Endocrine Regulation of the Testes
FSH regulation of inhibin secretion by Sertoli cells
Sertoli cells are intimately involved in regulating spermatogenesis (Fig 1). These cells secrete inhibin, a hormone with diverse actions in the testis and brain. Our studies indicate that in both juvenile and adult rams, natural rises in FSH secretion induce rises in inhibin secretion. Inhibin in rams is highest at 2 months of age when the testes are starting to grow rapidly. As the testes of the adult ram begin to redevelop in the summer, high FSH per se promotes inhibin secretion. FSH then "up regulates" the synthesis of its own receptor causing Sertoli cells to become more responsive to the FSH signal, thus maintaining inhibin at a high level.
LH regulation of steroidogenesis in Leydig cells
Studies were designed to determine why testosterone secretion in adult rams increases several fold as they pass from the nonbreeding season (spring) to the breeding season (autumn). The seasonal rise in testosterone was shown to be due to a higher frequency of pulsatile LH releases and, equally as important, to a greater number of LH receptors on the surface of Leydig cells (Fig 2). We have also learned that stronger LH action in the autumn enables Leydig cells to secrete more testosterone by increasing the activity of key proteins regulating intracellular cholesterol transport and steroid biosynthesis.