Paracrine Autocrine Agents

Paracrine agents are synthesized by cells and released, once given the appropriate stimulus, into the extracellular fluid. They then diffuse to neighboring cells, some of which are their target cells. (Note that, given this broad definition, neurotransmitters theoretically

Local Chemical Messengers

FIGURE 7-5

Categories of chemical messengers. (a) Reflexes. Note that chemical messengers that are secreted by nerve cells and act on adjacent nerve cells or effector cells are termed neurotransmitters, whereas those that enter the blood and act on distant effector cells (synonymous with target cells) are classified as hormones (also termed neurohormones). (b) Local homeostatic responses. With the exception of autocrine agents, all messengers act between cells—that is, intercellularly.

FIGURE 7-5

Categories of chemical messengers. (a) Reflexes. Note that chemical messengers that are secreted by nerve cells and act on adjacent nerve cells or effector cells are termed neurotransmitters, whereas those that enter the blood and act on distant effector cells (synonymous with target cells) are classified as hormones (also termed neurohormones). (b) Local homeostatic responses. With the exception of autocrine agents, all messengers act between cells—that is, intercellularly.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Homeostatic Mechanisms and Cellular Communication CHAPTER SEVEN

could be classified as a subgroup of paracrine agents, but by convention they are not.) Paracrine agents are generally inactivated rapidly by locally existing enzymes so that they do not enter the bloodstream in large quantities.

There is one category of local chemical messengers that are not intercellular messengers—that is, they do not communicate between cells. Rather, the chemical is secreted by a cell into the extracellular fluid and then acts upon the very cell that secreted it. Such messengers are termed autocrine agents (Figure 7-5). Frequently a messenger may serve both paracrine and au-tocrine functions simultaneously—that is, molecules of the messenger released by a cell may act locally on adjacent cells as well as on the same cell that released the messenger.

One of the most exciting developments in physiology today is the identification of a seemingly endless number of paracrine/autocrine agents and the extremely diverse effects they exert. Their structures span the gamut from a simple gas (nitric oxide) to fatty acid derivatives (the eicosanoids, see below) to peptides and amino acid derivatives. They tend to be secreted by multiple cell types in many tissues and organs. According to their structures and functions, they can be gathered into families; for example, one such family constitutes the "growth factors," encompassing more than 50 distinct molecules, each of which is highly effective in stimulating certain cells to divide and/or differentiate.

Stimuli for the release of paracrine/autocrine agents are also extremely varied, including not only local chemical changes (for example, in the concentration of oxygen), but neurotransmitters and hormones as well. In these two latter cases, the paracrine/au-tocrine agent often serves to oppose the effects induced locally by the neurotransmitter or hormone. For example, the neurotransmitter norepinephrine strongly constricts blood vessels in the kidneys, but it simultaneously causes certain kidney cells to secrete paracrine agents that cause the same vessels to dilate. This provides a local negative feedback, in which the paracrine agents keep the action of norepinephrine from becoming too intense.

In other cases, a neurotransmitter or hormone may stimulate the local release of a paracrine/autocrine agent that actually is responsible for causing the cellular response to that neurotransmitter or hormone. For example, most of the growth-promoting effects of growth hormone on bone are not exerted directly on bone cells by this hormone; rather, growth hormone stimulates the release from the bone cells of a paracrine/autocrine agent that then stimulates the bone growth.

Eicosanoids The general approach of this text is to describe the specific chemical messengers in the context of the functions they influence. However, one set of paracrine/autocrine agents exerts such a wide variety of effects in virtually every tissue and organ system that it is best described separately at this point. These are the eicosanoids, a family of substances produced from the polyunsaturated fatty acid arachidonic acid, which is present in plasma-membrane phospho-lipids. The eicosanoids include the cyclic endoperoxides, the prostaglandins, the thromboxanes, and the leukotrienes (Figure 7-6).

Stimulus

Membrane phospholipid

Phospholipase A2 J

Arachidonic acid r i

Cyclooxygenase Cyclic endoperoxides r

Prostaglandins

Thromboxanes

Lipoxygenase

Leukotrienes

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • marjorie
    Do paracrine and autocrine enter the bloodstream?
    7 years ago
  • lisa
    Where is paracrine agent produced?
    7 years ago
  • rosa
    What does an autocrine secretion act on?
    7 years ago
  • Monica
    Are autocrine chemical messengers local hormones?
    7 years ago
  • Sebhat
    Where are autocrine agents secreted?
    7 years ago
  • carmela fiorentini
    Is testosterone autocrine agent?
    6 years ago
  • Linda
    What are paracrine agents?
    6 years ago
  • MARGIT
    How chemical massanger classfiy as paracrine agent?
    4 months ago

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