The key difference between direct and indirect hormone action is that direct hormone action occurs when hormones directly act on non-endocrine tissues while indirect hormone action occurs when hormones modulate the secretory activity of other glands.
Hormones are chemical messenger secreted into the bloodstream. They can be amino acid derivatives, peptide hormones or lipid derivatives. Plants and animal secrete hormones, which regulate physiological functions and maintain body homeostasis. There are several endocrine glands (pituitary, pineal, testes, ovaries, thymus, thyroid, adrenal glands, and pancreas) that secrete hormones. Then these hormones travel via the bloodstream and act on target organs and tissues.
Hormones affect many different processes such as growth and development, metabolism, sexual function, mood, appetite, puberty, maintenance of body temperature, thirst and fertility, etc. Generally, hormones affect cells by binding with a specific receptor on the target cell and activating a signal transduction pathway. This is known as direct hormonal action. However, hormones can also act indirectly by stimulating another gland to secrete another hormone.
CONTENTS
1. Overview and Key Difference
2. What is Direct Hormone Action
3. What is Indirect Hormone Action
4. Similarities Between Direct and Indirect Hormone Action
5. Side by Side Comparison – Direct vs Indirect Hormone Action in Tabular Form
6. Summary
What is Direct Hormone Action?
Hormones directly act on target cells and tissues. The direct action of hormones on non-glandular tissues is known as direct hormone action. Target cells have receptors for that particular hormone. Hormones bind to the receptor directly and initiate a chain of events that leads to the target cell’s response. Upon hormone binding, some receptors directly influence gene expression. They are called nuclear receptors.
Figure 01: Direct Hormone Action – Hormone Receptor Binding
Some receptors cause a cascade of changes that leads to actions. Human growth hormone shows direct action on target tissues and organs. Therefore, growth hormone directly binds with the target cells to stimulate a response.
What is Indirect Hormone Action?
Hormones indirectly act on other endocrine glands to regulate their secretory activity. In other words, some hormones modulate the secretion of other hormones. This is the indirect hormone action. Growth hormone acts and exerts its effect directly and indirectly.
Figure 02: Indirect Hormone Action – IGF-1
Indirect effects are mainly mediated by the increased production of insulin-like growth factor 1. Insulin-like growth factor 1 (IGF-1), also called somatomedin C, is a hormone secreted from the liver and other tissues in response to growth hormone. Growth hormone increases circulating IGF-1. Hence, most growth-promoting actions of growth hormone are mediated by IGF-1, and it’s the indirect action of growth hormone.
What are the Similarities Between Direct and Indirect Hormone Action?
- Direct and indirect actions of hormones are two types of mechanisms of hormones.
- Growth hormone act via both direct and indirect actions.
What is the Difference Between Direct and Indirect Hormone Action?
Direct hormone action is the binding of hormones with target cells to stimulate the target cell’s response, while indirect hormone action is the modulation of other glands to secrete hormones. Thus, this is the key difference between direct and indirect hormone action.
The below infographic summarizes the differences between direct and indirect hormone action in tabular form.
Summary – Direct vs Indirect Hormone Action
Hormones are chemical or chemical messengers released into the blood by an endocrine gland. Hormones utilize two mechanisms to exert their effects. They are direct hormone actions and indirect hormone action. In direct hormone action, hormones directly bind with the receptors on target cells and tissues and regulate target cell’s responses. In indirect hormone action, hormones modulate the secretion of other hormones by stimulating other glands. Thus, this is the summary of the difference between direct and indirect hormone action.
Reference:
1. “Growth Hormone.” StatPearls, StatPearls Publishing, 16 May 2020, Available here.
2. “Hormone.” Lumen, Available here.
Image Courtesy:
1. “Hormone Receptor Binding” By Alexkeir – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Protein IGF1 PDB 1bqt” By Emw – Own work (CC BY-SA 3.0) via Commons Wikimedia
Leave a Reply