Negative and Positive Feedback Regulation

Assuming that adequate numbers of function­ing receptors are available, the biologic effect of any hormone is directly proportional to the concentration of the hormone in the body fluids available to bind to the receptors.

The release, and thus the plasma concentra­tion, of most hormones is controlled by some type of negative feedback regulation. In this type of regulation, the rising levels of the hormone bring about a biologic response that inhibits further hormone release. For example, β-cells in the pancreatic islets are directly affected by the concentration of glucose in the body fluids. An increase in glucose concentra­tion causes the β-cells to increase their release of insulin. one effect of insulin is to promote the uptake of glucose by skeletal muscle cells. As glucose is removed from the body fluids, the stimulus for insulin release is removed, and this has a negative effect on insulin release. This negative feedback regulation of insulin release is a major factor in determining a normal plasma concentration of glucose.

The negative feedback regulation of insulin by changes in plasma glucose is a relatively simple and straightforward feedback loop. The plasma constituent, glucose, being regulated by the hormone, insulin, has a direct effect on the cells releasing the hormone. However, negative feedback loops can be quite complex and have multiple organs in the loop.

A second type of feedback regulation, that is seen much less frequently than negative feed­back, is positive feedback regulation. In this case, the hormone brings about a biologic response that produces a further increase in the release of the hormone. This type of regulation is unusual, and it is not designed to maintain a stable or homeostatic level of some activity or blood constituent. one of the few examples of this type of regulation is the relationship between oxytocin release and dilation of the uterine cervix. An increase in oxytocin release is associated with dilation of the uterine cervix during parturition (details in chapters on repro­duction), and oxytocin acts on the smooth muscle of the uterus to increase uterine con­tractions. When the cervix dilates during par­turition and oxytocin is released, the contractions of the uterus move the fetus out of the uterus through the cervix. This further dilates the cervix, providing a greater stimulus for secre­tion of oxytocin. The overall effect is to expel the fetus when the cervix is dilated.