Ngi

Figure 13.5

Adenylate cyclase catalyzes conversion of (a) ATP molecules into cyclic AMP (fa). The atoms forming the new bond are shown in red.

Figure 13.d

(a) Body fluids carry nonsteroid hormone molecules to the target cell, where they combine (b) with receptor sites on the cell membrane. (c) This may activate molecules of adenylate cyclase, which (d) catalyze conversion of ATP into cyclic AMP. (e) Cyclic AMP promotes a series of reactions leading to the cellular changes associated with the hormone's action. The dashed arrow indicates breakdown of cAMP, which turns off the cellular response to the hormone.

Because many hormones utilize the cAMP-mediated second messenger system to exert their effects, an abnormality in this signaling system can lead to symptoms from many endocrine glands. In McCune-Albright syndrome, for example, a defect in the G protein that activates adenylate cyclase results in the conversion of ATP to cAMP even without hormonal stimulation. As a result, cells in the pituitary, thyroid, gonads, and adrenal glands secrete hormones in excess. These conditions are inherited because the components of the second messenger system are proteins.

Another enzyme, phosphodiesterase, quickly inactivates cAMP, so that its action is short-lived. For this reason, a continuing response in a target cell depends upon a continuing signal from hormone molecules combining with receptors in the target cell membrane. Since enzymes continuously degrade hormones, a sustained response depends on continued hormone secretion.

Hormones whose actions depend upon cyclic AMP include releasing hormones from the hypothalamus; TSH, ACTH, FSH, and LH from the anterior pituitary gland; ADH from the posterior pituitary gland; PTH from the

Using Hormones to Improve Athletic Performance

In the 2000 summer Olympic games held in Sydney, Australia, 36 athletes and coaches were dismissed for using banned performance enhancing substances — after many more individuals had been ejected for the same reason following drug tests given in the weeks preceding the games. Among those seeking to go beyond biology to win were runners, weight lifters, wrestlers, cyclists, and rowers.

The general focus of performance enhancement is misuse of certain powerful hormones of the human endocrine system. Three types of approaches are described here.

steroids

In the 1988 summer Olympics held in Seoul, South Korea, Canadian Ben Johnson flew past his competitors in the 100-meter run. But seventy-two hours later, officials rescinded the gold medal he won for his record-smashing time of 9.79 seconds, after a urine test revealed traces of the drug stanozolol, a synthetic stand-in for the steroid hormone testosterone (fig. 13A). Johnson's natural testosterone level was only 15% of normal — evidence of negative feedback acting because of an outside supply of the hormone. Yet Johnson's experience was soon forgotten. In the 1992 summer games in Barcelona, Spain, several athletes were dismissed for using drugs that they thought would have steroid-like effects. And in the 2000 summer games, a urine test on U.S. shot-putter C.J. Hunter revealed 1000 times the allowable limit of nan-drolone, a testosterone metabolite.

Athletes who abuse steroids do so to take advantage of the hormone's ability to increase muscular strength. But improved performance today may have consequences tomorrow. Steroids hasten adulthood, stunting height and causing early hair loss. In males, excess steroid hormones lead to breast development, and in females to a deepened voice, hairiness, and a male physique.

Figure 13A

Canadian track star Ben Johnson ran away with the gold medal in the 100-meter race at the 1988 Summer Olympics—but then had to return the award when traces of a steroid drug showed up in his urine.

Figure 13A

Canadian track star Ben Johnson ran away with the gold medal in the 100-meter race at the 1988 Summer Olympics—but then had to return the award when traces of a steroid drug showed up in his urine.

The kidneys, liver, and heart may be damaged, and atherosclerosis may develop because steroids raise LDL and lower HDL — the opposite of a healthy

table •

Sequence of Actions of Nonsteroid Hormone Using Cyclic AMP

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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