Endocrine cells release hormones in response to signals from hormones, signals from nerve cells, changes in local chemical conditions, and signals from the environment. Protein hormones are synthesized before they are needed and released from storage vesicles via exocytosis. Lipid-soluble hormones such as steroids are not stored, but are released upon synthesis. An example of homeostasis is the maintenance of blood glucose within a range of values compatible with normal cell function. An example of an event is ovulation.
Hormone receptors are protein molecules.
Receptors for lipid-soluble hormones are found inside cells. A major effect of lipid-soluble hormones is to alter gene transcription. Receptors for water-soluble hormones are transmembrane proteins that span the plasma membrane. A major effect of water-soluble hormones is to stimulate intracellular second messenger systems. New signaling molecules are still being discovered.