Ans. A. Calcitonin. (Ref. Guyton & Halls Textbook of Medical Physiology ll,h/pg. 913; Table 74-2)Some Hormones That Use the Adenylyl Cyclase-cAMP Second Messenger SystemSome Hormones That Use the Phospholipase C Second Messenger System# Adrenocorticotropic hormone (ACTH)# Angiotensin II (epithelial cells)# Angiotensin II (vascular smooth muscle)# Catecholamines (a receptors)# Calcitonin Catecholamines (b receptors)# Gonadotropin-releasing hormone (GnRH)# Corticotropin-releasing hormone (CRH)# Growth hormone-releasing hormone (GHRH)# Follicle-stimulating hormone (FSH)# Oxytocin# Glucagon# Thyroid-releasing hormone (TRH)# Human chorionic gonadotropin (HCG)# Vasopressin (VI receptor, vascular smooth muscle)# Luteinizing hormone (LH) # Parathyroid hormone (PTH) # Secretin # Somatostatin # Thyroid-stimulating hormone (TSH) # Vasopressin (V2 receptor, epithelial cells) Mechanisms of Action of Hormones:Ion Channel-Linked Receptors.# Virtually all the neurotransmitter substances, such as acetylcholine and norepinephrine, combine with receptors in the postsynaptic membrane.G Protein-Linked Hormone Receptors.# Many hormones activate receptors that indirectly regulate the activity of target proteins (e.g., enzymes or ion channels) by coupling with groups of cell membrane proteins called heterotrimeric GTP-binding proteins (G proteins).Enzyme-Linked Hormone Receptors.# Some receptors, when activated, function directly as enzymes or are closely associated with enzymes that they activate. These enzyme-linked receptors are proteins that pass through the membrane only once, in contrast to the seven-transmembrane G protein-coupled receptors. One example of an enzyme-linked receptor is the leptinreceptor. In the case of the leptin receptor, one of the signaling pathways occurs through a tyrosine kinase of the janus kinase (JAK) family, JAK2.Intracellular Hormone Receptors and Activation of Genes.# Several hormones, including adrenal and gonadal steroid hormones, thyroid hormones, retinoid hormones, and vitamin D, bind with protein receptors inside the cell rather than in the cell membrane.Additional educational points;# cAMP is not the only second messenger used by the different hormones. Two other especially important ones are (1) calcium ions and associated calmodulin and (2) products of membrane phospholipid breakdown.# Binding of norepinephrine with its receptor on the outside of many cells increases the activity of the enzyme adenylyl cyclase on the inside of the cell, and this causes formation of cyclic adenosine monophosphate (cAMP). The cAMP in turn can initiate any one of many different intracellular actions, the exact effect depending on the chemical machinery of the effector cell.# For a few peptide hormones, such as ANP, cGMP, which is only slightly different from cAMP, serves in a similar manner as a second messenger.# The normal calcium ion concentration in most cells of the body is 10-8 to 10-7 mol/L, which is not enough to activate the calmodulin system. But when the calcium ion concentration rises to 10-6 to 10-5 mol/L, enough binding occurs to cause all the intracellular actions of calmodulin. It is interesting that troponin C is similar to calmodulin in both function and protein structure.# Most hormones are present in the blood in extremely minute quantities; some concentrations are as low as one billionth of a milligram (1 picogram) per milliliter. Therefore, it was very difficult to measure these concentrations by the usual chemical means. An extremely sensitive method that revolutionized the measurement of hormones, precursors, and their end product is called radioimmunoassay (RIA).