Hormones (First Messengers) and Their Partners Inside Cells Help
Introduction to Hormones (First Messengers) and Their Partners Inside Cells
We have now traced the operation of several examples of the neuroendocrine system. Within such nerve–gland connections, it is often difficult to tell which part of the connection (the nerve or the gland) comes first in communicating and controlling various aspects of homeostasis. In certain instances, however, the endocrine system, alone, provides the message for initiating specific body responses to stimuli ( STIM -you- lie ) or “goads.”
Insulin Secretion And Blood Glucose Concentration
An important example of such hormone-directed responses is the stimulation of the beta cells of the pancreas by a rise in blood glucose concentration. As Figure 15.5 shows, eating a sugar-rich meal soon results in a rise in blood glucose concentration above the set-point level, and towards the upper limit of its normal range. There is only one major gland (the beta cells of the pancreas) that is strongly excited by this particular stimulus.
The beta cells respond by increasing their rate of secretion of insulin molecules into the bloodstream. Insulin hormone molecules are, then, the “First Messengers” within the bloodstream. By this we mean that the presence of so many insulin molecules in the blood sends a powerful “First Message” to the rest of the body. In words, this chemical message might read: “Hey! Listen up, target cells! The blood glucose concentration has risen near the upper limit of its normal range! Let’s do something about it!”
This “First Message” has an effect upon many cells in the body. But the chemical signal is especially strong for the main insulin target cells: adipocytes ( AH -dih-poh-sights) or “fat” (adipo-) “cells,” hepatocytes (heh- PAT -oh-sights) or “liver” (hepat) “cells,” and skeletal muscle fibers. When an insulin molecule binds (attaches) to the plasma membrane of one of these target cells, it has essentially delivered the “First Message .”
But the insulin molecule, like most hormone molecules, is far too large to actually enter the target cells. Instead, it operates by activating a “Second Messenger” – one that lies within the target cell. The “Second Messenger” is an intracellular helper molecule. The “Second Messenger” is thus the chemical that actually causes a change in target cell metabolism. In an insulin target cell, for example, “Second Messengers” activated by the binding of insulin to the cell surface help carry out two important changes in cell metabolism:
1. These molecules speed up the glucose carrier proteins that act to aid or “facilitate” the diffusion of glucose out of the bloodstream, and into the target cells.
2. Intracellular “Second Messenger” molecules activate a key enzyme required for glycolysis – the utilization of glucose for making ATP energy.
As a result of these two intracellular processes directed by “Second Messengers,” glucose leaves the bloodstream in large quantities, enters the insulin target cells, and gets broken down for free energy. Finally, the blood glucose concentration falls back toward its set-point level.
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