transport of hormones

7/30/2019 Transport of Hormones

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Transport of hormonesBasics

certain hormones are bound for the transport of carrier proteins in the blood.

the carrier proteins are synthesized in the liver.

the proportion of the bound hormone is different depending on Hormonart.

only the free hormone capable of binding to the receptor. there is an equilibrium between free and bound hormone.

Hydrophobic hormones

poorly water-soluble hormones (eg, steroid and thyroid hormones) are tied for thetransport through the bloodstream to specific carrier proteins.

Hydrophilic hormones

highly water-soluble hormones (eg insulin, TSH, LH) do not require transport proteins

in general.

Exception: IGF-1 has different binding proteins.

Factors which affect the binding of proteins leading to a change of the concentrationof bound hormone: change in the concentration of the binding proteins

o Estrogens increase (pregnancy), the concentration of sex hormone-binding

globulin (SHBG), thyroxine-binding globulin (TBG), corticosteroid-bindingglobulin (CBG) increase of total hormone concentration.

o Hyperthyroidism increases the concentration of SHBG.o Protein deficiency (malnutrition, nephrotic syndrome) leads to decrease in the

concentration of binding proteins. decrease of total hormone concentration

Displacement from the protein binding

o eg by drugs with high affinity plasma protein binding.

Since an equilibrium between free and bound hormone, does not cause changes inbinding proteins for endocrine or over functions. Diagnostic consequence: if possible,determine the concentration of free hormone

MODES OF HORMONE TRANSPORTTABLE OF CONTENTS


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Introduction

Traditional endocrinology

Function of the endocrine system

Glands and hormones of the human endocrine system

Growth and development

Endocrine-related developmental disorders

Ectopic hormone and polyglandular disorders

Endocrine changes with aging

Related

Contributors & Bibliography

Most hormones are secreted into the general circulation to exert their effects on appropriate

distant target tissues. There are important exceptions, however, such as self-contained portal

circulations in which blood is directed to a specific area. A portal circulation begins inacapillarybed. As the capillaries extend away from the capillary bed, they merge to form a set

ofveins, which then divide to form a second capillary bed. Thus, blood collected from the first

capillary bed is directed solely into the tissues nourished by the second capillary bed.

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Two portal circulations in which hormones are transported are present in the human body. One

system, the hypothalamic-hypophyseal portal circulation, collects blood from capillariesoriginating in thehypothalamusand, through a plexus of veins surrounding the pituitary stalk,directs the blood into the anterior pituitary gland. This allows theneurohormonessecreted by the

neuroendocrine cells of the hypothalamus to be transported directly to the cells of the anterior

pituitary. These hormones are largely, but not entirely, excluded from the general circulation. In

the second system, thehepatic portal circulation, capillaries originating in the gastrointestinaltract and thespleenmerge to form theportal vein, which enters the liver and divides to form

portal capillaries. This allows hormones from the islets of Langerhans of the pancreas, suchasinsulinandglucagon, as well as certain nutrients absorbed from the intestine, to be transported

into the liver before being distributed through the general circulation.

In serum, many hormones exist both as free, unbound hormone and as hormone bound to a serum

carrier ortransport protein. These proteins, which are produced by the liver, bind to specifichormones in the serum. Transport proteins include sex hormone-binding globulin, which binds

estrogens and androgens; corticosteroid-binding globulin, which binds cortisol; and growth

hormone-binding protein, which binds growth hormone. There are two specific thyroid hormonebinding proteins, thyroxine-binding globulin and transthyretin (thyroxine-binding prealbumin),

and at least six binding proteins for insulin-like growth factor-1 (IGF-1).
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In serum, protein-bound hormones are in equilibrium with a much smaller concentration of free,

unbound hormones. As free hormone leaves the circulation to exert its action on a tissue, bound

hormone is immediately freed from its binding protein. Thus, the transport proteins serve as a

reservoir within the circulation to maintain a normal concentration of the biologically importantfree hormone. In addition, transport proteins protect against sudden surges in hormone secretion

and facilitate even distribution of a hormone to all of the cells of large organs such as the liver.The production of many transport proteins is hormone-dependent, being increased by estrogensand decreased by androgens; however, the biological importance of this sensitivity to sex steroids

is not well understood.

The affinity (attraction) of hormones for binding proteins is not constant. The thyroid

hormonethyroxine, for example, binds much more tightly to thyroxine-binding globulin than does

triiodothyronine. Therefore, triiodothyronine is readily released as a free molecule and has easier

access to tissues than thyroxine. Similarly, among the sex steroids,testosteronebinds more

tightly to sex hormone-binding globulin than do other androgens or estrogens.

BIORHYTHMS

Some hormones, such as insulin, are secreted in short pulses every few minutes. Presumably, thetime between pulses is a reflection of the lag time necessary for the insulin-secreting cell to sense

a change in the blood glucose concentration. Other hormones, particularly those of the pituitary,are secreted in pulses that may occur at one- or two-hour intervals. Pulsatile secretion is anecessary requirement for the action of pituitary gonadotropins. For example, pituitary

gonadotropin secretion increases substantially and is maintained at increased levels when

gonadotropin-producing cells (gonadotrophs) are stimulated at 90- to 120-minute intervals by theinjection of hypothalamic gonadotropin-releasing hormone. If, however, the gonadotrophs are

subjected to a continuous injection of gonadotropin-releasing hormone, gonadotropin secretion is

inhibited.

In addition to pulses of secretion, many hormones are secreted at different rates at different times

of the day and night. These longer periodic changes are calledcircadian rhythms. One example ofa circadian rhythm is that ofcortisol, the major steroid hormone produced by the adrenal cortex.

Serum cortisol concentrations rapidly increase in the early morning hours, gradually decrease

during the day, with small elevations after meals, and remain decreased for much of the night.This particular rhythm is dependent on night-day cycles and persists for some days after airplane

travel to different time zones. The transitional period is reflected in the well-known phenomenon

ofjet lag. Other hormones follow different circadian rhythms. For example, serum concentrationsof growth hormone, thyrotropin, and the gonadotropins are highest shortly after the onset of

sleep. In the case of gonadotropins, this sleep-related increase is the first biochemical sign of the

onset of puberty. In addition, women have monthly biorhythms, which are reflected in their

menstrual cycles.

Endocrine dysfunction

ENDOCRINE HYPOFUNCTION AND RECEPTOR DEFECTSIn some cases, a decrease in hormone production, known ashypofunction, is required tomaintainhomeostasis. One example of hypofunction is decreased production of thyroid hormones

during starvation and illness. Because the thyroid hormones control energy expenditure, there is
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survival value in slowing the bodysmetabolismwhen food intake is low. Thus, there is a

distinction between compensatory endocrine hypofunction and true endocrine hypofunction.
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transport of hormones

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