Figure 632

Hormonal interrelationships between the hypothalamus, anterior pituitary, and testes. Solid arrows, Excitatory effects; dashed arrows, inhibitory effects. GnRH, gonadotropin-releasing hormone; FSH, follicle-stimulating hormone; LH, luteinizing hormone. (Modified with permission from Fox SI. Human Physiology (3rd ed.). Copyright 1990 Wm. C. Brown, Dubuque, IA. All rights reserved.)

testes, where they regulate testosterone synthesis and spermatogenesis, respectively. The resultant increases in serum testosterone levels exert a negative feedback at both the hypothalamic and the pituitary levels.

The hypothalamus releases GnRH in a pulsatile manner. The pulse frequency is sex specific, with males exhibiting a 120-minute frequency and females exhibiting a 60- to 90-minute frequency. The pulsating levels of GnRH from the pituitary modulate LH and FSH release. Androgens and estrogens can modulate go-nadotropin release at both the hypothalamus and pituitary levels. In this regard, the gonadal steroids modulate GnRH pulse frequency and amplitude at the hypothalamus level while simultaneously modifying pituitary responses to GnRH by influencing GnRH receptor levels in the pituitary. Increases in GnRH recep tor levels with a resultant increased sensitivity to GnRH is termed up-regulation, while a decrease in GnRH receptors is termed down-regulation. In the hypothalamus, the negative feedback of testosterone involves both the conversion to dihydrotestosterone (DHT) and aromatization into estradiol.

A separate protein hormone produced primarily in the testis, called inhibin, also affects the secretion of FSH. Inhibin has been isolated primarily from testicular extracts but also may be found in the antral fluid of ovarian follicles in females. Inhibin decreases the release of FSH from the pituitary but does not affect hypothalamic production of GnRH.

The catabolism of plasma testosterone and other androgens occurs primarily in the liver (Fig. 63.3), where they are conjugated into water-soluble compounds that are excreted by the kidney as the urinary 17-ketosteroids.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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