regression of the testes during the long day:short day:long day lighting regimen in adult Soay rams, confirming our previous observations with immunoreactive inhibin. During the growth phase of the testes in the first 8 weeks of exposure to short days there was a positive correlation between plasma FSH and inhibin A concentrations, indicating that during this phase the secretion of inhibin A is stimulated by FSH and that inhibin A did not act as a negative feedback hormone on FSH secretion. From week 8.5 to week 16.0 of exposure to short days, there was a negative correlation between FSH and testosterone concentrations, but not inhibin, indicating that when inhibin concentrations are high, testosterone acts as the negative regulator of FSH secretion. Thus, in intact adult rams, when the testes are fully active it appears that inhibin A may sensitize the pituitary to the negative feedback effects of testosterone, at which time they act synergistically to maintain plasma concentrations of FSH.