Chapter 45 Hormones and the Endocrine System Signaling molecules called HORMONES influence biological processes Hormones are secreted into the Extracellular fluid and communicate messages by binding specific receptors in the body target cells Chemical Signaling by hormones is the function of the endocrine system and the nervous system and regulate neurons muscle cells and endocrine cells 45 1 HORMONES AND OTHER SINGLAING MOLECULES BIND TO TARGET RECEPTORS TRIGGERING SPECIFIC RESPONSE PATHWAYS INTRACELLULAR COMMUNICATION Endocrine Signaling In endocrine signaling hormones secreted into the extracellular fluid by endocrine cells reach target cells via the bloodstream or hemolymph One of the main functions is to maintain homeostasis This includes energy metabolism blood pressure and volume solute concentrations and regulation of growth and development Paracrine and Autocrine Signaling Many types of cells produce and secrete LOCAL REGULTORS molecules that act over short distances and reach their target cells solely by diffusion This includes cytokines which enable communication between immune cells for immunity and growth factors for cell growth division and development Depending on target cell signaling by local regulators can be either paracrine or autocrine Paracrine signals involve target cells lie near the secreting cell Autocrine signaling has the secreting cells as the actual target cells PROSTAGLANDINS help sperm reach the egg by contracting the uterus and prostaglandins produced in the placenta induce labor Synaptic and Neuroendocrine Signaling Secreted molecules are crucial for 2 SYNAPTIC SIGNALING and NEUROENDOCRINE SIGNALLING Synaptic Signaling occurs when neurons form specialized specialized junctions called synapses with target cells These synapses then secrete neurotransmitters that diffuse and a short distance and fuse to receptors n the target cells Neurotransmitters are central to sensation memory cognition and movement Neuroendocrine signaling has specialized neurons called neurosecretory cells secrete neurohormones which diffuse onto nerve cell endings into the blood stream Neurohoromnes are important for endocrine signaling Signaling by Pheromones Some animal species have members use pheromones to communicate with each other and are released into the external environment Pheromones are important for defending territories warning of predators and attracting mates CHEMICAL CLASSES OF LOCAL REGULATORS AND HORMONES Classes of Local Regulators Once class of local regulators includes PROSTAGLANDINS which are modified fatty acids Nitric Oxide is both a local regulator and a neurotransmitter It s release results in vasodilation and erections Classes of Hormones Hormones fall into 3 major groups Polypeptides Steroids and Amines Hormones vary in their solubility in aqueous and lipid rich environments Polypeptides and most amine hormones are water soluble while steroid hormones are lipid soluble CELLULAR RESPONSE PATHWAYS There are several differences between the response pathways for water soluble and lipid soluble signaling molecules One difference in cellular hormone response pathways is the location of the target cells receptor proteins Water soluble hormones are secreted by exocytosis and travel freely in the blood however they cannot diffuse across the plasma membrane and must bind to cell surface receptors membrane inducing changes in cytoplasmic molecules and sometimes altering gene transcription Lipid Soluble Hormones can diffuse across membranes of endocrine cells and bind to transport proteins that keep them soluble in blood Upon leaving the blood they diffuse into target cells and typically bind to receptors in the cytoplasm or nucleus Pathway for Water Soluble Hormones The binding of a water soluble hormone to a receptor protein in the plasma membrane triggers events at the plasma membrane that result in a cellular response such as activation of an enzyme uptake or secretion of molecules or even alteration of transcription for specific genes This leads to a series of changes that results in SIGNAL TRANSDUCTION The pathway for Water Soluble Hormones could be best exemplified using the pathway of epinephrine adrenaline Epinephrine binds to G protein couple receptor Binding results in synthesis of cyclic AMP cAMP as a short lived second messenger Protein Kinase A is activated by CAMP an leads to activation of enzyme required for glycogen breakdown and inactivation of enzyme for glycogen synthesis Liver releases glucose for fuel Pathway for Lipid Soluble Hormones Intracellular receptors for lipid soluble hormones perform the entire task of transducing a signal within a target cell The hormone activates the receptor which ten directly triggers the cell s response which can mostly results in a change in gene expression Most steroid hormone receptors are predominantly ocated in the cytosol prior to binding to a hormone and after binding to the receptor the hormone receptor complex moves into the nucleus to alter transcription of particular genes by interacting with a specific DNA binding protein MULTIPLE EFFECTS OF HORMONES Many hormones directly form than one type of response in the body Epinephrine can increase blood flow to skeletal muscles decrease blood flow to the digestive tract and also breakdown glycogen in the liver A hormone can have widely varying effects by having target cells differ in receptor type or in the molecules that produce the response ENDOCRINE TISSUES AND ORGANS Some endocrine cells are found in organs that are part of other organ systems Endocrine cells can be grouped into ductless organs called ENDOCRINE GLANDS thyroid and parathyroid glands and gonads Endocrine glands secrete hormones directly into the surrounding fluid while Exocrine glands have ducts that carry secreted substances onto body surfaces or into body cavities such as salivary glands DUCTLESS TISSUES SECRETE HORMONES WHILE TKSSES WITH DUCTS SECRETE ENZYMES AND BICARBONATE 45 2 FEEDBACK REGULATION AND COORDINATION WITH THE NERVOUS SYSTEM ARE COMMON IN ENDOCRINE SIGNALING SIMPLE HORMONE PATHWAYS There are 2 basic types of organization for the regulation of hormone secretion Simple Endocrine and Simple Neuroendocrine Pathways In SIMPLE ENDOCRINE PATHWAYS endocrine cells respond directly to an internal or environmental stimulus by secreting a particular hormone which travels thorough the bloodstream and interacts with specific receptors to bring a response NEUROENDOCRINE pathways