Hormone Metabolism and Excretion

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A hormone's concentration in the plasma depends not only upon its rate of secretion by the endocrine gland but also upon its rate of removal from the blood, either by excretion or by metabolic transformation. The liver and the kidneys are the major organs that excrete or metabolize hormones.

The liver and kidneys, however, are not the only routes for eliminating hormones. Sometimes the hormone is metabolized by the cells upon which it acts. Very importantly, in the case of peptide hormones, en-docytosis of hormone-receptor complexes on plasma membranes enables cells to remove the hormones rapidly from their surfaces and catabolize them intra-cellularly. The receptors are then often recycled to the plasma membrane.

In addition, catecholamine and peptide hormones are excreted rapidly or attacked by enzymes in the blood and tissues. These hormones therefore tend to remain in the bloodstream for only brief periods—minutes to an hour. In contrast, because protein-bound hormones are less vulnerable to excretion or metabolism by enzymes, removal of the circulating steroid and thyroid hormones generally takes longer, often several hours (with thyroid hormone remaining in the plasma for days).

In some cases, metabolism of the hormone after its secretion activates the hormone rather than inactivates

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Principles of Hormonal Control Systems CHAPTER TEN

Principles of Hormonal Control Systems CHAPTER TEN

TABLE 10-2 Categories of Hormones

Major Form Location of Signal Transduction Rate of

Types in Plasma Receptors Mechanisms Excretion/Metabolism

Peptides and catecholamines Free Plasma membrane Receptors alter: Fast (minutes to

Channels intrinsic an hour)

to the receptors Enzymatic activity intrinsic to the receptor Enzymatic activity of cytoplasmic JAK kinases associated with the receptor G proteins in the plasma membrane. These control plasma-membrane channels or enzymes that generate second messengers (cAMP, DAG, IP3).

Steroids and thyroid Protein- Cell interior Receptors directly alter Slow (hours to days)

hormones bound gene transcription

Hormone Metabolism

FIGURE 10-7

Possible fates and actions of a hormone following its secretion by an endocrine cell. Not all paths apply to all hormones. % [<n>]

FIGURE 10-7

Possible fates and actions of a hormone following its secretion by an endocrine cell. Not all paths apply to all hormones. % [<n>]

it. In other words, the secreted hormone may be relatively or completely unable to act upon a target cell until metabolism transforms it into a substance that can act. We have already seen one example of hormone activation—the conversion of circulating T4 to the far more active T3. Another example is provided by testosterone, which is converted either to estradiol or dihydrotestosterone in certain of its target cells. These molecules, rather than testosterone itself, then bind to receptors inside the target cell and elicit the cell's response.

There is another kind of "activation" that applies to a few hormones. Instead of the hormone itself being activated after secretion, it acts enzymatically on a completely different plasma protein to split off a pep-tide that functions as the active hormone. The best known example of this is the renin-angiotensin system, described in Chapters 14 and 16; renin is not technically a hormone but an enzyme that participates in the generation of the hormone angiotensin.

Figure 10-7 summarizes the fates of hormones after their secretion.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

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