Determinants of Hormone Action

Transport of Hormones

  • Two chemical classes dominate reproductive endocrinology; their polarity dictates transport strategy:

    • Protein /peptide hormones

    • Hydrophilic → readily dissolve in aqueous plasma.

    • Circulate largely “free” (unbound) → immediately available to interact with surface receptors.

    • Steroid hormones

    • Lipophilic & water-insoluble → require carrier (transport) proteins for solubility.

    • Binding = “conjugation.” Multiple proteins display overlapping but unequal affinities.

  • Key plasma carrier proteins & their average binding shares (human data representative of table):

    • Albumin (ALB) – high capacity, low affinity.

    • Sex Hormone–Binding Globulin (SHBG) – lower capacity, high affinity for androgens/estrogens.

    • Cortisol-Binding Globulin (CBG / transcortin) – high affinity for glucocorticoids & progestogens.

    • Typical distribution (% bound):

  • Biological activity derives almost exclusively from the tiny “free fraction,” because only unbound steroid can diffuse across membranes to nuclear receptors.

  • Clinical / pathophysiological note:

    • ↓SHBG → ↑free testosterone → androgen excess signs

    • ↑SHBG → ↓free testosterone → diminished androgen action.

Blood Concentration & Clearance

  • Instantaneous plasma concentration of a hormone results from a balance:
    [H]blood=Secretion (production) rateClearance rate[H]_{blood} = \frac{\text{Secretion (production) rate}}{\text{Clearance rate}}

  • Clearance routes & kinetics

    • Hepatic metabolism → conjugation/inactivation; metabolites excreted by kidney (urine).

    • Pulmonary “blow-off” into expired air for some volatile or small molecules.

    • Target-cell utilization – quantitatively minor (only a fraction of free hormones are utilized).

  • Half-life ( t1/2t_{1/2} ) = time required for plasma concentration to fall to 12\tfrac12 starting value.

    • Steroid hormones t1/2≈2–3 mint_{1/2}\approx2\text{–}3\ \text{min}.

    • Prostaglandins t1/2≈3–10 mint_{1/2}\approx3\text{–}10\ \text{min}.

    • Gonadotrophins (LH, FSH) t1/2≈1–3 ht_{1/2}\approx1\text{–}3\ \text{h}. - longer half life

  • Functional consequence: rapid signal termination for steroids/prostaglandins vs prolonged action of glycoprotein hormones.

Metabolic Activation at Target Tissues

  • Enzymatic conversion of circulating hormone by target cell into more usable form.

  • Classic example: Testosterone → 5α5\alpha-Dihydrotestosterone (DHT)

    • Enzyme: 5α5\alpha-reductase within target cells.

    • DHT affinity for androgen receptor ≈ 10× that of testosterone → amplified response.

    • Developmental relevance: masculinization of external genitalia (penis, scrotum) during embryogenesis depends on local DHT production.

Secretory Patterns

  • Reproductive hormones are rarely secreted as smooth, steady outputs.

  • Three temporal motifs influence biological effectiveness:

    1. Pulsatility

    • Discrete peaks separated by troughs.

    • Each peak - Pulse

    • Defined by amplitude & frequency.

    • Example: Testosterone pulses across 24 h; each peak lasts minutes.

    1. Circadian rhythm

    • Superimposed daily oscillation; e.g.
      Testicular testosterone gradually rises during night–early morning.

  • GnRH–LH paradigm (gold-standard for pulse-dependency)

    • GnRH released from hypothalamic neurons into hypophyseal portal blood only in pulses.

    • Every GnRH pulse triggers an LH pulse from the anterior pituitary; frequency helps set relative LH vs FSH output.

    • Exogenous pulsatile GnRH → maintains LH/FSH.

    • Clinical correlation: Excessive GnRH pulse frequency → favors LH > FSH → ovarian androgen excess → Polycystic Ovary Syndrome (PCOS).

A pulsatile secretion of GnRH is need for secretion of gonadotrophin.

If its continues secretion - LH and FSH drops.

Receptor Expression & Regulation

  • Hormone can act only where cognate receptors are expressed.

  • Receptor density & sensitivity are determinants.

Down-Regulation

  • Chronic high ligand exposure → degradation of receptors or decreased synthesis.

  • Consequences

    • ↓Receptor number

    • ↓Cell sensitivity

    • ↓Physiological response

  • Mechanism behind suppressed LH/FSH during continuous GnRH infusion.

Up-Regulation

  • Opposite process; another hormone or paracrine factor increases receptor synthesis or membrane trafficking.

  • First - low receptor density and weak response.

  • This causes increase in sensitivity of receptors

  • Relevance: Dominant (pre-ovulatory) follicle up-regulates LH receptors in granulosa cells → heightened responsiveness → ovulation trigger.

Agonists & Antagonists

  • Agonist = exogenous molecule that binds the receptor - promote the receptor.

  • Antagonist = binds receptor but blocks activation (competitive or inverse).

  • Therapeutic / experimental toolset (selected examples):

    • Progesterone analogs

    • Agonists: synthetic progestins (contraceptives, luteal support).

    • Antagonist: RU-486 (mifepristone) – emergency contraception, medical abortion.

    • Estrogen system

    • Agonists: ethinylestradiol (oral contraceptive component).

    • Antagonists / SERMs: Tamoxifen – blocks estradiol in breast tissue (ER-positive cancer therapy).

    • Androgen system

    • Agonists: anabolic steroids (therapeutic for cachexia, abused in sport).

Environmental Endocrine Disruptors

  • Industrial chemicals with estrogenic or anti-estrogenic activity:

    • Pesticide residues, plasticizers (e.g.
      Bisphenol-A – BPA), phytoestrogens, pharmaceutical metabolites in wastewater.

  • Potential impacts: altered sexual differentiation, puberty timing, fertility, hormone-dependent cancers.

  • exogenous antagonist) can derail reproductive function and underlies numerous clinical disorders (PCOS, infertility, hormone-dependent cancers).