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The Hepatocytes play an extremely important role in the me- reduced complex combines with molecular oxygen to form an tabolism of drugs and xenobiotics—compounds that are oxygenated intermediate trusted eriacta 100 mg. One atom of the molecular oxygen foreign to the body cheap eriacta 100 mg fast delivery, some of which are toxic purchase eriacta 100 mg fast delivery. The kidneys ultimately dispose of these substances buy eriacta 100 mg lowest price, but for effective elimination purchase 100mg eriacta visa, the drug or its metabolites must be made hydrophilic (polar, water-soluble). This is because re- absorption of a substance by the renal tubules is dependent on its hydrophobicity. The more hydrophobic (nonpolar, lipid-soluble) a substance is, the more likely it will be reab- sorbed. Many drugs and metabolites are hydrophobic, and the liver converts them into hydrophilic compounds. The Liver Converts Hydrophobic Drugs and Xenobiotics to Hydrophilic Compounds Two reactions (phase I and II), catalyzed by different en- zyme systems, are involved in the conversion of xenobi- otics and drugs into hydrophilic compounds. In phase I re- actions, the parent compound is biotransformed into more polar compounds by the introduction of one or more polar groups. The enzymes involved are mostly located in the smooth ER; some are located in the cytoplasm. For example, alcohol dehydrogenase is located Phase I reactions in the metabolism of FIGURE 28. CHAPTER 28 The Physiology of the Liver 517 volatile fatty acids), whereas cellulose is not well digested then combines with two H and two electrons to form water. Only a small amount of long-chain fatty 3 acids, bound to albumin, is transported by the portal blood; P450 -drug complex and is transferred from the cy- 3 the most is transported in intestinal lymph as triglyceride- tochrome P450 to the drug molecule. The drug product with an oxygen atom incorporated is released from the com- rich lipoproteins (chylomicrons). The cytochrome P450 released can then be recycled for the oxidation of other drug molecules. In phase II reactions, the phase I reaction products un- The Liver Is Important in dergo conjugation with several compounds to render them Carbohydrate Metabolism more hydrophilic. Glucuronic acid is the substance most The liver is extremely important in maintaining an ade- commonly used for conjugation, and the enzymes involved quate supply of nutrients for cell metabolism and regulating are the glucuronyltransferases. Glucose is taken up by hepatocytes by a facilitated carrier-mediated process Aging, Nutrition, and Genetics and is converted to glucose 6-phosphate and then UDP- Influence Drug Metabolism glucose. UDP-glucose can be used for glycogen synthesis, The enzyme systems in phase I and II reactions are age-de- or glycogenesis. These systems are poorly developed in is the major precursor of glycogen. However, recent evi- human newborns because their ability to metabolize dence seems to indicate that the lactate in blood (from the any given drug is lower than that of adults. Older adults also peripheral metabolism of glucose) is also a major precursor have a lower capacity than young adults to metabolize drugs. Insufficient protein in the diet to Glycogen is the main carbohydrate store in the liver, and sustain normal growth results in the production of fewer of may amount to as much as 7 to 10% of the weight of a nor- the enzymes involved in drug metabolism. The glycogen molecule resembles a tree It is well known that drug-metabolizing enzymes can be with many branches (see Fig. Glucose units are linked induced by certain factors, such as polycyclic aromatic hy- via -1,4- (to form a straight chain) or -1,6 (to form a drocarbons. Persons who smoke inhale polycyclic aromatic branched chain) glycosidic bonds. The advantage of such a hydrocarbons, increasing the metabolism of certain drugs, configuration is that the glycogen chain can be broken down such as caffeine. Briefly, drug me- During fasting, glycogen is broken down by glyco- tabolism by the liver can be controlled by a single gene or genolysis. The enzyme glycogen phosphorylase catalyzes several genes (polygenic control). Careful study of the me- the cleavage of glycogen into glucose 1-phosphate. Glyco- tabolism of a certain drug by the population can provide gen phosphorylase acts only on the -1,4-glycosidic bond, important clues as to whether its metabolism is under sin- gle gene or polygenic control.

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They also penetrate into the lower in- of neurons that originate in the supraoptic nuclei and par- fundibular stem buy eriacta 100 mg low cost, where they form another important capil- aventricular nuclei of the hypothalamus cheap eriacta 100mg otc. The capillaries of this network converge into these neurons are large compared to those of other hypo- short hypophyseal portal vessels buy discount eriacta 100mg on-line, which also deliver blood thalamic neurons; hence generic 100mg eriacta, they are called magnocellular neu- into the sinusoids of the anterior pituitary discount 100mg eriacta visa. The hormones arginine vasopressin (AVP) and oxy- blood supply to the anterior lobe of the pituitary gland is tocin are synthesized as parts of larger precursor proteins known as the hypophyseal portal circulation. Prohor- When a neurosecretory neuron is stimulated to secrete, mones are then packaged into granules and enzymatically the releasing hormone is discharged into the hypophyseal processed to produce AVP and oxytocin. Releasing hormones travel transported down the axons by axoplasmic flow; they accu- only a short distance before they come in contact with their mulate at the axon terminals in the posterior lobe. Only the amount of releas- Stimuli for the secretion of posterior lobe hormones may ing hormone needed to control anterior pituitary hormone be generated by events occurring within or outside the secretion is delivered to the hypophyseal portal circulation body. These stimuli are processed by the central nervous by neurosecretory neurons. Consequently, releasing hor- system (CNS), and the signal for the secretion of AVP or mones are almost undetectable in systemic blood. Secretory granules containing the hor- synthesis and secretion of a particular anterior pituitary CHAPTER 32 The Hypothalamus and the Pituitary Gland 583 M hormone. Corticotropin-releasing hormone (CRH), thy- rotropin-releasing hormone (TRH), and growth hor- 2 Hypothalamus mone-releasing hormone (GHRH) stimulate the secretion Third and synthesis of ACTH, TSH, and GH, respectively ventricle (Table 32. Luteinizing hormone-releasing hormone 1 (LHRH), also known as gonadotropin-releasing hormone Superior (GnRH), stimulates the synthesis and release of FSH and hypophyseal LH. In contrast, somatostatin, also called somatotropin artery release inhibiting factor (SRIF), inhibits GH secretion. All Median of the releasing hormones are peptides, with the exception eminence of dopamine, which is a catecholamine that inhibits the Long portal synthesis and secretion of PRL. Releasing hormones can be Stalk vessels produced synthetically, and several are currently under Anterior study for use in the diagnosis and treatment of diseases of lobe the endocrine system. For example, synthetic GnRH is Posterior Hormone- now used for treating infertility in women. These signals are gen- Hormone Hormone erated by external events that affect the body or by changes occurring within the body itself. For example, sensory nerve excitation, emotional or physical stress, biological rhythms, changes in sleep patterns or in the sleep-wake cy- cle, and changes in circulating levels of certain hormones or Vein metabolites all affect the secretion of particular anterior pi- Short portal Vein tuitary hormones. Signals generated in the CNS by such vessels events are transmitted to the neurosecretory neurons in the Inferior hypothalamus. Depending on the nature of the event and hypophyseal the signal generated, the secretion of a particular releasing artery hormone may be either stimulated or inhibited. In turn, this response affects the rate of secretion of the appropriate an- terior pituitary hormone. M represents a magnocellular neuron releasing AVP or oxytocin at its axon terminals into capillaries that give HORMONES OF THE POSTERIOR PITUITARY rise to the venous drainage of the posterior lobe. Neurons 1 and 2 are secreting releasing factors into capillary networks that give Arginine vasopressin (AVP), also known as ADH, antidi- rise to the long and short hypophyseal portal vessels, respec- uretic hormone, and oxytocin are produced by magnocel- tively. Releasing hormones are shown reaching the hormone-se- lular neurons in the supraoptic and paraventricular nuclei of creting cells of the anterior lobe via the portal vessels. This action of AVP works to counteract the conditions that stimulate its secretion. For example, re- Proteolytic cleavage ducing water loss in the urine limits a further rise in the os- molality of the blood and conserves blood volume. Low blood AVP levels lead to diabetes insipidus and the exces- AVP + NP-II + GP sive production of dilute urine (see Chapter 24). Oxytocin Stimulates the Contraction of Smooth AVP, arginine vasopressin; NP-II, neurophysin II; GP, glycoprotein. Muscle in the Mammary Glands and Uterus Two physiological signals stimulate the secretion of oxy- tocin by hypothalamic magnocellular neurons. The axons of these neurons form ing stimulates sensory nerves in the nipple. Afferent nerve the infundibular stem and terminate on the capillary net- impulses enter the CNS and eventually stimulate oxytocin- work in the posterior lobe, where they discharge AVP and secreting magnocellular neurons. Oxytocin stimulates the contraction of myoepithe- each consisting of nine amino acid residues.

As we become drowsy and pass into sleep the EEG waves become more synchronised with 8±12 Hz alpha waves (b) order 100mg eriacta mastercard, sleep spindles then appear (c) before the EEG becomes even more synchronised with slow (about 1±2 Hz) high-voltage waves characteristic of deep slow-wave sleep (SWS) discount eriacta 100mg. About every 90 min this pattern is disrupted and the EEG becomes more like that in arousal (d) except that the subject remains asleep buy generic eriacta 100mg on line. This phase of sleep is also characterised by rolling buy eriacta 100 mg otc, rapid eye movements cheap eriacta 100 mg mastercard, the so-called REM sleep. These tracings have been drawn to show the main features of the different EEG phases of sleep and as such are much simpler than those that are actually recorded focus of sleep research but is usually complemented by measurements of muscle tone (the electromyogram, EMG) and eye movements (the electro-occulogram, EOG) which also show marked changes during the sleep cycle. When we are aroused and awake, the EEG is random (desynchronised) with multiple high-frequency (of at least 15 Hz), low-amplitude g(gamma)-wave forms. As we become drowsy and close our eyes, the EEG becomes more synchronised and a clear rhythm emerges (stage 1 sleep):this is a(alpha)-rhythm which has a frequency of 8±12 Hz. The former are single spikes whereas the latter are short trains of pulses (12±14 Hz). Progressing still further into the sleep state (as assessed by the EMG and EOG), the EEG becomes even more synchronised so that slower (about 1±2 Hz) and larger waves become more prominent. At the same time as all these changes are developing, the threshold for arousal by sensory stimuli increases. SLEEP AND WAKING 483 It was not until much later (1953) that another phase of the sleep cycle was discovered. However, in adults, other physiological changes that occur during REM sleep are quite different from those of stage 1. In particular, there is a flaccid paralysis of the limb muscles together with a loss of fine control of body temperature and other homeostatic mechanisms. It is often maintained that dreaming is restricted to these periods of REM sleep, which occur some three or four times during the night, each lasting about 30 min. However, it is now thought that dreams also occur during SWS but that these are more logical and more consistent with normal life events than are those occurring during REM sleep. This sleep pattern, seen in adults, takes some time to develop and appears in infants only around 6 months to one year after birth. Instead, as new parents will testify, young babies have a sleep cycle that lasts only around 3±6 h. The functions of these different phases of sleep are not at all clear but chronic sleep deprivation does eventually lead to death. It seems to be the slow-wave component of sleep (SWS) that is vital and it is thought to serve a restorative purpose. This would be consistent with its greater occurrence during the early stages of the sleep cycle when hormone secretion supports anabolic metabolism. If subjects are wakened every time they enter a period of REM sleep (evidenced by the EEG) there appears to be no overt harmful effect on their behaviour. In fact, REM sleep deprivation has even been used, with some claims of success, as a treatment for minor depression. However, there is an unproven belief that REM sleep is important for memory consolidation. ORIGIN OF THE EEG It appears that the voltage waves recorded in the EEG represent the summation of synaptic potentials in the apical dendrites of pyramidal cells in the cortex. These cells generate sufficient extracellular current for it to reach, and be recorded from, the cranium and scalp. Although these waves originate from the cortex rather than the SCN, the distinctive REM and non-REM phases of sleep still remain after destruction of the SCN but they then occur randomly over the 24-h cycle. This is a further indication that the SCN is at least partly responsible for setting the overall circadian rhythm of the sleep cycle. The more synchronised the activity of the cortical neurons, the greater the summation of currents and the larger and slower the EEG wave, as in the sleep pattern (Fig. While there are some dissociations between EEG pattern and behavioural states, the EEG offers one way of determining experimentally the pathways (and neurotransmitters) that control arousal and sleep, and can be regarded as an important objective measurement of the cortical correlates of sleep and waking. However, the rhythm and appearance of spindles in earlier phases of the sleep cycle do depend on links with the thalamus (see Steriade 1999). Unlike stimulation of the specific sensory relay nuclei in the thalamus, which only affects neurons in the appropriate sensory areas of the cortex, the non- specific nuclei can produce responses throughout the cortex and may not only control, but also generate, cortical activity. Certainly, in vitro studies show that neurons of the non-specific reticular thalamic nucleus (NspRTN) can fire spontaneously at about 8± 12 Hz (equivalent to EEG a-rhythm) or lower, and that low-frequency stimulation of this area can induce sleep. Maintenance of these frequencies relies on the degree of depolarisation of the thalamic neurons (Jahnsen and Llinas 1985) and this, in turn, depends on the nature and intensity of their afferent inputs. The NspRTN and other thalamic nuclei receive reciprocal inputs from the cortex and it is possible that it is the ensuing oscillations in neuronal activity in this circuit between the cortex and thalamus that give rise to the sleep spindle waves in stages 2±4.

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