Chapter 39: endocrine physiology and mechanisms of hypothalamic-pituitary regulation. Hormones are chemical messengers that travel via the bloodstream to exert effects on target cells distant from the secreting glands. Hormones regulate complex functions including reproduction, growth and development, fluid homeostasis, and metabolism. Hormones may be classified according to chemical structure as water-soluble or lipid- soluble. Receptors for water-soluble hormones are located on the cell surface (catecholamines, peptides), and those for lipid-soluble hormones are located intracellularly (thyroid hormones, steroids). Activation of g-protein linked receptors on the cell surface leads to the production of second messengers within the cell and alteration in intracellular functions. Most endocrine hormones affect their targets through g-protein linked receptors. Some endocrine hormones, such as insulin, growth hormone, and prolactin interact with receptors on the cell surface that are protein kinases or directly associated with kinases. Activation of kinase receptors results in phosphorylation of target proteins, which changes their activity and initiates intracellular signaling cascades.