Lecture Notes on Growth Hormone Synthesis and Secretion

Overview

• Growth Hormone (GH): A peptide hormone synthesized and secreted with notable circadian rhythms.
• Stimuli for GH Release:
  • High plasma amino acid levels.
  • Low plasma glucose levels.
  • Circadian rhythm (higher secretion at night).

Synthesis and Secretion Process

1. Role of Hypothalamus

   • Located in the center of the brain; contains neurosecretory neurons.
   • Neurosecretory neurons synthesize and secrete Growth Hormone Releasing Hormone (GHRH).

2. Pathway

   • GHRH travels down the axon of neurosecretory neurons to the axon terminal.
   • Released into the hypophyseal portal system, connecting the hypothalamus to the anterior pituitary.
   • GHRH binds to endocrine cell receptors in the anterior pituitary.

3. Outcome in Anterior Pituitary

   • Binding of GHRH prompts the synthesis and release of GH into systemic circulation.

Effects of Growth Hormone

• Targets and Actions:
  • Liver:
    • Triggers synthesis and secretion of Insulin-like Growth Factor 1 (IGF-1), also known as somatomedin.
    • IGF-1 targets bone, cartilage, and skeletal muscle, promoting growth.
  • Adipose Tissue:
    • Induces lipolysis, producing glycerol and fatty acids.
    • Glycerol converts to glucose in the liver, increasing plasma glucose levels.
  • Other Tissues:
    • Reduces glucose utilization, maintaining elevated glucose levels in circulation.

Negative Feedback and Regulation

• Insulin-like Growth Factor 1 (IGF-1)

  • Inhibits anterior pituitary's responsiveness to GHRH.
  • Acts on the hypothalamus to reduce GHRH secretion.

• Growth Hormone Inhibiting Hormone (GHIH)

  • Also synthesized by the hypothalamus.
  • Inhibits GH synthesis and secretion from the anterior pituitary.

Summary

• Regulation Mechanisms:
  • Negative feedback involving IGF-1.
  • Direct inhibition through GHIH.
• Purpose: Ensures balanced GH levels pertinent for growth and metabolism, aligning with energy needs and nutrient availability.