1- Principles of Endocrinology, MPhysio S24-1.pdf

Principles of Endocrinology Costanzo, 7th Ed., pages 399-410 RA Augustyniak LMU-DCOM Spring 2024 Objectives: 1. Discuss the major functions of the endocrine system and discriminate between nervous system and endocrine system regulation of metabolic reactions. 2. Describe the different types of cell-...

Principles of Endocrinology Costanzo, 7th Ed., pages 399-410 RA Augustyniak LMU-DCOM Spring 2024 Objectives: 1. Discuss the major functions of the endocrine system and discriminate between nervous system and endocrine system regulation of metabolic reactions. 2. Describe the different types of cell-to-cell signaling and associate which signaling mechanism is associated with which hormone. 3. Describe the four class of hormones and how each class is synthesized, stored, secreted & metabolized. 4. Discuss how hormone action is initiated for both water and lipid soluble hormones and explain the various signal transduction mechanisms involved for each water-soluble hormone discussed in class, especially for cAMP, cGMP, IP3 & DAG, calcium-calmodulin, and RTKs & RATKs. 5. Explain how hormone secretion is regulated through both negative and positive feedback mechanisms. 6. Differentiate between responsiveness and sensitivity of a hormone in terms of a dose-response curve and explain how expression of hormone receptors are controlled through up- and down- regulation. 1 By the end of this session, you should be able to answer the following questions from Units XIV of Guyton and Hall Physiology Review 4th Ed. Questions #’s: Unit XIV- 1, 18, 55, 81, 90, 102, 112 2 Optional Lecturio Videos Relevant to Lecture : Endocrine System and Hypothalamic-Pituitary Axis (HPA)- 02:11 min https://lmu.lecturio.com/#/lecture/c/27344/27330/27095/25076 Feedback Loops and Pituitary Gland- 04:11 min https://lmu.lecturio.com/#/lecture/c/27344/27330/27095/25078 NOTE: Must be logged into Lecturio for the weblinks to work Cell Surface Receptors- 17:19 min https://lmu.lecturio.com/#/lecture/c/7814/7534/7012/21290 Steroid Hormone Receptors- 06:41 min https://lmu.lecturio.com/#/lecture/c/7814/7534/7012/21292 Hormone Binding Proteins- 03:17 min https://lmu.lecturio.com/#/lecture/c/7814/7534/7012/21294 3 Endocrine and Nervous Systems Work Together to Maintain Homeostasis Redrawn from Berne and Levy, Principles of Physiology, 2nd ed. Page 587 SYSTEM Internal and External Chemical and Physical Stimuli Endocrine ENDOCRINE NERVOUS Nervous Neurocrine SIGNALS Hormones Neurotransmitters Blood Axon CONVEYANCE RECEPTORS RESPONSE Integrated Independent Independent 4 Overview of Endocrine Physiology Endocrine glands secrete chemical messengers, called hormones, into the extracellular fluid which then act on target cells to regulate biological processes Hormones regulate: Growth/Development Reproduction Blood pressure Behavior, etc. Growth, development, reproduction, blood pressure, concentrations of ions and other substances in blood, and even behavior are all regulated by the endocrine system. Endocrine physiology involves the secretion of hormones and their subsequent actions on target tissues. A hormone is a chemical substance that is secreted into the circulation in small amounts and delivered to target tissues where they produce physiologic responses. Hormones are synthesized and secreted by endocrine cells usually found in endocrine glands. This is just a summary of the organs that are involved in hormone production. In addition, endocrine cells reside as a minor component (in terms of mass) in other organs, either as groups of cells (islets of Langerhans in the pancreas) or as individual cell spread throughout several glands, including the GI tract, kidney, heart, adipose tissue and liver. 5 Redrawn from Berne The Endocrine and Nervous Principles of Systems Work Together to ed. Page Regulate Blood Glucose Glucose In Plasma ANS Hypothalamus Pancreas Adrenal Medulla CRH Pituitary Glucagon EPI Insulin NE ACTH Adrenal Cortex Cortisol Release Glucose Liver Rebuild Glucose Glucose In Plasma EPI Cortisol Insulin Glucose Utilization Endocrine and nervous communication can be integrated to provide for homeostasis. 6 Four Methods of Hormone Delivery Autocrine Paracrine  A cell that releases a hormone that either impact itself or nearby cells  Only occurs over short distances  Released hormones have very short half-lives Endocrine Neurocrine  A cell/nerve that releases a hormone wherein the target cells are a great distance away  Only occurs over long distances  Released hormones have medium to long half-lives Chemical Signaling mechanisms can be organized into 4 basic categories as to the distance and intended target cell or cells: Autocrine/Paracine Signaling - Cells secrete Local Chemical Mediators or local hormones that are so quickly bound or enzymatically destroyed that they affect only cells located in a very small region around the secreting cell. para = "around or surrounding Endocrine/Neurocrine Signaling - Endocrine glands or nerves secrete Circulating Hormones into the blood stream that disseminate widely through out the body and may affect specific target cells located far from the site of hormonal secretion. The presence and activity of Circulating Hormones may last for only a few minutes or even hours. 7 Principal Classes of Hormones Peptide and Protein hormones Synthesized from amino acids Account for most hormones Steroid hormones All steroid hormones are derivatives of cholesterol Amine hormones Catecholamines Dopamine Norepinephrine Epinephrine Thyroid hormones All amine hormones are derivatives of the amino acid tyrosine 3 Chemical Classifications of Hormones: Proteins are composed of individual units called Amino Acids. Peptides are short chains of amino acids while proteins are much larger and more complex arrangements of Amino Acids (or peptides). Because of their protein make-up, these kinds of hormones are not well suited for ingestion. In the presence of acid and catalytic enzymes, peptide and protein hormones breakdown into their constituent amino acids. Insulin is in this class of hormones and must be injected rather than ingested. Steroids are distinguished structurally by their basic backbone that is comprised of four rings of carbon atoms. All steroids contain these four rings of carbon atoms and it is the addition of various chemical groups to various locations on the rings that differentiates between individual steroids. In the common vernacular, the word "steroid" has come to mean "Anabolic Testosterone-derived Steroid". In medicine how ever, the term steroid is properly used to describe the category of hormones as a whole. For instance, testosterone is just one kind of steroid and differs significantly in it's function and use in medicine from Cortisol. Amines are structurally the simplest hormones. Modifying single Amino Acids forms these hormones. 8 Hormones Can Circulate Either Freely or Bound to Carrier Proteins Solubility in plasma determines the way hormones are carried in the blood  Peptide/protein hormones and catecholamines dissolve readily in plasma Thus, they are water soluble They do not require binding proteins for transport  Steroid and thyroid hormones are insoluble in plasma 1. 2. 3. 4. Thus, they are lipid soluble They require binding proteins for transport Facts about hormones bound to carrier proteins 1-10% of total hormone exists free in solution Only hormone that is free in solution is biologically active Large amount of bound hormones provides a reservoir Protein binding greatly slows the rate of clearance of hormones from plasma The relatively large amounts of hormones bound to proteins serve as reservoirs, replenishing the concentration of free hormones when they are bound to target receptors or lost from the circulation. 9 What do you need to know about hormones for the exam and for Level 1 Which hormones are derived from tyrosine vs cholesterol vs amino acids? Which hormones bind intracellular receptors? Which hormones bind extracellular receptors? Which have second messengers and what are they? Which hormones are water soluble? Which hormones insoluble in water and require a transport protein to circulate in the blood? 10 Characteristic PEPTIDES Solubility Water Soluble Biosynthesis Preprohormone, Prohormone, Hormone Storage Binding Proteins Half-Life Receptor Location Peptide and Protein Hormones: (e.g. TRH and Insulin) Synthesis: DNA - RNA - Preprohormone - prohormone – hormone Storage: Secretory Granules Secretion: Stimulus followed by exocytotic process 11 Characteristic PEPTIDES Solubility Water Soluble Biosynthesis Preprohormone, Prohormone, Hormone Storage Substantial Binding Proteins Rare Half-Life Short (minutes) Receptor Location Cell Membrane Exocytosis Peptide and Protein Hormones: (e.g. TRH and Insulin) Synthesis: DNA - RNA - Preprohormone - prohormone – hormone Storage: Secretory Granules Secretion: Stimulus followed by exocytotic process 12 Characteristic CATECHOLAMINES Solubility Water Soluble Biosynthesis Multiple Enzymes Catecholamine Synthesis Storage Binding Proteins Half-Life Receptor Location Amine Hormones: (e.g. Catecholamines) Synthesis: Occurs through a series of enzymatic processes Storage and Secretion: Same as for peptides and proteins 13 Characteristic CATECHOLAMINES Solubility Water Soluble Biosynthesis Multiple Enzymes Storage Substantial Binding Proteins Rare Half-Life Very Short (seconds) Receptor Location Cell Membrane Norepinephrine Removal Amine Hormones: (e.g. Catecholamines) Synthesis: Occurs through a series of enzymatic processes Storage and Secretion: Same as for peptides and proteins 14 Characteristic THYROID HORMONES Solubility Lipid Soluble Biosynthesis Multiple Enzymes Storage Substantial Binding Proteins Yes Half-Life Very Long (days) Receptor Location Nucleus Thyroid Hormone Synthesis & Secretion Thyroid Hormones: (e.g. Thyroxine and Triiodothyronine) Synthesis: The amino acid Tyrosine and Iodine are linked on a protein; thyroglobulin Storage: The protein incorporating the thyroid hormones is stored in follicles surrounded by endocrine cells. In addition, bound thyroid hormones in plasma serve as a reservoir. Secretion: Retrieval from follicle and enzymatic release from its protein storage form. 15 Characteristic STEROIDS Solubility Lipid Soluble Biosynthesis Multiple Enzymes Storage Origins of Cholesterol Used to Synthesize Steroid Hormones Binding Proteins Half-Life Receptor Location Steroid Hormones: (e.g. glucocorticoids, estrogens, androgens eicosanoids, vitamin D3) Synthesis: Cholesterol - series of enzymatic reactions leading to the steroid of interest. Storage: No appreciable storage - on demand synthesis of steroid from cholesterol, or bound steroid in plasma. Secretion: Since there is no storage, an increase in synthesis is in effect equal to an increase in release. Approximately 80% is taken up as LDL particles via receptor-mediated endocytosis. The cell synthesizes the remaining cholesterol de novo from acetyl coenzyme A (Acetyl CoA). LDL, low-density lipoprotein; VLDL, very-low-density lipoprotein. Steroid family hormones are small molecules synthesized from cholesterol in a series of reactions in both cytosol and microsomes. Low solubility in aqueous blood means that circulating hormone is generally coupled with a more soluble binding protein (note that the chemical binding equilibrium means that there is always some circulating free as well, and it is in fact this “free’ fraction that is active), but high solubility in lipid environment of cell and nuclear membranes means that they easily diffuse through membranes, although “facilitated diffusion” by interactions with cell membrane proteins has been reported for some. Because of this easy hormone access to the interior of the cell, steroid hormone family receptors are intracellular. Similarly, delivery of steroid hormones into the blood is primarily through simple diffusion out of the cells where they are produced, and this easy 16 diffusibility means that steroid hormones are generally not stored, but are rather released into circulation as they are made. Since these hormones circulate bound to large proteins, they cannot be freely filtered by the kidney glomerulus; generally before excretion, additional modifications (e.g., sulfation, glucuronidation) occur in the liver and kidney which tend to both inactivate the hormones and make them more soluble without the necessity of large binding proteins. In this form, the small soluble modified hormone can be eliminated in urine. Most (but not all) effects of these hormones involve hormone-receptor complexes binding to DNA hormone response elements (HREs). This binding causes increases or decreases of transcription of the particular genes containing the specific HREs, thus causing changes in expression of particular proteins. Both this time lag (synthesis of mRNA, synthesis of protein necessary) and the fact that the steroid hormones themselves have to be synthesized de novo (there is no stored supply of steroid hormones), means that full effects from stimulation of steroid hormone family members may take many hours to occur. 16 Characteristic STEROIDS Solubility Lipid Soluble Biosynthesis Multiple Enzymes Storage Minimal Binding Proteins Yes Half-Life Long (hours) Receptor Location Intracellular (Cytoplasm or Nucleus) Steroid Hormones: (e.g. glucocorticoids, estrogens, androgens eicosanoids, vitamin D3) Synthesis: Cholesterol - series of enzymatic reactions leading to the steroid of interest. Storage: No appreciable storage - on demand synthesis of steroid from cholesterol, or bound steroid in plasma. Secretion: Since there is no storage, an increase in synthesis is in effect equal to an increase in release. 17 Summary of Characteristics of the Principal Classes of Hormones 18 Hormone Action of Water Soluble Hormones 1. Hormone Binds to Receptor 2. Signal Transduction 3. Alterations in intracellular processes 19 Peptide/Protein Hormones: Mechanism of ActionAdenylyl Cyclase When as binds to adenylate cyclase (AC), ATP is converted to cAMP cAMP is a second messenger cAMP molecules bind protein kinase A (PKA) PKA then phosphorylates downstream enzymes ACTH LH FSH TSH ADH (V2 receptor) HCG MSH CRH Calcitonin PTH Glucagon This leads to the physiologic actions Activity of AC o Gai inhibits AC o Gas stimulates AC Downstream effects of adenylyl cyclase activation/inhibition. When a ligand binds to a receptor coupled to as, adenylyl cyclase is activated, whereas when a ligand binds to a receptor coupled to ai, the enzyme is inhibited. The activated enzyme converts ATP to cAMP, which then can activate protein kinase A. The second messenger is cAMP. Since AC is an enzyme it can convert many ATP to cAMP. cAMP can bind and activate PKA and PKA can activate (usually phosphorylate) many other proteins. Note, a single hormone molecule binding to its receptor can cause phosphorylation to many proteins downstream amplifying signal. This can happen pretty quick. 20 Peptide/Protein Hormones : Mechanisms of ActionPhospholipase C Hormone binds GTP (Gq) coupled receptor PLC converts PIP2 to IP3 and DAG from membrane lipids DAG and IP3 are second messengers IP3 mobilizes Ca2+ from the endoplasmic reticulum Together, Ca2+ and DAG activate protein kinase C (PKC) PKC phosphorylates proteins This leads to physiologic actions GnRH TRH Angiotensin II ADH (V1 receptor) Oxytocin 21 Peptide/Protein Hormones: Mechanisms of ActionTyrosine Kinase Receptors Insulin IGF-1 Growth hormone Prolactin Figures 9-6 from Costanzo’s Physiology, 6th Ed. All tyrosine kinase and tyrosine kinase associated receptors cause some sort of autophosphorylation inside the cell after the ligand binds. This may also cause receptor dimerization or receptor may already exist as a dimer. The main point here is there is no adenylate cyclase or PLC that is the second messenger. Also most TK’s will’s will be some sort of growth factor mediated mechanism-in Endocrinology-e.g insulin, IGF’s, leptin and GH (also note that GH acts on jak/stat). Receptor tyrosine kinases have an extracellular binding domain that binds the hormone or ligand, a hydrophobic transmembrane domain, and an intracellular domain that contains tyrosine kinase activity. When activated by hormone or ligand, the intrinsic tyrosine kinase phosphorylates itself and other proteins. One type of receptor tyrosine kinase is a monomer (e.g., nerve growth factor [NGF] and epidermal growth factor receptors, see Fig. 9.6A). In this monomeric type, binding of ligand to the extracellular domain results in dimerization of the receptor, activation of intrinsic tyrosine kinase, and phosphorylation of tyrosine moieties on itself and other proteins, leading to its physiologic actions. Another type of receptor tyrosine kinase is already a dimer (e.g., insulin and insulin-like growth factor [IGF] receptors, see Fig. 9.6B). In this dimeric type, binding of the ligand (e.g., insulin) activates intrinsic tyrosine kinase and leads to phosphorylation of itself and other proteins and ultimately the hormone’s physiologic actions. The mechanism of the insulin 22 receptor is also discussed later in the chapter. Tyrosine kinase–associated receptors (e.g., growth hormone receptors, see Fig. 9.6C) also have an extracellular domain, a hydrophobic transmembrane domain, and an intracellular domain. However, unlike the receptor tyrosine kinases, the intracellular domain does not have tyrosine kinase activity but is noncovalently “associated” with tyrosine kinase such as those in the Janus kinase family (JAK, Janus family of receptor-associated tyrosine kinase, or “just another kinase”). Hormone binds to the extracellular domain, leading to receptor dimerization and activation of tyrosine kinase in the associated protein (e.g., JAK). The associated tyrosine kinase phosphorylates tyrosine moieties on itself, the hormone receptor, and other proteins. Downstream targets of JAK include members of the STAT (signal transducers and activators of transcription) family, which cause transcription of mRNAs and ultimately new proteins involved in the hormone’s physiologic actions. 22 Peptide/Protein Hormones : Mechanism of ActionCalcium-Calmodulin (CM) Ca2+-CM mediates smooth muscle contraction Smooth Muscle Cell Hormone binds to a receptor coupled G-protein: 1) Opens cell membrane Ca2+ channels and it releases Ca2+ from the endoplasmic reticulum. 2) These two actions produce an increase in intracellular [Ca2+] 3) Ca2+ binds to calmodulin and the Ca2+-calmodulin complex produces physiologic actions. 23 Amine Hormones: Mechanism of Action  Epinephrine and norepinephrine act on adrenergic receptors (a1, a2, 1, 2)  Dopamine works on dopaminergic receptors (DA-2) Figure 47-5 from Boron and Boulpaep’s Medical Physiology, 2nd Ed. Beta adrenergic receptors: Beta 1 receptors are prominent on the heart Beat 2 receptors are in vascular smooth muscle of skeletal muscle, walls of GI tract and bladder and in the bronchioles Alpha adrenergic receptors: Alpha 1 are in vascular smooth muscle of skin, skeletal muscle and splanchnic regions, in sphincters of GI tract and bladder and in the radial muscle of the iris Alpha 2 are inhibitory and are located both presynaptically and postsynaptically 24 Hormone Action of Steroid and Thyroid Hormones (Lipid Soluble) 1. Hormone penetrates plasma membrane 2. Binds to cytoplasmic or nuclear receptor 3. Alterations in gene regulation 25 Steroid and Thyroid Hormones: Mechanism of Action Glucocorticoids Aldosterone 1,25-Dihydroxycholecaldiferol Thyroid hormones Estrogen Progesterone Testosterone Figures 9-7, 8 from Costanzo’s Physiology, 5th Ed. Steroid hormones and thyroid hormones have the same mechanism of action. In contrast to the adenylyl cyclase and phospholipase C mechanisms utilized by peptide hormones and involving cell membrane receptors and generation of intracellular second messengers, the steroid hormone mechanism involves binding to cytosolic (or nuclear) receptors that initiate DNA transcription and synthesis of new proteins. In further contrast to peptide hormones, which act quickly on their target cells (within minutes), steroid hormones act slowly (taking hours). 1. The steroid hormone diffuses across the cell membrane and enters its target cell (Step 1), where it binds to a specific receptor protein (Step 2) that is located in either the cytosol or nucleus. Steroid hormone receptors are monomeric phosphoproteins that are part of a gene superfamily of intracellular receptors. Each receptor has six domains. The steroid hormone binds in the E domain located near the C terminus. The central C domain is highly conserved among different steroid hormone receptors, has two zinc fingers, and is responsible for DNA binding. With hormone bound, the receptor undergoes a conformational change and the activated hormone-receptor complex enters the nucleus of the target cell. 2. The hormone-receptor complex dimerizes and binds (at its C domain) via the zinc fingers to specific DNA sequences, called steroidresponsive elements (SREs) located in the 5′ region of target genes (Step 3). 3. The hormone-receptor complex has now become a transcription factor that regulates the rate of transcription of that gene (Step 4). New messenger RNA (mRNA) is transcribed (Step 5), leaves the nucleus (Step 6), and is translated to new proteins (Step 7) that have specific physiologic actions (Step 8). The nature of the new proteins is specific to the hormone and accounts for the specificity of the hormone’s actions. For example, 1,25-dihydroxycholecalciferol induces the synthesis of a Ca2+-binding protein that promotes Ca2+ absorption from the intestine; aldosterone induces synthesis of Na+ channels (ENaC) in the renal principal cells that promote Na+ reabsorption in the kidney; and testosterone induces synthesis of skeletal muscle proteins. 26 Mechanisms of Hormone Action 27 Negative Feedback vs. Positive Feedback Solid lines and plus (+) signs indicate stimulation; dashed lines and minus (-) signs indicate inhibition. Regulation of Hormone Secretion (Costanzo – pages 399-401) Hormone secretion can be regulated by neural mechanisms, humoral mechanism and feedback mechanisms. An example of a neural mechanism is when preganglionic fibers stimulate the adrenal medulla to enhance secretion of the catecholamines. An example of a humoral mechanism is when glucose levels in the blood become too high, β cells of the pancreatic islets release insulin in order to lower the plasma glucose. Feedback mechanisms are more prevalent than neural mechanisms and can be either negative or positive. Usually, these hormonal feedback systems involve three tiers of the neurendocrine system; the hypothalamus, the pituitary and the target endocrine gland. The hypothalamus produces neurocrines which are released from hypothalamic peptidergic neurons to stimulate (if they are releasing neurocrines) or inhibit (if they are inhibiting neurocrines) hormones of the anterior pituitary. In turn, the hormones of the anterior pituitary stimulate the release of hormones from the target endocrine gland. The hormones of the target endocrine glands then produce a physiological response at the target tissues. The type feedback involved depends on if the hormones of the target 28 endocrine gland or the anterior pituitary inhibit (negative feedback) or stimulate (positive feedback) the hormones in the up tiers (i.e. the pituitary of hypothalamus). With negative feedback the physiological response of a hormone is reduced. With positive feedback, the physiological response of a hormone is reinforced. Positive feedback in comparison to negative feedback is rare and usually produce explosive events, such as the LH surge induced by increasingly higher levels of estradiol. In negative feedback systems there are three types of inhibitory loops. Long-loop negative feedback is when hormones of the target endocrine gland inhibit the release of hormones from the anterior pituitary or of neurocrines from the hypothalamus. Short-loop negative feedback is when hormones of the anterior pituitary inhibit the release neurocrines from the hypothalamus. Ultrashort (or short-short) negative feedback is when neurocrines from the hypothalamus inhibit their own release. 28 Dose-Response Curves are an Index of Hormone Activity % Hormone Effect Dose-Response Curves 100 50 Threshold 0 Sensitivity (Half-maximal concentration) HormoneConcentration Concentration Hormone Decreased Responsiveness Decreased Sensitivity % Hormone Effect Responsiveness – amount of hormone bound to receptor Maximal Response Sensitivity – affinity of hormone for receptor Hormone Concentration Regulation of Hormone Receptors (Costanzo – page 401) Action of hormones at the target tissues is not the only means by which physiological responses at the target cells can be regulated. Hormone receptors must also be present on the target cells in order for a physiological response to occur. The physiological response at the target cells depends on the binding of a hormone to its receptor and a dose-response curve can be used as an index to hormonal activity. The responsiveness of a hormone refers to the amount of hormone bound to the receptor. The plateau portion of a doseresponse curve indicates saturation of the receptor by the hormone and hence maximum responsiveness. A downward shift in the dose-response curve indicates a decrease in responsiveness of the hormone. The sensitivity of a hormone refers to the affinity of a hormone for its receptor. A shift to the right in a dose-response curve indicates a decrease in the hormone’s sensitivity for its receptor. 29 Why would Responsiveness Decrease? Hormone + Receptor -----> Hormone/Receptor 1. Decrease in number of target cells 2. Decrease in the total number of receptors 3. Decreased concentration of precursors or enzyme activated by hormone 4. Increase in noncompetitive binding 30 Why would Sensitivity Decrease? Hormone + Receptor -----> Hormone/Receptor 1. Decrease in the affinity of the receptors for hormone 2. Alterations in cofactors 3. Increased rate of hormone degradation 4. Increase in competitive inhibition 31 Receptors: UP- and Down-Regulation When a hormone increases the total number of receptors it can bind to or increases the sensitivity of the receptor it binds to, this is called upregulation. When a hormone decreases the total number of receptors it can bind to or decreases the sensitivity of the receptor it binds to, this is called down-regulation. A change in the number of receptors or the affinity of the receptors may be regulated hormonally in processes referred to as up- or down-regulation. When a hormone increases the total number of receptors it can bind to or increases the sensitivity of the receptor it binds to, this is called up-regulation. When a hormone decreases the total number of receptors it can bind to or decreases the sensitivity of the receptor it binds to, this is called down-regulation. Hormones can even up- or down-regulate the number and sensitivity of receptors for other hormones. 32 Question The half-life of a lipid –soluble hormone in the blood is: A. directly proportional to its rate of secretion B. directly proportional to the affinity of the hormone for its plasma protein carrier C. directly proportional to the number of hormone receptors present in the target tissue D. inversely proportional to the concentration of the hormone’s plasma protein carrier E. inversely proportional to the molecular weight of the hormone Answer: B 33