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Enhanced renal ammonia synthesis and excretion 1) Hydration of CO2 in the cells buy aleve 500 mg low cost, forming H2CO3 and is a lifesaving adaptation because it allows the kidneys to yielding H for secretion remove large H excesses and add more new HCO3 to 2) Dehydration of H2CO3 to H2O and CO2 in the the blood. Also, the excreted NH4 can substitute in the proximal tubule lumen, an important step in the reabsorp- urine for Na and K , diminishing the loss of these cations. In the proximal tubule, the two ions Several factors influence the renal excretion of H , includ- are directly linked, both being transported by the Na /H ing intracellular pH, arterial blood PCO , carbonic anhy- exchanger in the luminal plasma membrane. Enhanced Na reabsorp- drase activity, Na reabsorption, plasma [K ], and aldos- terone (Fig. The avid renal reabsorption of Na observed in states of volume depletion is accompanied by a factor influencing the secretion and, therefore, the excretion parallel rise in urinary H excretion. An increase in PCO increases the [K ] influence the renal excretion of H. A fall in plasma 2 2 [K ] favors the movement of K from body cells into in- formation of H from H2CO3, leading to enhanced renal terstitial fluid (or blood plasma) and a reciprocal move- H secretion and excretion—a useful compensation for any condition in which the blood contains too much H CO. In the kidney tubule cells, these 2 3 (This will be discussed later, when we consider respiratory movements lower intracellular pH and increase H se- acidosis. K depletion also stimulates ammonia synthesis 2 and, consequently, less complete reabsorption of filtered by the kidneys. The result is the complete reabsorption HCO and a loss of base in the urine (a useful compensa- of filtered HCO3 and the enhanced generation of new 3 tion for respiratory alkalosis, also discussed later). Consequently, hypokalemia (or a decrease in body K stores) leads to increased plasma [HCO3 ] Carbonic Anhydrase Activity. Hyperkalemia (or excess K in the hydrase catalyzes two key reactions in urinary acidification: body) results in the opposite changes: an increase in in- tracellular pH, decreased H secretion, incomplete reab- sorption of filtered HCO3 , and a fall in plasma [HCO3 ] (metabolic acidosis). Aldosterone stimulates the collecting ducts to secrete H by three actions: + 1) It directly stimulates the H -ATPase in collecting Increased H plasma H+-ATPase duct -intercalated cells.
For experimental detail see text Thus B Bmax B À X K K If B/X is plotted against B (the Scatchard plot) it should give a straight line (Fig discount aleve 250mg with amex. In many binding studies the relative abilities of a series of unlabelled drugs to displace a labelled ligand from a particular receptor is taken as a guide to their affinity for that receptor. This is normally represented as Ki, the concentration of drug required to displace half of the labelled ligand. Its accuracy depends on the chosen ligand only binding to the receptor it is intended to study and no other receptor. It must be emphasised that binding studies only measure the ability of a drug to combine with a receptor, they do not indicate whether it is an agonist or antagonist. Also compared with an antagonist the binding of an agonist may be affected in an uncertain manner by the change in state caused by the activation of the receptor. DRUG ANTAGONISM One drug can overcome the effect of another or reduce the activity of an endogenously released and active substance such as a neurotransmitter, either by competing with that substance for its receptor site (receptor antagonism) or stimulating a different receptor to induce an opposing effect (physiological or functional antagonism). The former may be regarded as true antagonism for in the latter case both drugs are actually agonists. It is epitomised by the use in asthma of beta adrenoceptor agonists like salbutamol to dilate bronchi that have been constricted by a cocktail of local mediators such as PHARMACOLOGY AND DRUG EFFECTS 109 110 NEUROTRANSMITTERS, DRUGS AND BRAIN FUNCTION histamine, acetylcholine and kinins. In the CNS the inhibitory NT (GABA) could be regarded as the physiological antagonist of the excitatory NT (glutamate). When the agonist and antagonist compete for the same receptor the binding of the agonist and the response it produces are both reduced. Thus to obtain the same response in the presence, as in the absence of antagonist, the concentration of agonist must be increased and over a range of agonist concentrations this results in a parallel shift to the right in the position of its DRC (Fig. The degree of this shift, the amount by which the agonist concentration has to be increased in order to produce the same response in the presence as in the absence of the antagonist, is known as the dose ratio (r). The larger this ratio, the greater the shift in the DRC and the more potent is the antagonist.
To some extent cheap aleve 500 mg with amex, this proposal is supported by microdialysis studies of changes in 5-HT efflux in the terminal fields of 5-HT neurons. A single swim stress also fails to increase 5-HT efflux in the medial prefrontal cortex of rats. Indeed, it would be interesting to know whether this reflects any long-term influence of 5-HT-moduline on regulation of 5-HT release (see above). Also, 5-HT efflux is increased in the Raphe nuclei and their terminal fields after handling of rats (Adell, Casanovas and Artigas 1997), hypo- glycaemic shock (Vahabzadeh, Boutelle and Fillenz 1995) and even during a conditioned fear response in which animals actually freeze (i. Finally, there is compelling evidence that 5-HT transmission in the amygdala affects emotional, rather than merely motor, components of anxiety (see Chapter 19). Overall, it remains to be seen whether or not changes in the release of 5-HT in the terminal field parallel changes in the firing rate of neurons in the Raphe nuclei. However, this question might be resolved by the recent discovery of a subpopulation of 5-HT neurons in the Raphe nuclei that is affected by aversive stimuli. It is proposed that these (presumed) 5-HT neurons form a distinct mesocorticolimbic group that, unlike neurons in other zones of the DRN and MRN, contribute to the stress response in vivo (Lowry et al. If so, this could explain the neuronal origin of the increase in 5-HT efflux in forebrain areas during stress and could suggest that certain 5-HT neurons have an important role in the stress response rather than merely governing motor activity. THE ROLE OF 5-HT IN GOVERNING FOOD INTAKE Obviously, regulation of food intake depends on many neurotransmitters and hormones but this final section will outline the role played by central 5-HT transmission in this process. It had been the belief for some time that increased 5-HT transmission in the brain reduces food intake (Blundell 1977) and this certainly explains the satiety in rats that follows infusion of 5-HT into the paraventricular nucleus (PVN) of the hypothalamus. However, recent studies using microdialysis have found that 5-HT efflux in the lateral hypothalamus is itself increased by food intake, suggesting the existence of a feedback control system. In fact, because the increase in 5-HT efflux is greater in genetically obese rats than in their lean counterparts, it has been proposed that there is a deficiency in the 5-HT inhibition of food intake in obesity. The increase in food intake induced by 5-HT1A receptor agonists is entirely consistent with the view that increased 5-HT transmission in the brain reduces food intake (hypophagia) since the activation of 5-HT1A autoreceptors in the Raphe nuclei would ensure a reduction in 5-HT release from the nerve terminals. Of course, this explanation does not tackle the question of what role, if any, is served by postsynaptic 5-HT1A receptors and, in fact, recent studies suggest that links between 5-HT1A receptor activation and food intake are far more complex than originally proposed.
Instead discount 250 mg aleve otc, they are formed by the chemical modification of tyrosine residues in the peptide Capillary structure of thyroglobulin as it is secreted by the follicular cells into the lumen of the follicle. Therefore, the T4 and T3 formed by this chemical modification are actually part of the amino acid sequence of thyroglobulin. The high concentration of thyroglobulin in the colloid provides a large reservoir of stored thyroid hormones for Parafollicular cell later processing and secretion by the follicle. The synthesis of T4 and T3 is completed when thyroglobulin is retrieved A cross-sectional view through a portion of through pinocytosis of the colloid by the follicular cells. Once inside follicular cells, the io- HO O COOH dide ions diffuse rapidly to the apical membrane, where they are used for iodination of the thyroglobulin precursor. A typical thyroglobulin molecule contains only H H 20 to 30 atoms of iodine. The iodination of thyroglobulin is catalyzed by the en- HO O C C COOH zyme thyroid peroxidase, which is bound to the apical membranes of follicular cells. Thyroid peroxidase binds H NH2 an iodide ion and a tyrosine residue in the thyroglobulin 5 precursor, bringing them in close proximity. The enzyme oxidizes the iodide ion and the tyrosine residue to short- Triiodothyronine (T3) lived free radicals, using hydrogen peroxide that has been generated within the mitochondria of follicular cells. The numbering of the iodine atoms on free radicals then undergo addition. A second iodine atom may be added to a MIT residue by this same to its constituent amino acids, releasing T4 and T3 mole- enzymatic process, forming a diiodotyrosine (DIT) cules from their peptide linkage. Iodinated tyrosine residues that are close together in the thyroglobulin precursor molecule undergo a coupling reaction, which forms the iodothyronine structure. Thy- Follicular Cells Synthesize roid peroxidase, the same enzyme that initially oxidizes Iodinated Thyroglobulin iodine, is believed to catalyze the coupling reaction The steps involved in the synthesis of iodinated thyroglob- through the oxidation of neighboring iodinated tyrosine ulin are shown in Figure 33.