Hormones are substances that control the functions of the different systems of our body. They are secreted either by the exocrine glands such as the pancreas and the endocrine glands such as the liver.
Hormones may be either polypeptide hormones or steroid hormones. They act as chemical messengers and cause a response in a target tissue or tissues. Hormones have different signaling methods: 1. Hormone will enter the cell and redirect how the gene acts. These are the steroid hormones, Vitamins D and A, and thyroxine. 2. Receptor-mediated will change the shape and signal the interior of the cell. This occurs in the cell membrane, NOT inside the cell. It causes a cascade effect. Ion-channels may open up next to the receptor, which allows ions to flow in or out of the cell.
Insulin, an anabolic hormone, acts by signaling the membrane receptor and activating tyrosine kinase. Glucagon acts by changing the shape of the G-proteins and acting on adenylate cyclase to convert ATP to cAMP. This excites protein kinase which phosphorylates proteins.
Insulin is a peptide hormone made by pancreatic islet β-cells, with anabolic effects. It binds to specific receptors in the cell membrane of tissues like liver, muscle, and adipose tissue, and increases the transport of glucose and amino acids across the plasma membrane. Insulin increases protein, triglyceride, and glycogen synthesis in the liver and muscle. By acting on the receptor, it activates or inhibits enzymes in the cell. For example, when insulin signals the membrane receptor, it activates and causes the phosphorylation of tyrosine kinase which then places phosphates on Tyrosine residues of target proteins. Another action of insulin is its ability to decrease the blood concentration of glucose. Furthermore, it decreases the fatty acid level by inhibiting hormone sensitive lipase in fat. As a response to stress, epinephrine will decrease the insulin release. Insulin secretion is increased by increased glucose amino acids, and secretin. Overall, insulin acts to increase the synthesis of glycogen, protein, and triacylglycerol. It is considered an anabolic hormone.
Glucagon is a peptide hormone that is secreted by pancreatic islet α-cells. It maintains blood glucose by increasing liver glycogenolysis and gluconeogenesis. Glucagon increases blood sugar, breaks down liver glycogen, and increases gluconeogenesis and does not break down muscle glycogen. It is increased in response to hypoglycemia, to increase the glucose level. Remember, glucagon is increased by: a decreased blood glucose level, an increased epinephrine (stress), and a high amino acid diet with a high protein meal needs to counter the hypoglycemic effect that insulin would create. A sudden increase of glucagon would increase liver cAMP levels and liver glycogenolysis. Glucagon increases the transformation of pyruvate to
phosphoenolpyruvate. It also increases fructose-1, 6-diphosphate conversion to fructose - 6 - phosphate.
Oxytocin causes uterine contraction and the release of milk during breast feeding ("milk let down"). This hormone is released by the posterior pituitary. Vasopressin is an Anti diuretic hormone (ADH) which increases sodium and water reabsorption in the kidneys. This hormone is released by the posterior pituitary.
Growth hormone increases amino acid uptake and growth. Somatotropin is considered a growth hormone. Growth hormone is released by the anterior pituitary.
Adrenocorticotropin stimulates growth of the adrenal cortex. This hormone is released by the anterior lobe of the pituitary or the hypophysis. The Parathyroid Hormone increases blood calcium levels. It decreases plasma phosphate level by increasing the excretion of phosphate. Cholecystokinin is a hormone made by the intestinal endocrine cells of the jejunum and the last part of the duodenum, and is increased in the presence of fat. Cholecystokinin causes the gallbladder to release bile and the pancreas to release enzymes. It decreases motility or causes less movement of gastric material to the small intestine. This peptide hormone stimulates the contraction of the gallbladder.
Secretin is released when there is an increase in fats, a low pH due to increased gastric hydrochloric acid, and chyme. Secretin causes the pancreas to release bicarbonate by pancreatic juice secretion into the duodenum. Gastrin hormone is secreted in the pyloric - antral area of the stomach. It increases the Hydrochloric acid secretion of parietal cells. A gastrinoma is a gastrin - secreting tumor that is associated with Zollinger - Ellison - syndrome.
Steroid hormones are lipid soluble hormones that cause effects at a slower rate. This is why you need to give corticosteroids for several days before you see an effect. They decrease the protein synthesis. They activate nuclear receptors of the target tissue and cause an increase in messenger RNA and protein synthesis to occur.
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