ENDOCRINOLOGY COORDINATION OF PHYSIOLOGICAL PROCESSES: -In a living organism there must be coordination of number of physiological activities taking place simultaneously such as: movement, respiration, circulation, digestion, excretion and metabolism. -The central nervous system and the endocrine system represent the two major means by which these functions are coordinated. LONG DISTANCE COMMUNICATION: -Communication between cells that are not in contact is achieved through a number of chemical substances, which are secreted by releasing cells and interact with specific receptors on distant target cells. -Signaling through these receptors leads to a specific physiological effect.
1. ENDOCRINE SIGNALING: Involves hormone secretion into the blood by an endocrine gland. The hormone is transported by the blood to a distant target site. Figure 1.1.
NEUROENDOCRINE SIGNALING Figure 1.2.
Paracrine Signaling Autocrine Signaling Figure 1.3 Figure 1.4
Communication by hormones (or neurohormones) can involve six steps. (1) Synthesis of the hormone by endocrine cells (or neurons in case of neurohormone). (2) Release of the hormone by the endocrine cells (or the neurohormones by the neurons) (3) Transport of the hormone or neurohormone to the target site by the blood stream. (4) Detection of the hormone or neurohormone by a specific receptor protein on the target cells. (5) A change in cellular metabolism triggered by the hormonereceptor interactions (6) Removal of the hormone, which often terminates the cellular response
Classical Endocrine Organs Figure 1.5.
Hypothalamic-Pituitary Signaling. -via blood vessels of the pituitary stalk. -Hypothalamic-Hypophyseal Portal System -from the hypothalamus to the the adenohypophysis (anterior pituitary). -hypothalamic neurohormones either activate or inhibit activity of one of the six types of hormone-producing cells in the anterior pituitary. Figure 1.6 -called either releasing hormones (releasing factors) or inhibiting hormones (inhibiting factors).
TABLE 2: CLASSES OF HORMONES BASED ON THEIR STRUCTURE PEPTIDES AND PROTEINS GLYCOPROTEINS POLYPEPTIDES STEROIDS AMINES - Follicle Stimulating - Adrenocorticotropin - Aldosterone -Epinephrine Hormone (FSH) Hormone (ACTH) - Cortisol -Thyroxine (T4) -Luteinizing - Growth Hormone (GH) - Estradiol -Triiodothyronine (T3) Hormone (LH) - Prolactin - Progesterone -Melatonin - Thyroid Stimulating - B-Lipotropin (B-LPH) - Testosterone Hormone (TSH) - B-Endorphin - Vitamin D - Human Chorionic - Insulin Gonadotropin (HCG) - Glucagon - Insulin-like growth factors (IGFs) or (Somatomedins) - Parathyroid Hormone (PTH) -Calcitonin -Oxytocin - Vasopressin - Angiotensin (ADH) - Relaxin - Somatostatin - Corticotropin Releasing Hormone (CRH) - Cholecystokinins -and others
SYNTHESIS OF PROTEIN HORMONES Figure 1.7
Structures of some steroid hormones Figure 1.8
STRUCTURES OF THYROID HORMONES Figure 1.9
lock and key mechanism for a hypothetical membrane receptor Figure 1.10
Properties of Hormone Receptors. (a) SPECIFICITY: recognition of single hormone or hormone family. (b) AFFINITY: High affinity for the hormone; i.e. binding hormone at its physiological concentration. (c) SATURABILITY: Should show saturability; i.e. a finite number of receptors. (d) MEASUREABLE BIOLOGICAL EFFECT: A measurable biological response due to interaction of hormone with its receptor. Receptor Regulation. Receptors can be upregulated either by increasing their activity in response to hormone or their synthesis. Receptors can be downregulated either by decreasing their activity or their synthesis
3 mechanisms by which a hormone can exert effects on target cells: (1) Direct effects on function at the cell membrane. (2) Intracellular effects mediated by second messenger systems. (3) Intracellular effects mediated by genomic or nuclear action. (1) Direct effect. Figure 1.11
(2) Signaling via an intracellular second messenger. Figure 1.12
(3) Intracellular genomic signaling. Figure 1.13