PITUITARY HORMONES YuliaKomarova, Ph.D. 312-996-1332 firstname.lastname@example.org
Knowledge Objectives 1. Understand the regulation of growth hormone (GH) biosynthesis and secretion including the roles of growth hormone releasing hormone (GH-RH) and GH releasing peptides. 2. Know the physiological conditions that elicit growth hormone secretion and how specific diagnostic maneuvers can elicit GH secretion. 3. Understand the regulation of prolactin biosynthesis secretion and release by suckling; effect of dopaminergic and serotonergic agonists and antagonists. 4. Understand the medical problems related to hypersecretion of prolactin in the female and in the male. 5. Know the structure and actions of oxytocin and roles in parturition and lactation. 6. Know the effects of vasopressin on receptor subtypes and signal transduction systems in vascular smooth muscle.
Growth Hormone (GH) • Structure • GH is a 191-amino-acid peptide with two sulfhydryl bridges. It composed of the four-helix bundle with two pairs of parallel helices joined antiparallel • Absorption, Metabolism, and Excretion • Circulating endogenous GH has a has t1/2~ 20–25 minutes and is cleared by the liver • Developmental Actions • is required during childhood for attainment of normal adult size and body composition • stimulates longitudinal bone growth • Metabolic Effects • increases the production of Insulin-like Growth Factor (IGF)-1 in the liver, bone, cartilage, muscle, and the kidney • controls lipid and carbohydrate metabolism, and lean body mass • reduces insulin sensitivity, which results in mild hyper-insulinemia
Molecular and Cellular Effects of GH hGH receptor contains 620 amino acids, approximately 250 of which are extracellular, 24 of which are transmembrane, and 350 of which are cytoplasmic. JAK2, a cytoplasmic tyrosine kinase of the Janus kinase family; STAT (Signal Transducers and Activators of Transcription), Shc (an adapter protein that regulates the Ras/MAPK signaling pathway), and IRS-1 and IRS-2 (insulin-receptor substrate proteins that activate the PI3K pathway).
Growth Hormone Deficiency • GH deficiency is a result of a genetic mutations or damage to the pituitary or hypothalamus by a tumor, infection, surgery, or radiation therapy. • In most patients, the deficiency is idiopathic, with normal production of other pituitary hormones and no obvious structural abnormalities. • Children with GH deficiency present with short stature, delayed bone age, a low age-adjusted growth velocity, hypoglycemia and adiposity. • Criteria for diagnosis are: (1) a growth rate below 4 cm per year and (2) the absence of a serum GH response to two GH secretagogues.
Therapeutic Uses of Somatropin • Somatropinis administered subcutaneously 6–7 times per week. Peak levels occur in 2–4 hours and active blood levels persist for approximately 36 hours. • In children, somatropin is administered in a dose of 25-50 g/kg per day subcutaneously in the evening; higher daily doses (e.g., 50-67 g/kg) are employed for patients with Noonan's syndrome or Turner's syndrome, who have partial GH resistance • For adults, a typical starting dose is 150-300 g per day, with higher doses used in younger patients transitioning from pediatric therapy; lower doses are used in older patients (e.g., >60 years of age). • Because estrogen inhibits GH action, women taking oral—but not transdermal—estrogen may require larger GH doses to achieve the target IGF-1 level.
Somatropin Toxicity & Contraindications • Children: • rarely intracranial hypertension, which may manifest as vision changes, headache, nausea • scoliosis as a result of rapid growth • patients with Turner syndrome have an increased risk of otitis media. • hypothyroidism , pancreatitis, gynecomastia, and nevus growth • Adults: • peripheral edema, myalgias, and arthralgias (especially in the hands and wrists) occur commonly but remit with dosage reduction. • rarely proliferative retinopathy • Contraindications: • known malignancy
Recombinant Human Insulin-like growth factor-1 (IGF1) (mecasermin) • impaired growth secondary to mutations in the GH receptor or postreceptor signaling pathway • in patients with GH deficiency who develop antibodies against GH • in patients with IGF-1 gene defects that lead to primary IGF-1 deficiency • 40-80 g/kg per dose twice daily by subcutaneous injection, with a maximum of 120 g/kg per dose twice daily
Excess Production of Growth Hormone • Acromegaly, a result GH-secreting pituitary adenomas, which is characterized by abnormal growth of cartilage and bone tissue, and many organs including skin, muscle, heart, liver, and the gastrointestinal tract. Gigantism is a result of GH-secreting adenoma occurring before the long bone epiphyses close.
Growth Hormone Antagonists • Somatostatin analogs: octreotide, octreotide acetate, lanreotide, vapreotide • the amino acid residues in positions 7-10 of the SST-14 peptide (Phe-Trp-Lys-Thr) are the major determinants of biological activity. Trp8 and Lys9 are essential, whereas conservative substitutions at Phe7 and Thr10 are permissible. • active SST analogs retain this core segment constrained in a cyclic structure by a disulfide bond • octreotide and lanreotide bind to the SST subtypes with the following order of selectivity: • SST2 > SST5 > SST3 ≫ SST1 and SST4.
Therapeutic Uses of Octreotide • acromegaly • metastatic carcinoidtumors • gastrinoma • glucagonoma • nesidioblastosis • the watery diarrhea • and diabetic diarrhea • 90Y-labled octreotide is used in selective destruction of SST2 receptor-positive tumors • Octreotide (50-200 g) is administered subcutaneously three times daily, peak effects are seen within 30 min, serum t1/2 is 90 min, and duration of action is 12 hour. • a long-acting, slow-release form (SANDOSTATIN-LAR DEPOT) in which the active species is incorporated into microspheres is administered intramuscularly in a dose of 20 or 30 mg once every 4 week. A lower dose of 10 mg per injection should be used in patients requiring hemodialysis or with hepatic cirrhosis.
Toxicity & Contraindications of SST analogs • GI side effects—including diarrhea, nausea, and abdominal pain—occur in up to 50% of patients • 25% of patients develop gallbladder sludge or even gallstones, presumably due to decreased gallbladder contraction and bile secretion. • cardiac effects include sinus bradycardia (25%) and conduction disturbances (10%).
Pegvisomant, GH Receptor Antagonist • Pegvisomant, high affinity antagonist of GH receptor, prevents activation of GH receptor downstream signaling • the polyethylene glycol (PEG) derivative of a mutant GH, which has increased affinity for one site of the GH receptor but a reduced affinity at its second binding site. • binds to the GH receptor but does not activate JAK-STAT signaling or stimulate IGF-1 • Pegvisomant(SOMAVERT) is FDA-approved therapy for the treatment of acromegaly and provides a highly effective alternative for use in patients who have not responded to SST analogs. • is administered subcutaneously as a 40-mg initial dose under physician supervision, followed by self-administration of 10 mg per day. The dose is titrated at 4- to 6-week intervals to a maximum of 40 mg per day • Contraindications: • an unexplained elevation of hepatic transaminases
Regulation of Prolactin Secretion • Hypothalamic regulation of prolactin secretion is predominantly inhibitory. • The major regulator of prolactin secretion is DA, which is released by tuberoinfundibular neurons and interacts with the D2 receptor on lactotropes to inhibit prolactin secretion
Excess Production of Prolactin • Hyperprolactinemiais developed as a result of impaired transport of dopamine (prolactin-inhibiting hormone) to the pituitary or more commonly, as a result of prolactin-secreting adenomas. • Hyperprolactinemia produces a syndrome of amenorrhea and galactorrheain women, and loss of libido and infertility in men. • Hypogonadism and infertility associated with hyperprolactinemia result from inhibition of release of Gonadotropin-releasing hormone (GnRH).
Dopamine Agonists • Quinagolide, a drug approved in Europe, is a nonergot agent with similarly high D2 receptor affinity. • Bromocriptine and cabergolineare ergot derivatives with a high affinity for dopamine D2 receptors. • Pharmacokinetics • All dopamine agonists are oral preparations, which are eliminated by metabolism. • Cabergoline, with a half-life of approximately 65 hours, has the longest duration of action. • Quinagolide has a half-life of about 20 hours, whereas of • Bromocriptine has the half-life about 7 hours.
Therapeutic Uses of Dopamine Agonists Hyperprolactinemia • dopamine agonists shrink pituitary prolactin-secreting tumors, lower circulating prolactin levels, and restore ovulation in approximately 70% of women with microadenomas and 30% of women with macroadenomas • Cabergoline is initiated at 0.25 mg twice weekly orally or vaginally. It can be increased gradually up to a maximum of 1 mg twice weekly. • Bromocriptine is generally taken daily after the evening meal at the initial dose of 1.25 mg; the dose is then increased as tolerated. Most patients require 2.5–7.5 mg daily. • Acromegaly • A dopamine agonist alone or in combination with pituitary surgery, radiation therapy, or octreotide administration can be used to treat acromegaly • The doses are 20–30 mg/d of bromocriptine unless the pituitary tumor secretes prolactin as well as GH.
Toxicity & Contraindications of Dopamine Agonists • nausea, headache, light-headedness, orthostatic hypotension, and fatigue • occasional psychiatric manifestations • high dosages can cause cold-induced peripheral digital vasospasm • pulmonary infiltrates have occurred with chronic high-dosage therapy • therapy during the early weeks of pregnancy has not been associated with an increased risk of spontaneous abortion or congenital malformations • patients with very large adenomas continue a dopamine agonist treatment throughout pregnancy • rare reports of stroke or coronary thrombosis in postpartum women taking bromocriptine to suppress postpartum lactation
Posterior Pituitary Hormones Oxytocinand Vasopressin are synthesized in neuronal cell bodies in the hypothalamus and transported via their axons to the posterior pituitary, where they are stored and then released into the circulation.
Oxytocin • Oxytocinis a peptide hormone secreted by the posterior pituitary that participates in labor and delivery and elicits milk ejection in lactating women. • Oxytocinacts via a specific GPCR, OXT, coupled to Gq/G11 and activating the PLC-IP3-Ca2+ pathway and enhancing activation of voltage-sensitive Ca2+ channels. • Oxytocinstimulates the release of prostaglandins and leukotrienes that augment uterine contraction. • Oxytocin causes contraction of myoepithelial cells surrounding mammary alveoli, which leads to milk ejection.
Therapeutic Uses of Oxytocin • Oxytocin is used to induce labor for conditions requiring early vaginal delivery such as Rh problems, maternal diabetes, preeclampsia, or ruptured membranes. • It is also used to augment abnormal labor that is protracted or displays an arrest disorder. • Oxytocin is usually administered intravenously via an infusion pump with an initial infusion rate of 0.5–2 mU/min. • For induction of labor, rate is increased every 30–60 minutes until a physiologic contraction pattern is established. The maximum infusion rate is 20 mU/min. • For postpartum uterine bleeding, 10–40 units are added to 1 L of 5% dextrose, and the infusion rate is titrated to control uterine atony. • Contraindications: • fetal distress • prematurity, abnormal fetal presentation • cephalopelvic disproportion
Vasopressin • Vasopressin is a peptide hormone released by the posterior pituitary in response to rising plasma tonicity or falling blood pressure. • A deficiency of this hormone results in diabetes insipidus • Vasopressin activates V1a subtypes of G protein-coupled receptors on vascular smooth muscle cells and mediate vasoconstriction. • V2 receptors are found on renal tubule cells and reduce diuresis through increased water permeability and water resorption in the collecting tubules. • Extrarenal V2-like receptors regulate the release of coagulation factor VIII and von Willebrandfactor.
Therapeutic Uses of Vasopressin • [Phe2, Ile3, Orn8] vasotocin is the most specific V1 vasoconstrictor agonist that is selective for vasoconstrictor activity. • Terlipressin (triglycyllysinevasopressin), a synthetic vasopressin analog that is converted to lysine vasopressin in the body, is more selective for V1 receptors and has a longer half-life. • Vasopressin and its analogues are effective in the treatment of vasodilatory shock states and in some cases of esophageal variceal bleeding and colonic diverticularbleeding due to its V1a agonist activity. • The V1aantagonists, the peptide antagonist d(CH2)5[Tyr(Me)2]AVP and nonpeptide, orally active V1a-receptor antagonists, relcovaptanand SRX251 are potential therapeutic agents for the treatment of Raynaud'sdisease, hypertension, heart failure, brain edema, motion sickness, cancer, preterm labor, and anger reduction.
Literature: Bertram G. Katzung, Susan B. Masters, Anthony J. Trevor Basic & Clinical Pharmacology, 11e, Chapter 37 Hypothalamic & Pituitary Hormones Laurence L. Brunton, Bruce A. Chabner, Björn C. KnollmannGoodman & Gilman's The Pharmacological Basis of Therapeutics, 12e Chapter 38 Introduction To Endocrinology: The Hypothalamic-Pituitary Axis