Endocrinology: Growth Hormone

Endocrinology: Growth Hormone

Overview of Growth Hormone Synthesis

  • Pre-mRNA (Precursor Messenger RNA): An initial transcript that includes both introns and exons.

  • Exons: Coding regions of the gene that are retained in the final mRNA.

  • Introns: Non-coding regions that are removed during RNA processing.

  • Growth Hormones:

    • Pre-22.5-kDa Growth Hormone: Initial precursor form produced in the nucleus.

    • Pre-20-kDa Growth Hormone: Processed form that refers to the final product which is smaller in size.

    • Pro-hormone: Intermediate form before becoming active hormone (often larger).

    • 22-kDa Growth Hormone: Active form consisting of 191 amino acids, secreted from the gland.

    • 20-kDa Growth Hormone: Smaller form with 176 amino acids, typically involved in physiological functions.

Hormone Regulation

  • Factors Influencing Growth Hormone Secretion:

    • Stress: Stress can increase growth hormone levels as part of the body's response.

    • Exercise: Physical activity stimulates GH release, aiding in recovery and muscle growth.

    • Starvation: Leads to increased secretion to mobilize energy reserves from tissues.

    • Acute Hypoglycemia: Low blood sugar levels trigger GH release to elevate glucose concentration.

    • Aging: GH secretion decreases with age, affecting growth and metabolism.

  • Hypothalamic Control:

    • GHRH (Growth Hormone-Releasing Hormone): 43-amino acid peptide secreted from the arcuate nucleus to stimulate GH release from pituitary somatotropes.

    • Somatostatin (SS): Released from the periventricular region, inhibits GH secretion.

  • Feedback Mechanisms:

    • Short Loop Feedback: GHRH and somatostatin exert modulating effects directly on GH secretion.

Growth Hormone Action and Signaling Pathways

  • Hormonal Interactions:

    • GHRH and Somatostatin Interactions:

    • GHRH enhances GH synthesis, while SS inhibits it via specific receptors.

  • Intracellular Signaling Mechanism:

    • Hormonal interactions result in the activation of G-proteins, which affect cyclic AMP (cAMP) levels via adenylyl cyclase (AC).

    • cAMP activates protein kinase A (PKA), which is involved in GH secretion processes:

    • GHRH leads to increased Ca²⁺ concentration that promotes GH release;

    • Somatostatin triggers inhibitory pathways through a different G-protein leading to reduced cAMP levels.

  • JAK-STAT Signaling Pathway:

    • Growth hormone binds to its receptor (GH-R) causing dimerization and activation of Janus kinases (JAKs).

    • These kinases phosphorylate the receptor and subsequently recruit and activate Signal Transducers and Activators of Transcription (STATs), which translocate to the nucleus to initiate gene expression related to growth and metabolism.

  • Roles of IGF-I (Insulin-like Growth Factor 1):

    • Produced mainly in the liver as a response to GH.

    • Plays critical roles in cell growth, development, and metabolic regulation.

Physiological Effects of Growth Hormone

  • Tissues Affected by GH:

    • Adipose Tissue:

    • Decreases glucose uptake, increases lipolysis, and reduces adiposity.

    • Liver:

    • Promotes RNA synthesis, protein synthesis, gluconeogenesis, and increases levels of Insulin-like Growth Factors (IGFs).

    • Muscle:

    • Decreases glucose uptake, increases amino acid uptake, and enhances protein synthesis for lean body mass.

    • Bone and Cartilage:

    • Increases cell size and number, promotes growth in size and function, and enhances collagen synthesis.

Growth Hormone and Thyroid Hormone Interactions

  • Effects of Thyroid Hormones on GH Function:

    • Normal/Abnormal Levels of Thyroid Hormones: GH secretion can be altered based on the thyroid hormone levels (hyperthyroid vs. hypothyroid states).

Diurnal Variation in GH Secretion

  • Secretion Patterns: GH secretion varies throughout the day, being highest during sleep periods.

  • Normal Levels:

    • GH secretion is highest during the deep sleep stages and significantly varies depending on the time of day.

IGF-1 and Growth Stages

  • Importance of IGF-1 in Growth:

    • IGF-1 concentrations correlate with the rate of growth during puberty and other growth stages.

  • Growth Hormone Dependence:

    • The extent to which growth stages are influenced by GH and other hormones.

  • Age and Growth Hormone Levels: GH levels fluctuate with age; peak in adolescents and decrease in older adults.

Bone Growth and Growth Plates

  • Growth Plates Histology:

    • Describe the epiphyseal plate structure and function in longitudinal bone growth.

    • Chondrocytes play a critical role in growth plate function.

  • Influences on Chondrocytes:

    • Factors such as cortisol, estradiol, IGF-1, thyroid hormone, and testosterone influence chondrocyte division, hypertrophy, and overall growth plate health.

  • Key Stages: Different growth stages show how GH interacts with bone through chondrocyte proliferation and differentiation, leading to overall skeletal growth.