Case study 2: Type 2 Diabetes1.
Whatcommonly performed tests would be able to confirm the likely diagnosis of thispatient, and what would normal results for those tests be? (5 marks)TheHbA1c test is the most commonly used test to identify type 2 diabetes and isrecommended by WHO now to help diagnose type 2 diabetes (WHO, 2011). The test shows your average plasma glucose levels over thepast 8 to 12 weeks. The value to diagnose diabetes is around 6.5% but if youfall below 6.5% you are not guaranteed to not be diabetic. At least 2 HbA1ctests must be done with both showing diabetic values for you to then bediagnosed with type 2 diabetes (WHO, 2011).
TheOGTT test is another commonly used test to diagnose type 2 diabetes. You mustavoid food and drinks for 8-12 hours before having the test. They will measureyour blood glucose before the test and after the test when you’re given a sweetglucose drink. They measure again after 2 hours (NHS, 2016). The test will determine whether you have impaired glucosetolerance (IGT) or diabetes (NHS, 2016). The values for someone without diabetes should be less than6mmol/l before the test and less than 7.8mmol/l two hours after the test. WithIGT the blood glucose levels should be 6-7mmol/l before the test and7.
9-11mmol/l two hours after the test (NHS, 2016). To be diagnosed with type 2 diabetes your blood glucose levelswill be more than 7mmol/l before the test and more than 11mmol/l two hoursafter the test (NHS, 2016). 2. Describe the endocrinological axis that regulates the release of insulin including details ofthe cells/tissues involved. (15 marks)Insulinsecretion is primarily controlled by a direct negative feedback system betweenthe pancreatic ?cells and the concentration of glucose in the blood flowing to them (Sherwood, 2016). ? cellsare in islet clusters and have capillaries surrounding them called fenestraewhich allow for unrestricted nutrient access and therefore allow for ? cells to sense changes in glucose levelsquickly (Fu, et al., 2016) (Sherwood, 2016). When insulin is at high levels it will promote blood glucoselevels back to normal and storage of glucose.
On the other hand, when bloodglucose levels are low insulin secretion is directly inhibited. Metabolism of insulinsecretion is shifted from being absorptive to post absorptive. This is thenegative feedback loop which maintains glucose levels without nerves and otherhormones being involved (Sherwood, 2016).
Glucose stimulates insulin secretion through changing themembrane potential of ? cells, which eventually lead to insulin secretion.GLUT2 which is expressed in ? cells is the first glucose sensor to beinteracted with. Glucose enters the ? cells via GLUT2-mediated diffusion (Fu, et al., 2016).
The glucose is then phosphorylated to glucose-6-phosphate byglucokinase. Glucose-6-phosphate undergoes oxidation which then generates ATP.The ATP generated causes the usually open K+ channels to close asATP binds to them. The voltage-gates Ca2+ channel is open at restingpotential. The decreased permeability of K+ ions leads todepolarization of ? cells due to the positively charged K+ being inhigh quantity (Sherwood, 2016). The depolarization of the membrane causes the voltage-gated Ca2+channel to open. Ca2+ enters the ? cells eventually triggeringexocytosis of secretory vesicles containing insulin. Insulin is then secreted.
There areother inputs which are involved in the regulation of insulin secretion. When youhave eaten a high protein meal your bloods amino acid level is high. This willdirectly stimulate the ? cells and increase insulin secretion. Amino acid entryinto cells increases as high insulin levels enhance this. This lowers bloodamino acid levels and promotes protein synthesis.
Amino acids generate ATP likeglucose does, so they increase insulin secretion the same way, using a negativefeedback loop. Incretinslike glucose-dependent insulinotrophic peptide (GIP) and glucagon-like peptide1 (GLP-1) stimulate pancreatic insulin secretion. This comes after there hasbeen a presence of food in the gastrointestinal tract (Sherwood, 2016).
Theautonomic nervous system directly influences insulin secretion (Sherwood, 2016). The parasympathetic and sympathetic nerve fibres innervate theislets. The presence of food in the digestive tract stimulates increasedparasympathetic activity and increased insulin secretion. Acetylcholine actsthrough the IP3-Ca2+ pathway. This and the GIP, GLP-1pathways are feedforward responses due to their anticipation of nutrientabsorption (Sherwood, 2016). Sympathetic stimulation and the increase in epinephrine inhibitinsulin secretion by decreasing cAMP. Allowing for blood glucose levels to rise (Sherwood, 2016).
3. Describe the cell signalling pathway activated by insulin during blood glucose regulation. (10 marks)Theinsulin receptor (IR) is a heterotrimeric bifunctional complex, consisting of 2extracellular ? subunits that bind insulin and 2 transmembrane ? subunits withtyrosine kinase activity (Chang, et al., 2005).
When insulin binds to a ? subunit it will inducetransphosphorylation of one ? subunit by another on specific tyrosine residuesin an activation loop. This results in increased catalytic activity of thekinase (Chang, et al., 2005). The next step is for the IR to undergo autophosphorylation whichis in the juxtamembrane regions and intracellular tail. The insulin receptorbecomes activated and then phosphorylates tyrosine residues on intracellularsubstrates like insulin receptor substrates (IRS), Shc isoforms and Gab-1.
Whenphosphorylated these substrates will interact with the SH2 domains thatspecifically recognize different phosphotyrosine motifs. Glucose uptake byinsulin will be mediated by phosphatidylinositol (PI) 3-kinase dependent andindependent pathway (Chang, et al., 2005). After the tyrosine is phosphorylated IRS proteins will interactwith the p85 regulatory subunit of PI3k which activate small G proteins bybinding to nucleotide exchange factors (Saltiel & Kahn, 2001). Phosphatidylinositol-3,4,5-triphosphate (PIP3) is phosphorylatedby PI3K.
It will regulate the localization and activity of many proteins. PI3Kis essential to glucose uptake and GLUT4 translocation. If it is blockedglucose uptake is no longer stimulated. PIP3 recruits and canactivate pleckstrin homology (PH) domain containing proteins like adaptermolecules, enzymes and their substrates. Ser/Thr kinase PDKI is one of these.
It phosphorylates and activates kinases like AKT1. PIP3 helps AKttravel to the plasma membrane using the PH domain. Akt will then phosphorylateother proteins at other subcellular locations Overexpression of a membranebound form of AKT in 3t3L1 adipocytes resulted in increased glucose transportand localization of GLUT4 to the plasma membrane (Chang, et al., 2005). Akt if interfered with inhibits GLUT4 translocation. ReducingAKT2 decreases insulin sensitivity and decreases glucose disposal (Chang, et al., 2005).
TheRas-MAPK pathway is also essential in the insulin signalling pathway. Shcactivates the Ras-MAPK pathway and this Shc-Grb2-SOS-Ras-Raf-MAPK pathwaycontrols cellular proliferation and gene transcription ( Boucher, et al., 2014). The activated receptors and IRS proteins will have docking sitesfor adaptor molecules that have SH2 domains like Shc and Grb2.
Gab-1 can bebound by the carboxy terminal SH3 domain of Grb2. The protein Son of theSevenless (SOS) will be bound by the amino terminal SH3 domain in proline-richregions. SOS is a guanine nucleotide exchange factor for Ras ( Boucher, et al., 2014). SOS will catalyse the switch of membrane bound Ras from itsinactive state being bound to GDP (Ras-GDP) to its active state bound to GTP(Ras-GTP). Ras-GTP will interact with and stimulate downstream effectors likeSer/Thr kinase Raf ( Boucher, et al.
, 2014). This stimulates the downstream target MEK1 and 2 which willphosphorylate and activate the MAP kinases ERK1 and 2. ERk1 and 2 are essentialto cell proliferation or differentiation, regulating gene expression orextranuclear events, such as cytoskeletal reorganization throughphosphorylation and activation of targets get in the cytosol and nucleus ( Boucher, et al.
, 2014).4. What are the physiological effects of insulin signalling activation that contribute to blood glucose regulation?(15 marks)Insulinis important to glucose, fatty acids and amino acids as it lowers theirpresence in blood and promotes their storage. When they enter the blood duringthe absorptive state, insulin promotes their cellular uptake and conversioninto glycogen, triglycerides and proteins. Insulin will increase the activityof glycogen synthase to achieve these effects. Insulin will also inhibitenzymes like hormone sensitive lipase which breakdown triglycerides back intofree fatty acids and glycerol (Sherwood, 2016).Insulinhas 4 effects on carbohydrates to lower blood glucose levels and promotecarbohydrate storage. Insulin facilitates glucose transport into most cellsglucose transport into most cells.
Glycogenesis, the production of glycogenfrom glucose, in both skeletal muscle and liver is stimulated by insulin.Insulin will inhibit glycogenolysis, the breakdown of glycogen into glucose. Itwill also inhibit gluconeogenesis. This is the conversion of amino acids intoglucose in the liver. The amino acids present in blood will be decreased, soless is available for gluconeogenesis and the hepatic enzymes needed forconverting amino acids into glucose inhibited.Tolower blood fatty acid levels and promote triglyceride storage insulin uses 4methods to this. Fatty acid entry from blood into adipose tissue cells isenhanced.
Using GLUT4 the transport of glucose into adipose tissue cells willincrease. Glucose is a precursor for the formation of fatty acids and glycerol,which are used for triglyceride synthesis. Lipolysis will be inhibited toreduce the number of fatty acids releasing into blood. They all promote fattyacid and glucose removal and storage of triglycerides (Sherwood, 2016).Tolower blood amino acid levels and enhance protein synthesis insulin willpromote 3 effects. Active transport of amino acids from the blood into musclesand other tissues is promoted.
This will decrease the blood amino acid leveland aid protein synthesis inside cells. Protein synthesizing machinery isstimulated so more amino acids are incorporated into protein. Proteindegradation is inhibited. This leads to a protein anabolic effect and showsinsulin is essential for growth (Sherwood, 2016).
5. What initial treatments would be recommended for this patient? How do these treatmentswork? (5 marks)Forthis patient initially, they will be recommended to eat more healthily tocontrol their glucose levels without medication if possible. So, they shouldeat more fibre and fruit and vegetables to lower their glucose levels. Theywill be recommended to lose 5-10% of their body fat as well if they’reoverweight, this again can lower blood glucose levels.
The first medication they are prescribed ifthis doesn’t work is metformin. This reduces the amount of glucose released bythe liver. Sulphonylureas (NHS, 2016) can be used to increase insulin secretion from the pancreas.
Examples include gliclazide. Gliptins (DPP-4 inhibitors) can be prescribed too.They prevent the breakdown of GLP-1 which helps the body produce insulin. Thisprevents high blood glucose levels. Insulin injections can be used too withusually 2-4 injections a day lowering blood glucose levels.
If hypoglycaemic,glucagon can be administered to help with tiredness and to increase bloodglucose levels (NHS, 2016). These treatments can help relieve symptoms like tiredness,excessive thirst and frequent urination at night. This is because theyare mainly due to high blood glucose levels.
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