Topic 21: COMMUNICATION BETWEEN CELLS – CHEMICAL (LECTURE 33)OBJECTIVES:1. Know the different kinds of chemical signal molecules.2. Understand the concept of second messenger and hormone signal amplification.3. Understand how the chemistry of a hormone determines where in the cell itsreceptor is located.4. Be able to compare and contrast the control of hormone secretion by the anteriorand posterior pituitary and be able to recognize which hormones are secreted bythese two regions.5. Understand the concept of negative feedback regulation as relates to thyroidhormone secretion.An animal consists of a complex assemblage of cells organized into tissues, organs andorgan systems; the activities of these cells must be coordinated. This is accomplishedby a diverse array of chemical signals.Fig. 11.3- kinds of chemical signal molecules1. hormone- a chemical signal molecule released by an endocrine gland into the bloodwhich is then carried long distance to have an effect on some target organ;neurohormone- a chemical signal molecule released by a specialized nerve cell(neuroendocrine gland) into the blood which is then carried long distance to have aneffect on some target organ2. neurotransmitter – a chemical signal molecule released by a nerve cell (neuron)which has impact on adjacent cell(s) like other neurons3. paracrine secretion- a molecule released by a cell other than a neuron which has alocal effectHormones & neurohormones- these molecules are released at very low concentrationsinto the blood; target organs have specific hormone receptors which bind the hormoneand subsequently the biological effect is produced. Hormone receptors are proteinswhich can be located inside the cell or on the exterior of the cell membrane. Location isdetermined by the chemistry of the hormone-1. lipid soluble hormones like steroids can dissolve into the cell membrane and passright on through; these hormones have cytoplasmic and/or nuclear receptors2. peptide or protein hormones are lipid-insoluble; they cannot pass through the cellmembrane; they interact with membrane-bound receptors.How can a few hormone molecules produce a large biological effect; the hormonesignal is amplified by signal transduction pathways (fig. 11.5). G- proteins are often involved in signal transduction- fig. 11-61Binding of signal molecule to membrane receptor initiates a series of changes whichleads to a biological effect. Examples of typical signal transduction systems- G proteinlinked (fig. 11.7)- binding of a signal molecule to receptor causes activation of a Gprotein by phosphorylation which in turn activates some other process. Second messenger systems in signal transduction pathways.A way of amplifying the interaction of a chemical signal with a receptor is toproduce a second messenger ( second messengers- small, water soluble molecules orions that are present in excess of the original signal chemical). These molecules areable to travel throughout the cell producing a variety of biological effects.Cyclic AMP based systems- fig. 11.12; cyclic AMP is produced by the enzyme adenylylcyclase and broken down by the enzyme phosphodiesterase.Fig. 11.13 – G protein linked activation of adenylyl cyclase with subsequent productionof cyclicAMP and activation of protein kinase A.Calcium ions and inositol triphosphate- fig. 11.15; activation of phospholipase C causesbreakdown of membrane phospholipids; inositol triphosphate IP3 is one of the products.This causes the endoplasmic reticulum to release calcium ions which act as a secondmessenger system.Hormones –transported long distances by the circulatory system; generally aresomewhat long-lasting in biological effects- minutes to a few hours.Fig. 45.3- Chemical signaling overviewSome examples of endocrine glands and hormone action in mammals- the major pointsto keep in mind: (1) targets may be very distant from endocrine gland/neuroendocrinecells, (2) hormones are very specific; specificity is determined by the receptors in thetarget organ, (3) often there are hormones that have opposite or antagonistic effects toa particular hormone and (4) hormone secretion is under precise control.Fig. 45.6- posterior and anterior pituitary gland; secretions of this gland are under directcontrol by a region of the brain known as the hypothalamus.Posterior pituitary- neuroendocrine cells in the hypothalamus extend all the way intothe pos. pituitary; release two hormones into the blood- (1) antidiuretic hormone and (2)oxytocin (milk ejection reflex; contraction of the uterus)Anterior pituitary- neuroendocrine cells in the hypothalamus produce neurohormonesthat are released into blood vessels going to the anterior pituitary. These agents controlthe secretion of many of the hormones of the anterior pituitary including:21. growth hormone- bone elongation, brain development2. gonadotropins- follicle stimulating hormone and luteinizing hormone; targets are theovary and testes3. thyroid stimulating hormone (TSH) - stimulates thyroid to release thyroid hormone4. adrenocorticotropic hormone- causes the cortical region of the adrenal gland torelease the hormone cortisol (important in carbohydrate and protein metabolism)An example of control of hormone secretion.Fig. 45.8- thyroid hormone consists of two very similar hormones T3 and T4 which arederived from the amino acid tyrosine; both stimulate metabolism and are also essentialfor normal brain development. Both are under very tight control in terms of secretion bywhat are known as negative feedback loops (the accumulation of a product in asequence inhibits its further production/release).1. hypothalamus releases TRH (TSH releasing hormone) which causes the anteriorpituitary to release TSH2. TSH causes the thyroid gland to release thyroid hormone.3. as TSH levels in the blood rise, it begins to inhibit the release of TRH by thehypothalamus4. as thyroid hormone levels in the blood rise, it begins to also inhibit the release ofTRH5. the net effect is to avoid over secretion of thyroid
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