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 its receptor is located.
4. Be able to compare and contrast the control of hormone secretion by the anterior and posterior pituitary and be able to recognize which hormones are secreted by these two regions.
5. Understand the concept of negative feedback regulation as relates to thyroid hormone secretion.
An animal consists of a complex assemblage of cells organized into tissues, organs and organ systems; the activities of these cells must be coordinated. This is accomplished by a diverse array of chemical signals.
Fig. 11.3- kinds of chemical signal molecules
1. hormone- a chemical signal molecule released by an endocrine gland into the blood which 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 an effect on some target organ
2. neurotransmitter – a chemical signal molecule released by a nerve cell (neuron) which has impact on adjacent cell(s) like other neurons
3. paracrine secretion- a molecule released by a cell other than a neuron which has a local effect
Hormones & neurohormones- these molecules are released at very low concentrations into the blood; target organs have specific hormone receptors which bind the hormone and subsequently the biological effect is produced. Hormone receptors are proteins which can be located inside the cell or on the exterior of the cell membrane. Location is determined by the chemistry of the hormone-
1. lipid soluble hormones like steroids can dissolve into the cell membrane and pass right on through; these hormones have cytoplasmic and/or nuclear receptors
2. peptide or protein hormones are lipid-insoluble; they cannot pass through the cell membrane; they interact with membrane-bound receptors.
How can a few hormone molecules produce a large biological effect; the hormone signal is amplified by signal transduction pathways (fig. 11.5).
G- proteins are often involved in signal transduction- fig. 11-6
Binding of signal molecule to membrane receptor initiates a series of changes which leads to a biological effect. Examples of typical signal transduction systems- G protein linked (fig. 11.7)- binding of a signal molecule to receptor causes activation of a G protein 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 to produce a second messenger ( second messengers- small, water soluble molecules or ions that are present in excess of the original signal chemical). These molecules are able 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 adenylyl cyclase and broken down by the enzyme phosphodiesterase.
Fig. 11.13 – G protein linked activation of adenylyl cyclase with subsequent production of cyclicAMP and activation of protein kinase A.
Calcium ions and inositol triphosphate- fig. 11.15; activation of phospholipase C causes breakdown of membrane phospholipids; inositol triphosphate IP3 is one of the products. This causes the endoplasmic reticulum to release calcium ions which act as a second messenger system.
Hormones –transported long distances by the circulatory system; generally are somewhat long-lasting in biological effects- minutes to a few hours.
Fig. 45.3- Chemical signaling overview
Some examples of endocrine glands and hormone action in mammals- the major points to keep in mind: (1) targets may be very distant from endocrine gland/neuroendocrine cells, (2) hormones are very specific; specificity is determined by the receptors in the target organ, (3) often there are hormones that have opposite or antagonistic effects to a particular hormone and (4) hormone secretion is under precise control.
Fig. 45.6- posterior and anterior pituitary gland; secretions of this gland are under direct control by a region of the brain known as the hypothalamus.
Posterior pituitary- neuroendocrine cells in the hypothalamus extend all the way into the 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 neurohormones that are released into blood vessels going to the anterior pituitary. These agents control the secretion of many of the hormones of the anterior pituitary including:
1. growth hormone- bone elongation, brain development
2. gonadotropins- follicle stimulating hormone and luteinizing hormone; targets are the ovary and testes
3. thyroid stimulating hormone (TSH) - stimulates thyroid to release thyroid hormone
4. adrenocorticotropic hormone- causes the cortical region of the adrenal gland to release 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 are derived from the amino acid tyrosine; both stimulate metabolism and are also essential for normal brain development. Both are under very tight control in terms of secretion by what are known as negative feedback loops (the accumulation of a product in a sequence inhibits its further production/release).
1. hypothalamus releases TRH (TSH releasing hormone) which causes the anterior pituitary to release TSH
2. 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 the hypothalamus
4. as thyroid hormone levels in the blood rise, it begins to also inhibit the release of TRH
5. the net effect is to avoid over secretion of thyroid hormone
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