What is the Difference Between Direct and Indirect Hormone Action

The main difference between direct and indirect hormone action is that the direct hormone action is the action of hormones as a result of their binding to a receptor on the target cell whereas indirect hormone action is the action exerted via stimulating the secretion of another hormone. Furthermore, direct hormone action is responsible for the main function of a hormone while indirect hormone action exerts a distinct effect in the body.

Direct and indirect hormone action are two types of actions of hormones secreted by the body. Indirect action of hormones always facilitates the direct action of hormones.

Key Areas Covered

1. What is Direct Hormone Action
      – Definition, Receptor Binding, Endocrine Glands
2. What is Indirect Hormone Action
      – Definition, Direct and Indirect Action of Growth Hormone
3. What are the Similarities Between Direct and Indirect Hormone Action
      – Outline of Common Features
4. What is the Difference Between Direct and Indirect Hormone Action
      – Comparison of Key Differences

Key Terms

Direct Hormone Action, Growth Hormone, Indirect Hormone Action, Insulin-like Growth Factor 1 (IGF-1), Receptor Binding

What is Direct Hormone Action

Direct hormone action is the main action of a hormone. It occurs through the binding of hormone to the receptor on the target cell. Based on the location inside the cell, there are two types of receptors as external receptors and internal receptors. Since peptide hormones cannot freely-diffuse into the cell, they use external receptors or the receptors embedded in the lipid bilayer of the cell membrane. The external receptors are fixed receptors that use an intracellular signaling mechanism to transduce the hormonal signal into the cell. Basically, this occurs through the second messengers.

However, steroid hormones can easily penetrate the cell membrane; hence, their receptors occur inside the cell. These are internal receptors. The two types of internal receptors are the receptors found in the cytoplasm and nuclear receptors found on the nuclear membrane. The receptors in the cytoplasm are mobile receptors, which are freely-floating. Here, the hormone-receptor complex usually serves as a transcription factor that regulates the expression of a particular gene inside the nucleus.

Figure 1: Some  Primary Endocrine Organs and Their Hormones

Furthermore, the hormones secreted by the primary endocrine glands including the hypothalamus, pituitary gland, thyroid, parathyroid, adrenal glands, pineal body, the ovaries, and the testis stimulate the secretion of other hormones from the secondary endocrine glands while exerting their main hormonal action. Moreover, some examples of secondary endocrine organs include heart, kidneys, skeleton, adipose tissue, skin, thymus, liver, etc.

What is Indirect Hormone Action

Indirect hormone action is the action of a particular hormone, exerted by stimulating the production of a second hormone with a related function to the main function of the hormone. For example, the anterior pituitary gland secretes growth hormone or somatotropin responsible for the stimulation of growth, reproduction, and regeneration of cells in the body. In fact, this is the main function or the direct hormone action of the growth hormone. In addition, growth hormone serves as a stress hormone, which stimulates the production and release of a second hormone, insulin-like growth factor 1 (IGF-1) formally called somatomedin C.

Figure 2: Direct and Indirect Action of Growth Hormone

Moreover, the second hormone increases the concentration of glucose and free fatty acids, which aid the direct action of the growth hormone.

Similarities Between Direct and Indirect Hormone Action

• Direct and indirect hormone action are two types of mechanism of actions of some hormones in the body.
• In addition, both types of actions are related to the general action of the hormone.

Difference Between Direct and Indirect Hormone Action

Definition

Direct hormone action refers to the hormonal action exerted via the binding of the hormone to a specific receptor while the indirect hormone action refers to the hormonal action exerted via the stimulation of another hormone, which facilitates the direct hormone action. Thus, this is the main difference between direct and indirect hormone action.

Significance

Moreover, direct hormone action is the main function of the hormone while indirect home action facilitates the main function of the hormone.

Growth Hormone

Direct hormone action of the growth hormone is to stimulate the growth of bones and soft tissue while regulating protein, carbohydrate, and lipid metabolism. In contrast, IGF-I mediates the indirect hormone action, stimulating the growth of different tissues. Hence, this is another difference between direct and indirect hormone action.

Conclusion

The direct hormone action is the main function of the hormone mediated by the binding of the hormone to either external or internal receptor. On the other hand, the indirect hormone action is the stimulation of the release of a second hormone with a related function to the main function of the hormone. Therefore, the main difference between direct and indirect hormone action is the mechanism of action of the hormone in both types of hormone actions.

References:

1. “Mechanism of Hormone Action: Hormones, Mechanisms, Videos, Example.” Toppr Bytes, Toppr.com, 12 Feb. 2018, Available Here

Image Courtesy:

1. “Endocrine central nervous en” By LadyofHats – made myself based on the information found on the wikipedia article (Public Domain) via Commons Wikimedia 
2. “Endocrine growth regulation” By Häggström, Mikael (2014). “Medical gallery of Mikael Häggström 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. (Public Domain) via Commons Wikimedia