Hormones exert their effects by acting on specific target organs or tissues with designated binding sites, known as hormone receptors. The mechanisms vary between steroid hormones and protein hormones.
Steroid Hormones:
1. Lipid-soluble steroid hormones easily traverse the lipoproteinaceous plasma membrane.
2. Receptors for steroid hormones are located in the cytoplasm or nucleus.
3. The hormone-receptor complex, formed in the cytoplasm, enters the nucleus to regulate gene expression or chromosome function.
4. In some instances, receptors reside within the nucleus, where the hormone-receptor complex is generated.
5. These complexes interact with the genome, eliciting biochemical changes that influence physiological and developmental functions.
Protein Hormones:
1. Receptors for protein hormones are situated on the cell membrane (membrane-bound receptors).
2. When a hormone binds to its receptor, it creates a hormone-receptor complex, each receptor being specific to a particular hormone.
3. Hormones interacting with membrane-bound receptors typically do not enter the target cell but generate second messengers like cyclic AMP, Ca++, or IP (Inositol triphosphate).
4. This initiates biochemical changes in the target tissue.
5. Consequently, tissue metabolism and physiological functions are regulated by hormones.