Hormones are chemical messengers secreted into the blood or extracellular fluid by one cell that affect the functioning of other cells. Most hormones circulate in blood, coming into contact with essentially all cells. However, a given hormone usually affects only a limited number of cells, which are called target cells. A target cell responds to a hormone because it bears receptors for the hormone.
Hormone receptors are found either exposed on the surface of the cell or within the cell, depending on the type of hormone. In very basic terms, binding of a hormone to a receptor triggers a cascade of reactions within the cell that affects function.
A traditional part of the definition of hormones described them as being secreted into blood and affecting cells at distant sites. However, many of the hormones known to act in that manner have also been shown to affect neighboring cells or even have effects on the same cells that secreted the hormone. Nonetheless, it is useful to be able to describe how the signal is distributed for a particular hormonal pathway, and three actions are defined:
- Endocrine – the hormone is distributed in blood and binds to distant target cells.
- Paracrine – the hormone acts locally by diffusing from its source to target cells in the neighborhood.
- Autocrine – the hormone acts on the same cell that produced it.
Two types of molecules bind to the hormone binding sites: Agonists are the hormones themselves. They bind the receptor and trigger all the post-receptor events that lead to a biologic effect. Antagonists are molecules that bind the receptor thereby blocking the agonist, preventing the intracellular signaling system.
Even if all hormone levels appear to be well within the ideal range, the receptor sites must be available for the hormones to function. If estrogen binding sites in bone are occupied by biphenyls (antagonists) then estrogen will not stimulate bone growth and osteoporosis follows. As noted in earlier blogs over 800 biphenyls are still used today, most of them in the food industry.
Hormone antagonists are widely used as drugs. Tamoxifen is an estrogen antagonist used to treat breast cancer. Similar drugs are used to treat prostate cancer, infertility and endometriosis. Phytoestrogens, like those found in soy can act as agonists or antagonists. They may stimulate some estrogen receptor sites, like those in breast and uterus, while blocking those in bone.
In a study published in the Proceedings of Nutrition Society in February, 2001 Dauncey and White, from the Developmental Genetics Program in Cambridge, reviewed the role of nutrition on receptor-gene interaction. They state that “nutrition profoundly alters the phenotypic expression of a given genotype. Many hormones act as nutritional signals and their receptors play a key role in mediating the effects of nutrition of numerous genes involved in differentiation, growth and metabolism.” It appears that both vitamin A and vitamin D play key roles in the potential interactions between these receptors.
The herbs Tribulus and Chaste Tree both stimulate receptor activity. Chaste Tree acts as an agonist on the pineal gland receptor sites to stimulate the production of melatonin. Melatonin exhibits agonist activity on receptor sites for hormone production in the adrenals, ovaries, and testicles. The net effect of Chaste Tree is to slightly modulate overall hormone production.
The action of Tribulus is a little more complex. In acts as an antagonist on hormonal receptor sites in the hypothalamus. However, those sites function to block hormone production when bound by the various endocrine hormones. This “negative feedback” helps control hormone levels. When a testosterone binding site in the hypothalamus is bound by testosterone the post-receptor effect is a reduction in testosterone production. When Tribulus binds to a testosterone receptor site in the hypothalamus it stimulates testosterone production.
The beauty of this herb is that the only hormonal binding sites that are open in the hypothalamus will be those with low serum levels, so Tribulus only stimulates production of low hormones be that testosterone, estrogen, or progesterone.
The Bottom Line:
Receptor site activity is an area of intense research and holds great promise for nutritional support of the endocrine system. Be cautious of drugs that act as antagonists as the effects are far reaching. Currently there are a few herbs that influence receptor site activity. Hopefully, new research will reveal more evidence on the effect of various herbs on hormone receptor sites.