Physiology Exam 2 Chapter 11 Endocrine System Endocrine glands lack the ducts that are present in exocrine glands The endocrine glands secrete hormones into the blood Exocrine glands secrete chemicals through a duct that leads to the outside of a membrane and thus to the outside of a body surface Both are developed in embryo in epithelia Endocrine System Epithelial tissue lining the inner surface of the trachea develops evagination vagination of epithelium of trachea Connection to the epithelial surface is cut off which forms the thyroid gland To retain connection to epithelial surface the salivary gland is formed The skin stomach heart and liver are all endocrine glands that produce hormones Chemical Classification of Hormones Hormones can be divided into a few chemical classes 1 Amines Hormones derived from the amino acids tyrosine and tryptophan They include the hormones secreted by the adrenal medulla epinephrine and norepinephrine 2 Polypeptides and proteins Antidiuretic hormone ADH is a polypeptide growth hormone is a protein insulin two polypeptide chains derived from a single larger molecule 3 Glycoproteins These molecules consist of a protein bound to one or more carbohydrate groups Examples are folliclestimulating hormone FSH and luteinizing hormone LH 4 Steroids Two basic types Sex steroids produced by gonads and corticosteroids produced by the adrenal cortex Hormones classified by Solubility Hormones can be divided into those that are polar and water soluble and those that are nonpolar and insoluble in water Because the nonpolar hydrophobic hormones are soluble in lipids they are called lipophilic hormones They are active when taken orally and have a genomic action can turn genes on or off Lipophilic hormones are steroid hormones contraceptive pills and thyroid hormones pills for people whose thyroid is deficient Polar hydrophilic water soluble hormones include polypeptides glycoproteins and the monoamines catecholamine hormones secreted by the adrenal medulla epinephrine and norepinephrine These cannot be taken orally cannot pass the plasma membrane of the target cell goes through the signal transduction pathway no genomic action for hormone to have its effect on the target cell Hormones Hormone molecules that affect the metabolism of target cells are often derived from precursor molecules In polypeptide hormones the precursor may be a long chained prohormone that is cut and spliced together to make a hormone Prehormones are sometimes used to indicate such precursors of prohormones Prehormones In some cases the hormone secreted by the endocrine gland from the prehormone is actually inactive in target cells To become active the target cells must modify the chemical structure of the secreted hormone Prehormone Thyroxine T4 is a principle hormone of the thyroid gland and must be changed into T3 within the target cells in order to affect the metabolism of those cells Comparing Neurotransmitters with Hormones Neurotransmitters do not travel in the blood as do hormones instead they diffuse across a narrow synaptic cleft to the membrane of the postsynaptic cell Hormones are released by an endocrine gland However the actions of neurotransmitters are very similar to the actions of hormones In order to function as physiological regulation they are both regulatory molecules that combine to receptor proteins and cause a specific sequence of changes in the target cell which must have a mechanism to turn off the action of the regulator Mechanism of action of Lipophilic hormones The mechanism of action of lipophilic hormones include the steroid hormone family sex steroids and corticosteroids and the thyroid hormone family thyroid hormones T3 and T4 Mechanism of action of Lipophilic hormones Hormones are delivered by the blood to every cell in the body but only the target cells are able to respond to these hormones In order to respond a target cell must have specific receptor proteins for that hormone Because the lipophilic hormones steroids and thyroxine can pass through the plasma membrane and enter their target cells the receptor proteins for them are located within the cytoplasm and nucleus Mechanism of action of Lipophilic hormones Unlike water soluble hormones the lipophilic steroid and thyroid hormones do not travel dissolved in the aqueous portion of the plasma instead they are transported to their target cells attached to plasma carrier proteins These hormones must then dissociate from their carrier proteins in the blood in order to pass through the lipid component of the plasma membrane and enter the target cell within which their receptor proteins are located Mechanism of action of Lipophilic hormones The receptors for the lipophilic hormones are known as nuclear hormone receptors because they function within the cell nucleus to activate genetic transcription The nuclear hormone receptors function as transcription factors that first must be activated by binding to their hormone ligands The newly formed mRNA produced by the activated genes directs the synthesis of specific proteins including enzyme proteins that change the metabolism of the target cell Mechanism of action of Lipophilic hormones Each nuclear hormone receptor has two regions or domains a ligand hormone binding domain which binds to a hormone molecule and a DNA binding domain which binds to the hormone response element of DNA composed of two half sites that are each 6 nucleotide bases long separated by a spacer segment that is 3 nucleotide bases long Binding to the hormone causes the receptor to dimerize on the half sites of the hormone response element This stimulates genetic transcription synthesis of RNA Mechanism of Steroid Hormone Action Steroid hormones exert their effects by entering their target cells and binding to nuclear receptor proteins They influence their target tissue by stimulating genetic transcription mechanism known as genomic action of steroids The receptors for the steroid hormones are located in the cytoplasm before the steroid arrives and unbound steroid receptors may be located in the nucleus as well When the cytoplasmic receptors bind to their specific steroid hormone ligands they translocate to the nucleus Once the steroid hormone receptor protein complex is in the nucleus its DNA binding domain binds to the specific hormone response element of the DNA One steroid receptor bound to one molecule of the steroid hormone attaches as a single unit to one of the half site Another