Endocrine Physiologylecture 3

Dale Buchanan Hales, PhD

Department of Physiology & Biophysics

Anterior pituitary

• Anterior pituitary: connected to the hypothalamus by hypothalmoanterior pituitary portal vessels.

• The anterior pituitary produces six peptide hormones: – prolactin, growth hormone (GH), – thyroid stimulating hormone (TSH), – adrenocorticotropic hormone (ACTH), – follicle-stimulating hormone (FSH), – luteinizing hormone (LH).

Anterior pituitary cells and hormones

Cell type Pituitary population

Product Target

Corticotroph 15-20% ACTH-lipotropin

Adrenal glandAdipocytesMelanocytes

Thyrotroph 3-5% TSH Thyroid gland

Gonadotroph 10-15% LH, FSH Gonads

Somatotroph 40-50% GH All tissues, liver

Lactotroph 10-15% PRL Breastsgonads

Anterior pituitary hormones

Feedback regulation of hypothalmus/pituitary

A prominent feature of each of the hormonal sequences initiated by the hypothalamic releasing hormones is negative feedback exerted upon the hypothalamic-pituitary system by the hormones whose production are stimulated in the sequence.

Hypothalamus-pituitary axis

Feedback control

Feedback control of

thyroid function

Feedback leads to

restoration of homeostasis

Feedback control of

growth hormone

Regulation of Growth Hormone Secretion

• GH secretion controlled primarily by hypothalamic GHRH stimulation and somatostatin inhibition

• Neurotransmitters involved in control of GH secretion– via regulation of GHRH and somatostatin

• Neurotransmitter systems that stimulate GHRH and/or inhibit somatostatin– Catecholamines acting via 2-adrenergic

receptors– Dopamine acting via D1 or D2 receptors– Excitatory amino acids acting via both NMDA

and non-NMDA receptors

Regulation of Growth Hormone Secretion

-adrenergic receptors stimulate somatostatin release and inhibit GH

-adrenergic receptors inhibit hypothalamic release of GHRH

Regulation of Growth Hormone Secretion

• Additional central mechanisms that control GH secretion include an ultra-short feedback loop exerted by both somatostatin and GHRH on their own secretion

Regulation of Growth Hormone Secretion

Growth hormone vs. metabolic state

• When protein and energy intake are adequate, it is appropriate to convert amino acids to protein and stimulate growth. hence GH and insulin promote anabolic reactions during protein intake

• During carbohydrate intake, GH antagonizes insulin effects-- blocks glucose uptake to prevent hypoglycemia. (if there is too much insulin, all the glucose would be taken up).

• When there is adequate glucose as during absorptive phase, and glucose uptake is required, then GH secretion is inhibited so it won't counter act insulin action.

• During fasting, GH antagonizes insulin action and helps mediate glucose sparing, ie stimulates gluconeogenesis

• In general, during anabolic or absorptive phase, GH facilitates insulin action, to promote growth.

• during fasting or post-absorptive phase, GH opposes insulin action, to promote catabolism or glucose sparing

Growth hormone vs. metabolic state

Growth hormone

and metabolic

state

Clinical assessment of GH

• Random serum samples not useful due to pulsatile pattern of release

• Provocative tests necessary– GH measurement after 90 min exercise– GH measurement immediately after onset of sleep

• Definitive tests– GH measurement after insulin-induced hypoglycemia– Glucose suppresses GH levels 30-90 min after

administration– patients with GH excess do not suppress– Measurement of IGF-1 to assess GH excess

Acromegaly and Gigantism

• Caused by eosinophilic adenomas of somatotrophs• Excess GH leads to development of gigantism if

hypersecretion is present during early life– a rare condition– Symmetrical enlargement of body resulting in true

giant with overgrowth of long bones, connective tissue and visceral organs.

• Excess GH leads to acromegaly if hypersecretion occurs after body growth has stopped.– Elongation of long bones not possible so there is over

growth of cancellous bones– protruding jaw, thickening of phalanges, and over growth of visceral organs

Acromegaly A) before presentation; B) at admission

Harvey Cushing’s first reported case

Acromegaly

Gigantism

Identical twins, 22 years old, excess GH secretion

ACTH: adrenocorticotropic hormone: synthesis and regulation of secretion

• Produced in corticotrophs• ACTH is produced in the anterior pituitary by

proteolytic processing of Prepro-opiomelanocortin (POMC).

• Other neuropeptide products include and lipotropin, -endorphin, and -melanocyte-stimulating hormone (-MSH).

• ACTH is a key regulator of the stress response

ACTH synthesis

Processing and cleavage of pro-opiomelanocortin (POMC)

ACTH

ACTH

ACTH is made up of 39 amino acidsRegulates adrenal cortex and synthesis of

adrenocorticosteroids-MSH resides in first 13 AA of ACTH-MSH stimulates melanocytes and can darken

skinOverproduction of ACTH may accompany

increased pigmentation due to -MSH.

Addison’s Disease

• Disease in which patients lack cortisol from zona fasiculata, and thus lacks negative feedback that suppresses ACTH production

• Result: overproduction of ACTH

• Skin color will darken

• JFK had Addison’s disease and was treated with cortisol injections

-endorphin

• Produced as a result of ACTH synthesis

• Binds to opiate receptors

• Results in “runner’s high”

• Role in anterior pituitary not completely understood

• One of many endogenous opioids such as enkephalins

Melanocyte-stimulating hormone(MSH)

• MSH peptides derived by proteolytic cleavage of POMC

-MSH has antipyretic and anti-inflammatory effects– Also inhibits CRH and LHRH secretion

• Four MSH receptors identified• May inhibit feeding behavior• ACTH has MSH-like activity • However– MSH has NO ACTH like activity

Regulation of ACTH secretion

Regulation of ACTH secretion

• Stimulation of release– CRH and ADH

– Stress

– Hypoglycemia

• CRH and ADH both synthesized in hypothalamus– ADH (a.k.a. vasopressin) is released by posterior

pituitary and reaches anterior pituitary via inferior hypophyseal artery.

• Deficiency of vasopressin (ADH) in hereditary diabetes insipidus is accompanied by decreased ACTH release.

• Vasopressin potentiates CRH at both hypothalamic and pituitary levels.

• Many vasopressinergic neurons also contain CRH resulting in co-release of two peptides into portal blood.

Regulation of ACTH secretion

ACTH

• Circadian pattern of release– Highest levels of cortisol are in early AM

following ACTH release– Depends on sleep-wake cycle, jet-lag can result

in alteration of pattern

• Opposes the circadian pattern of growth hormone secretion

Regulation of ACTH

ACTH

• Acts on adrenal cortex– stimulates growth of cortex (trophic action)– Stimulates steroid hormone synthesis

• Lack of negative feedback from cortisol results in aberrantly high ACTH, elevated levels of other adrenal corticosteroids– adrenal androgens

• Adrenogenital syndrome: masculization of female fetus

Glycoprotein hormones

LH, FSH, TSH and hCG and subunitsEach subunit encoded by different gene

subunit is identical for all hormones subunit are unique and provide biological

specificity

Glycoprotein hormones

Glycoprotein hormones contain two subunits, a common subunit and a distinct subunit:

TSH, LH, FSH and hCG.

Gonadotrophs

• Cells in anterior pituitary that produce LH and FSH

• Synthesis and secretion stimulated by GnRH– major effect on LH

• FSH secretion controlled by inhibin • Pulsitile secretion of GnRH and inhibin cause

distinct patterns of LH and FSH secretion

LH/FSH

• Pulsatile pattern of secretion– LH pulses are biphasic (every 1 minute, then large

pulse at 1 hour)– FSH pulses are uniphasic

• Diurnal– LH/FSH more pronounced during puberty

• Cyclic in females– ovarian cycle with LH surge at time of ovulation

• Males are not cyclic, but constant pulses of LH cause pulses of testosterone to be produced

Pulsitile secretion of GnRH and LH

Regulation of LH/FSH

• Negative feed-back– Inhibin produced by testes and ovaries Decreases FSH

-subunit expression

– Testosterone from Leydig cells– synthesis stimulated by LH, feedsback to inhibit GnRH production from hypothalamus and down-regulates GnRH receptors

– Progesterone– suppresses ovulation, basis for oral contraceptives. Works at both the level of pituitary and hypothalamus.

• Dopamine, endorphin, and prolactin inhibit GnRH release. – Prolactin inhibition affords post-partum contraceptive

effect

• Overproduction of prolactin via pituitary tumor can cause amenorrhea– shuts off GnRH– Treated with bromocryptine (dopamine agonist)

– Surgical removal of pituitary tumor

Regulation of LH/FSH

• Positive feedback– Estradiol at high plasma concentrations in late

follicular phase of ovarian cycle stimulates GnRH and LH surge– triggers ovulation

Regulation of LH/FSH

Regulation of gonadotropin

secretion

Thyrotrophs

• Site of TSH synthesis

• Pattern of secretion is relatively steady

• TSH secretion stimulated by TRH

• Feedback control by T3 (thyroid hormone)

Feedback control of

thyroid function

Grave’s disease

• Hyperthyroidism caused by circulating antibodies to the TSH receptor.

• Associated with diffuse goiter.

• Autoantibodies bind to TSH receptor and mimic the action of TSH itself leads to persistent stimulation of thyroid and elevated levels of thyroid hormones.

Lacotrophs

• Site of production of prolactin• Lactogenesis (milk synthesis) requires prolactin• Tonically inhibited

– Of the anterior pituitary hormones, the only one– Multifactoral control, balance favors inhibition

• Dopamine inhibits prolactin• Prolactin releasing hormone is TRH

– Ocytocin also stimulates prolactin release– Estradiol enhances prolactin synthesis

Prolactin

• Stimulates breast development and lactogenesis

• May be involved in development of Leydig cells in pre-pubertal males

• Immunomodulatory effects– stimulates T cell functions– Prolactin receptors in thymus

Clinical assessment of PRL

• Single basal serum PRL measurement sufficient to determine excess– PRL deficiency not a usual clinical concern

• PRL is only anterior pituitary with predominant negative control by hypothalamus– often elevated by lesions that interfere with portal blood flow.

• Elevated by primary PRL adenomas of pituitary

Posterior pituitary hormones: ADH (AVP) and Oxytocin (hypothalamic hormones)

Both are synthesized in the cell bodies of hypothalamic neurons

ADH: supraoptic nucleus Oxytocin: paraventricular nucleus Both are synthesized as preprohormones and

processed into nonapeptides (nine amino acids). They are released from the termini in response to

an action potential which travels from the axon body in the hypothalamus

Hypothalamus and posterior pituitary

Structures of ADH and oxytocin

In uterus during parturitionIn mammary gland during

lactation

Oxytocin: stimulates myoepithelial contractions

Oxytocin: milk ejection from lactating mammary gland

suckling is major stimulus for release.

sensory receptors in nipple connect with nerve fibers to the spine, then impulses are relayed through brain to PVN where cholinergic synapses fire on oxytocin neurons and stimulate release.

Oxytocin: uterine contractions

• Reflexes originating in the cervical, vaginal and uterus stimulate oxytocin synthesis and release via neural input to hypothalamus

• Increases in plasma at time of ovulation, parturition, and coitus

• Estrogen increases synthesis and lowers threshold for release

Oxytocin secretion is stimulated by nursing