Regulating Plasma Hormone Levels

Factors Involved: Secretion versus RemovalRegulation of Secretion

Metabolic Clearance Rate & Half-lifeRole of Carrier Proteins

Role of Glycosylation

Notes on the First Midterm

GlandRegulatory Hormone

GlandBlood Hormone

BindingTransportation

Active MetaboliteTarget Cell

Receptor BindingSignal Transduction

ActivationResponse

OR

(-)

A linear control sequence with each collection of cells secreting a hormone to

control subsequent cells.

Regulation of Hormone Secretion

Sensing and signaling: a biological need is sensed, the endocrine system sends out a signal to a target cell whose action addresses the biological need. Key features of this stimulus response system are: receipt of stimulus synthesis and secretion of hormone delivery of hormone to target cell evoking target cell response degradation of hormoneThis is relevant to the status of the hormone itself, rather than to

the type of system that regulates it (neural, endocrine, humoral), as we will see later in the hour.

Control of Endocrine Activity

•The physiologic effects of hormones depend largely on their concentration in blood and extracellular fluid. •Almost inevitably, disease results when hormone concentrations are either too high or too low, and precise control over circulating concentrations of hormones is therefore crucial.

Control of Endocrine Activity

The concentration of hormone as seen by target cells is determined by three factors:

•Rate of production•Rate of delivery•Rate of degradation and elimination

Control of Endocrine Activity

Rate of production: Synthesis and secretion of hormones are the most highly regulated aspect of endocrine control. Such control is mediated by positive and negative feedback circuits, as described below in more detail.

Control of Endocrine Activity

Rate of delivery: An example of this effect is blood flow to a target organ or group of target cells - high blood flow delivers more hormone than low blood flow.

Control of Endocrine Activity

Rate of degradation and elimination: Hormones, like all biomolecules, have characteristic rates of decay, and are metabolized and excreted from the body through several routes. Shutting off secretion of a hormone that has a very short half-life causes circulating hormone concentration to plummet, but if a hormone's biological half-life is long, effective concentrations persist for some time after secretion ceases.

Feedback Control of Hormone Production

Feedback loops are used extensively to regulate secretion of hormones in the hypothalamic-pituitary axis. An important example of a negative feedback loop is seen in control of thyroid hormone secretion

Inputs to endocrine cells

Neural control

• Neural input to hypothalamus stimulates synthesis and secretion of releasing factors which stimulate pituitary hormone production and release

Chronotropic control

• Endogenous neuronal rhythmicity• Diurnal rhythms, circadian rhythms (growth

hormone and cortisol), Sleep-wake cycle; seasonal rhythm

Episodic secretion of hormones

• Response-stimulus coupling enables the endocrine system to remain responsive to physiological demands

• Secretory episodes occur with different periodicity

• Pulses can be as frequent as every 5-10 minutes

Episodic secretion of hormones• The most prominent episodes of release occur with a

frequency of about one hour—referred to as circhoral

• An episode of release longer than an hour, but less than 24 hours, the rhythm is referred to as ultradian

• If the periodicity is approximately 24 hours, the rhythm is referred to as circadian – usually referred to as diurnal because the increase in

secretory activity happens at a defined period of the day.

Circadian (chronotropic) control

Circadian Clock

Physiological importance of pulsatile hormone release

• Demonstrated by GnRH infusion • If given once hourly, gonadotropin secretion and

gonadal function are maintained normally • A slower frequency won’t maintain gonad function • Faster, or continuous infusion inhibits gonadotropin

secretion and blocks gonadal steroid production

Clinical correlate

• Long-acting GnRH analogs (such as leuproline) have been applied to the treatment of precocious puberty, to manipulate reproductive cycles (used in IVF), for the treatment of endometriosis, PCOS, uterine leiomyoma etc

Endocrine Feedback Signals

• The strength of the feedback signal depends upon:- the levels of hormone available- the numbers of receptors for the hormone on

the target tissue• The level of hormone available depends upon three

factors:- rate of hormone production- rate of hormone secretion- rate of hormone clearance (breakdown,

excretion)

Types of Factors Influencing Secretion Rates

• In general, there are 3 types of factors involved in the regulation of secretion:

- neural- endocrine- humoral (glucose, osmolarity, blood pressure,

etc.).

• The rate of hormone secretion can be regulated by one or more types of factors.e.g., Insulin secretion is stimulated by glucose levels, parasympathetic nervous input, and gastric hormones.

Neural Regulation of Hormone Secretion

• Secretion of hormones from cells can be influenced by neuronal activity.

• Example: Release of norepinephrine and epinephrine from the adrenal medulla.

stress CNS

sympathetic nervoussystem adrenal medulla

norepinephrine release

Preganglionicfibers

Neural Regulation of Hormone Release

• Another Example: Release of vasopressin from the posterior pituitary

osmoreceptors (supraoptic nucleus of the hypothalamus)

posterior pituitary

vasopressin

Endocrine Control of Hormone Secretion

There are many examples in which a hormone is secreted in response to another hormone.

• ACTH acts on the zona fasciculata to stimulate the production of cortisol.

• LH acts on the Leydig cells to stimulate the production of testosterone.

• Thyroid-Stimulating Hormone acts on the thyroid to stimulate the release of T3, T4.

Neuroendocrine Regulation of Hormone Release

• A number of releasing factors are secreted from the hypothalamus, and travel to the anterior pituitary to regulate hormone secretion. This is termed neuroendocrine regulation (NOT neural regulation).- GnRH: stimulates LH, FSH release- CRF: stimulates ACTH release- GHRH: stimulates GH release- somatostatin: inhibits GH release- TRH: stimulates TSH release

Feedback Control of Endocrine Secretion

Feedback control of Endocrine Secretion

Humoral Control of Secretion

• Hormones are also secreted in response to changing levels of certain ions and nutrients.

• E.g., Parathyroid gland responds to decreased Ca2+ levels with increased parathyroid hormone release.

Humoral Control of Secretion

• Another example: aldosterone secretion

zona glomerulosa

aldosterone

decreased [Na+],increased [K+]

Mechanisms of Regulated Release

• The effects described so far typically influence both synthesis of hormone (last lecture) and release of hormone.

• For steroid hormones, the rate of synthesis and rate of production of a hormone are roughly the same (no hormone storage).

For peptide hormones, hormone can be synthesized and stored in secretory vesicles until there is a need for release.

Mechanisms of Release

• Peptide hormones are released from cells via migration of secretory vesicles toward the cell membrane. The vesicles fuse with the cell membrane, releasing contents by exocytosis.

Regulated versus Constitutive Release• Constitutive release: in many cells in the body, the

migration of vesicles to the cell surface is constant and not regulated.

• Regulated release: in endocrine cells, the migration of vesicles to the cell surface occurs when there is a signal telling the cell to release hormone.

Receptor

constitutive release

regulated release

What are the postreceptor signals regulating movement and release of vesicles?

• The detailed mechanisms are not well understood. May involve movement along microtubules.

• Secretion is often dependent upon influx of calcium into the cell.- Influx of calcium results in cell depolarization.- Cell depolarization is also sufficient to cause hormone release.- Calcium can act as a second messenger in cells, via calmodulin (effects on enzyme activation via phosphorylation)-Calcium may influence microtubule contraction.

Influence of Second Messengers on Secretion• In addition, there appears to be involvement of cyclic

AMP, at least in some cases (cAMP increases in response to signal for release)

• Calcium may stimulate cAMP formation• Example: Aplysia californica: California sea slug

- Secretes egg-laying hormone from bag cells- Secretion is stimulated in cell culture by electical depolarization of cells.- Secretion can be inhibited by blockers of cAMP-dependent protein kinase A

cyclic AMP protein kinase A phosphorylation of enzymes

Role of Protein Kinase A in Stimulated Secretion of ELH

ELH

causes egg-laying behavior in whole animals

Aplysia californica bag cellsdepolarizing

current(+)

PKA inhibitor(-)

Synthesis versus Secretion• The signal pathways resulting in increased synthesis

of a peptide hormone may be different from the signals causing increased release.

• Example: Actions of GnRH on LH synthesis and release.

GnRH R

Calcium release

Protein KinaseC

LH mRNA

synthesis

Why Regulate Peptide Hormone Release?

• Allows for large, rapid changes in peptide hormone levels.

• Allows for the release of peptide hormones at a greater rate than the hormone is synthesized.- peptide can be accumulated in secretory granules, and rapidly released when necessary

Clearance of Hormone from the Body• Since hormones are released in response to specific

conditions, they must be inactivated so they do not continue to exert their effects for an indefinite period.

• Hormones are broken down, modified, and/or removed from the blood and the body at different rates.

• Hormones are excreted primarily from the kidney into urine.

Endocrine Gland Hormone Target Cell

Action

Clearance Rates of Hormones

• The clearance rate of a hormone (how fast it is broken down and/or removed from the blood) can be expressed in two ways:1) Metabolic Clearance Rate (MCR): the volume of blood from which a hormone is completely removed in a given period of time (ie, milliters/hour).2) Circulating Half-life: The time it takes for 50% of a hormone to be removed from the circulation.

Metabolic Clearance Rate

• The larger the MCR number, the faster the hormone is removed from the blood.

• Example: a hormone with a MCR of 100 ml/minute is removed faster than a hormone with a MCR of 50 ml/min.

• Theoretical calculation of clearance rate:urine production x [concentration in urine] (ml/min)

[concentration in plasma]• However, hormones appear in urine after being metabolized,

and are thus difficult to measure.

Circulating Half-Life

• It is easier to determine the half-life of a hormone (T1/2; how long it takes for 50% of the hormone to be removed from the blood).

Hor

mon

eC

once

ntra

tion 40

20

0

Time (minutes)

0 10 20 30 40

T1/2

Determining T1/2 of a Hormone

• BUT: Hormone is constantly made in the body. Thus, the rate of decline depends upon not only clearance, but synthesis as well.

• So, how is T1/2 calculated (answer given in lecture!).

Some Typical Half-lives of Hormones

Hormone T1/2

small peptides 4-40 minutes

large proteins 15 - 180 minutes(TSH, LH, FSH)

steroids 5 - 120 minutes

What Happens to Hormones During Clearance?

• A very small amount of total circulating hormone is degraded within cells by internalization following binding to membrane receptors.

• Here’s what happens:- peptide hormone binds to receptor on cell surface- the hormone:receptor complex is internalized by endocytosis to form a vesicle- the vesicle may fuse with a lysosome, resulting in degradation of the hormone

Ligand-Induced Internalization and Degradation of Receptors and Hormones

H

Hreceptor

H

lysozome

What Happens to Hormones During Clearance?

• A very small amount (< 1%) of hormone is secreted in the urine as intact hormone.

• The majority of peptide hormones and steroid hormones are first metabolized (broken down or modified) before secretion in the urine (mostly) and feces (< 10%).

• Most metabolism of hormones occurs in the liver and kidneys.

• Peptide hormones: enzymes cut between peptide bonds in a specific manner- endopeptidases cut within the peptide- exopeptidases cut from each end (aminopeptidases and carboxypeptidases)- breaking disulfide bonds

NH2 COOHaminoacid 1

aminoacid 2

aminoacid 3

aminoacid 4

What Happens to Hormones During Clearance?

exopeptidase exopeptidase

endopeptidase

Metabolism of Steroid Hormones and Thyroid Hormones

• A key step in the metabolism of steroid hormones by the liver is the conjugation (adding on) of glucuronic acid or sulfate groups.

• This conjugation makes the steroids more water soluble (easier to excrete from the body).

• Conjugated steroids are excreted from the liver in bile. They may then be reabsorbed into the blood and excreted via kidneys into urine. Some is excreted with bile in feces.

• Thyroid hormones (nonpeptide) are similarly metabolized by the liver.

Factors Influencing the Half-life of Hormones

• There are three factors which appear to influence the rate of clearance of hormones:- size of the hormone (smaller peptides have short half-lives)- whether it binds to a binding protein (mostly steroids)- the glycosylation pattern

Influence of Binding Proteins on T1/2

• Many steroid hormones are found in the circulation bound to carrier proteins (binding proteins produced from the liver), due to their insoluble nature.

• Bound hormone has a longer half-life (protected from degradation).

• For example, most aldosterone is in the plasma in free (not bound) form, and aldosterone has a very short half-life.

Specificity of Binding Globulins• Some binding proteins are highly specific, with high

binding affinity for their hormone:-Testosterone Binding Globulin (TeBG; also called SHBG); binds T and E2 - Cortisol Binding Globulin (CBG): binds cortisol- Thyroid Hormone Binding Globulin (TBG): binds T3, T4

• Other proteins, such as albumin, bind a wider variety of hormones, and bind less tightly

• While testosterone bound to albumin is biologically available to tissues (loose binding), T bound to TeBG is not biologically available.

Regulation of Binding Globulins

• The presence of carrier proteins influences half-life and biological availability of steroid hormones.

• Interestingly, the production of these carrier proteins from the liver is regulated in different physiological states.

• Example: During pregnancy, there is a stimulation of CBG levels, DECREASING negative feedback of cortisol on ACTH release. ACTH release INCREASES, resulting in more cortisol production, until the relative level of free/bound cortisol is normal (restoration of negative feedback).Why would you want this?

Regulation of CBG during Pregnancy

pregnancy

increased CBG

decreased free cortisol

decreased negative feedback on ACTH

increased ACTH release increased cortisol release

normal free cortisol

effects on fetal adrenal function?

Effects of Glycosylation on Half-life of Peptide Hormones

• Some peptide hormones are glycosylated (glycoproteins)

• There is a strong correlation between the amount of sialic acid in the glycoprotein chains, and the half-life of the hormone.

• Removal of sialic acid dramatically decreases the half-life of the hormone.

Relationship Between Sialic Acid Content and Glycoprotein Hormone Half-life

Hormone Half-life %Sialic AcidLH 45 min 2%FSH 180 min 5hCG 360 min 10

Glycosylation Pattern of LH, FSH, hCG

alpha subunit

FSH Beta

LH Beta

hCG Beta

Making “SuperFSH”Using molecular techniques, one can add the

carboxyl terminus of hCG onto FSH, making an FSH molecule with increased sialic acid content (and increased half-life)

hCG

FSH

FSH+CTP

Why Does Sialic Acid Matter?

• Sialic acid appears to protect the hormone from metabolism by the kidney.

Is There Regulation of Sialic Acid Content of Glycoproteins?

• Changes in sialic acid content have been reported during aging in men and women (LH, FSH).

• Steroid hormones (testosterone and estradiol) appear to influence sialic acid content.