9.3 Endocrine System & Hormone Action

Endocrine vs. Exocrine Glandular Secretion

Intercellular communication relies on chemical signaling mechanisms categorized by how secretor cells release their products and how those signals reach target cells:

1. Endocrine Secretion: Endocrine glands are ductless. They synthesize and release chemical signals called hormones directly into interstitial fluid, which rapidly diffuse into adjacent capillary beds for systemic distribution through the bloodstream. Endocrine hormones act specifically on target cells that express high-affinity receptors for that hormone.
2. Exocrine Secretion: Exocrine glands possess specialized ducts lined with epithelial cells. They secrete non-hormonal substances (e.g., digestive enzymes, sweat, sebum, mucus, saliva, bicarbonate) directly onto an external or internal epithelial surface (e.g., skin, lumen of gastrointestinal tract).
3. Paracrine & Autocrine Secretion: Localized non-systemic signaling. Paracrine signals diffuse through interstitial fluid to act on neighboring adjacent cells of a different type (e.g., pancreatic delta cell somatostatin inhibiting alpha/beta cells). Autocrine signals bind to receptors on the exact cell that secreted them (e.g., T-lymphocyte interleukin-2 self-stimulation).

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Chemical Classification of Hormones

Hormones fall into three primary structural classes, which dictate their solubility, transport in blood, receptor location, and cellular mechanism of action:

1. Peptide Hormones: Synthesis & Second Messenger Signaling

Peptide hormones are synthesized on ribosomes as inactive preprohormones, cleaved in the endoplasmic reticulum to prohormones, and packaged in the Golgi apparatus into secretory vesicles where final cleavage yields active peptides. Because they cannot cross the hydrophobic lipid bilayer, peptide hormones bind extracellular cell-surface receptors:

• cAMP / Protein Kinase A Pathway ($G_s / G_i$ GPCRs): Hormone binding to a $G_s$-coupled receptor stimulates $G_{\alpha s}$ to exchange GDP for GTP, activating Adenylate Cyclase. Adenylate cyclase converts ATP into cyclic AMP (cAMP). cAMP binds to regulatory subunits of Protein Kinase A (PKA), releasing catalytic subunits that phosphorylate specific target enzymes. Phosphodiesterases (PDEs) degrade cAMP to terminate signal.
• $\text{IP}_3$ / DAG / $\text{Ca}^{2+}$ Pathway ($G_q$ GPCRs): Hormone binding to a $G_q$-coupled receptor activates Phospholipase C (PLC), which cleaves membrane $\text{PIP}_2$ into Inositol 1,4,5-trisphosphate ($\text{IP}_3$) and Diacylglycerol (DAG). $\text{IP}_3$ diffuses to the endoplasmic reticulum and opens ligand-gated $\text{Ca}^{2+}$ channels, releasing $\text{Ca}^{2+}$ into the cytosol. Cytosolic $\text{Ca}^{2+}$ and DAG synergistically activate Protein Kinase C (PKC).

2. Steroid Hormones: Transcriptional Regulation

Steroid hormones are synthesized on demand from cholesterol in the smooth ER and mitochondria. Because they are lipophilic, they diffuse freely out of secretor cells and across target cell plasma membranes. In blood, steroid hormones are bound to specific transport proteins (e.g., Corticosteroid-Binding Globulin, Sex Hormone-Binding Globulin). Only the free (unbound) fraction can cross membranes. Upon entering target cells, steroids bind intracellular receptors. The hormone-receptor complex dimerizes, translocates into the nucleus, and binds to Hormone Response Elements (HREs) on DNA, altering transcription rates of specific mRNA transcripts.

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The Hypothalamic-Pituitary Axes

The hypothalamus serves as the master neuroendocrine integrator, converting neural inputs from the brain into hormonal outputs that regulate the pituitary gland (hypophysis).

                    HYPOTHALAMUS
                         |
         +---------------+--------------+
         |                              |
   Hypophyseal Portal             Hypothalamo-Hypophyseal
   Vascular System                Axonal Tract
         |                              |
         v                              v
 ANTERIOR PITUITARY             POSTERIOR PITUITARY
 (Adenohypophysis)              (Neurohypophysis)
         |
  +------+------+               (Stores & Secretes ADH & Oxytocin)
  |             |
FLAT           PEG
(Tropic)      (Direct)

1. Anterior Pituitary (Adenohypophysis) & Hypophyseal Portal System

The anterior pituitary is derived from oral ectoderm (Rathke's pouch) and is controlled by hypothalamic releasing and inhibiting hormones. These hypothalamic peptides are secreted into the Hypophyseal Portal System—a specialized capillary network consisting of a primary capillary plexus in the median eminence, hypophyseal portal veins, and a secondary capillary plexus in the anterior pituitary. This allows hypothalamic hormones to reach anterior pituitary cells directly at high concentrations without systemic dilution.

Anterior pituitary hormones are remembered using the classic mnemonic FLAT PEG:

Tropic Hormones (FLAT) — Act on other endocrine glands:

• FSH (Follicle-Stimulating Hormone): Stimulated by hypothalamic GnRH. Target: Gonads (promotes ovarian follicle development in females; stimulates Sertoli cells and spermatogenesis in males).
• LH (Luteinizing Hormone): Stimulated by hypothalamic GnRH. Target: Gonads (triggers ovulation and corpus luteum formation in females; stimulates Leydig cell testosterone production in males).
• ACTH (Adrenocorticotropic Hormone): Stimulated by hypothalamic CRH. Target: Adrenal cortex (stimulates synthesis and release of glucocorticoids, primarily cortisol).
• TSH (Thyroid-Stimulating Hormone): Stimulated by hypothalamic TRH. Target: Thyroid follicular cells (stimulates thyroid hormone $T_3$ and $T_4$ synthesis and release).

Direct Hormones (PEG) — Act directly on target non-endocrine tissues:

• Prolactin: Inhibited by hypothalamic Dopamine (Prolactin-Inhibiting Hormone / PIH). Target: Mammary glands (stimulates milk production). Suckling suppresses dopamine release, disinhibiting prolactin.
• Endorphins: Target: CNS opioid receptors (inhibit pain perception, produce euphoria).
• GH (Growth Hormone / Somatotropin): Stimulated by GHRH, inhibited by Somatostatin. Target: Muscle, bone, liver. Stimulates hepatic IGF-1 release, bone growth, protein synthesis, lipolysis, and decreases glucose uptake (diabetogenic effect).

2. Posterior Pituitary (Neurohypophysis)

The posterior pituitary is a direct neural extension of the hypothalamus. It does NOT synthesize any hormones. Instead, it consists of unmyelinated axon terminals of magnocellular neurons whose cell bodies reside in the supraoptic and paraventricular nuclei of the hypothalamus.

Hormones are synthesized in hypothalamic soma, packaged into vesicles, transported down the hypothalamo-hypophyseal tract, and stored in posterior pituitary terminals until nerve impulses trigger release into systemic capillaries:

1. ADH (Antidiuretic Hormone / Vasopressin): Synthesized primarily in the supraoptic nucleus. Secreted in response to elevated blood osmolarity (detected by hypothalamic osmoreceptors) or low blood volume/pressure. Acts on principal cells of the renal collecting duct to insert Aquaporin-2 channels, increasing water reabsorption and concentrating urine.
2. Oxytocin: Synthesized primarily in the paraventricular nucleus. Secreted during labor (cervical stretch) and lactation (nipple stimulation). Stimulates uterine smooth muscle contraction during labor (via a positive feedback loop) and mammary myoepithelial cell contraction for milk ejection ("let-down" reflex).