45. Hormonal Control of Reproduction in the Male
Inhibin, which was originally purified from follicular fluid of the pig ovary, is a disulfide-linked heterodimer comprised of an a subunit and either of two forms of a p subunit, pA or pB. The physiologically important form of inhibin secreted by the human testis is the apB dimer called inhibin B. Its concentration in blood plasma is reflective of the number of functioning Sertoli cells and spermatogenesis. Both inhibin A and inhibin B are produced by the ovary (see Chapter 46). Little is known about the significance of alternate p subunits or the factors that determine when each form is produced. All three subunits are encoded in separate genes and presumably are regulated independently. They are members of the same family of growth factors that includes AMH and TGF-p. Of additional interest is the finding that dimers formed from two p subunits produce effects that are opposite those of the ap dimer and stimulate FSH release from gonadotropes maintained in tissue culture. These compounds are called activins and function in a paracrine mode in the testis and many other tissues. While the production of the a subunit is largely confined to male and female gonads, p subunits are produced in many extragonadal tissues where activins mediate a variety of functions. Activins are produced in the pituitary and appear to play a supportive role in FSH production. The pituitary and other tissues also produce an unrelated protein called follistatin, which binds activins and blocks their actions.
The feedback relations that fit best with our current understanding of the regulation of testicular function in the adult male are shown in Fig. 14. Pulses of GnRH originating in the arcuate nuclei evoke secretion of both FSH and LH by the anterior pituitary. FSH and LH are positive effectors of testicular function and stimulate release of inhibin and testosterone, respectively. Testosterone has an intratesticular action that reinforces the effects of FSH. It also travels through the circulation to the hypothalamus, where it exerts its negative feedback effect primarily by slowing the frequency of GnRH pulses. Because secretion of LH is more sensitive to frequency of stimulation than is secretion of FSH, decreases in GnRH pulse frequency lower the ratio of LH to FSH in the gonadotropic output. In the castrate monkey, the hypothalamic pulse generator discharges once per hour and slows to once every 2 hours after testosterone is replaced. This rate is about the same as that seen in normal men. The higher frequency in the castrate triggers more frequent bursts of gonadotropin secretion, resulting in higher blood levels of both FSH and LH. Testosterone may also decrease the amplitude of the GnRH pulses somewhat and may also exert some direct restraint on LH release from gonadotropes. In high enough concentrations, testosterone may inhibit GnRH release sufficiently to shut off secretion of both hypothalamus
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