Regulating Plasma Hormone Levels

1. Regulating Plasma Hormone Levels Factors Involved: Secretion versus Removal Regulation of Secretion Metabolic Clearance Rate & Half-life Role of Carrier Proteins Role of Glycosylation Notes on the First Midterm

2. GlandRegulatory HormoneGlandBlood HormoneBindingTransportationActive MetaboliteTarget CellReceptor BindingSignal TransductionActivationResponse OR (-)

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

4. 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 hormone This 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.

5. 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.

6. 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

7. 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.

8. 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.

9. 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.

10. 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

11. Inputs to endocrine cells

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

13. Chronotropic control • Endogenous neuronal rhythmicity • Diurnal rhythms, circadian rhythms (growth hormone and cortisol), Sleep-wake cycle; seasonal rhythm

14. 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

15. 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.

16. Circadian (chronotropic) control

17. Circadian Clock

18. 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

19. 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

20. 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)

21. 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.

22. stress CNS sympathetic nervous system adrenal medulla norepinephrine release Preganglionic fibers 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.

23. osmoreceptors (supraoptic nucleus of the hypothalamus) posterior pituitary vasopressin Neural Regulation of Hormone Release • Another Example: Release of vasopressin from the posterior pituitary

24. 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.

25. 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

26. Feedback Control of Endocrine Secretion

27. Feedback control of Endocrine Secretion

28. 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.

29. decreased [Na+], increased [K+] Humoral Control of Secretion • Another example: aldosterone secretion zona glomerulosa aldosterone

30. 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.

31. 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.

32. Receptor 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. constitutive release regulated release

33. 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.

34. cyclic AMP protein kinase A phosphorylation of enzymes 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

35. Aplysia californica bag cells depolarizing current (+) (-) PKA inhibitor Role of Protein Kinase A in Stimulated Secretion of ELH ELH causes egg-laying behavior in whole animals

36. Calcium release R synthesis LHb mRNA Protein Kinase C 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

37. 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

38. Endocrine Gland Hormone Target Cell Action 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.

39. 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.

40. 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.

41. 40 20 0 Hormone Concentration T1/2 0 10 20 30 40 Time (minutes) 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).

42. 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!).

43. 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

44. 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

45. H receptor H H lysozome Ligand-Induced Internalization and Degradation of Receptors and Hormones

46. 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.

47. amino acid 1 amino acid 2 amino acid 3 amino acid 4 NH2 COOH What Happens to Hormones During Clearance? • 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 endopeptidase exopeptidase exopeptidase

48. 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.

49. 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

50. 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.