Ординатура / Офтальмология / Английские материалы / Eye Essentials Diabetes and the Eye_Steele, Steel_2008

Type 1 diabetes mellitus

Fig. 3.3a Diagrammatic illustration of the structure of the islets of Langerhans.

Conversely adrenergic sympathetic nerves inhibit insulin and stimulate glucagon secretion. Other nerves that originate in the pancreas contain peptides such as vasoactive intestinal peptide (VIP).This is responsible for the stimulation and release of all islet hormones and neuropeptide Y (NPY) which inhibits insulin secretion. Exactly how all these different neuropeptides all interact is still not fully understood.

The regulation of insulin secretion

The most influential factor in the regulation of insulin secretion is the change in blood glucose concentrations.The responses to

Insulin

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Constitutive pathway

Proinsulin

> insulin

Fig. 3.3b Diagrammatic representation of the intra-cellular production of insulin.

these changes are both sensitive and rapid — usually within 30 seconds of the arrival of a glucose stimulus.

Glucose-stimulated insulin release is biphasic — a rapid first phase lasting some 5–10 minutes followed by a more prolonged second phase which continues for as long as the duration of the stimulus (see Fig. 3.4).

The half-life of insulin in the circulation in humans is about 5 minutes.

The effects of insulin

Insulin’s physiological effects are far reaching and comprehensive and affect the metabolism of carbohydrates, fats and proteins. They can conveniently be divided into rapid, intermediate and delayed.These are shown in Box 3.1.

The effects of insulin on adipose tissue, muscle and liver are detailed in Box 3.2.

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Raised glucose levels

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1st phase

Insulin level

Basal

2nd phase

05

Time (min)

Fig. 3.4 Insulin biphasic half-life of insulin released from islet cells.

Thus all the effects are anabolic. It encourages the storage of carbohydrate as glycogen and the building up of lipid and protein stores.

Box 3.1 The principal actions of insulin

Rapid (seconds)

Increased transport of glucose, amino acids and potassium into insulin-sensitive cells.

Intermediate (minutes)

Stimulation of protein synthesis.

Inhibition of protein breakdown.

Activation of glycolytic enzymes and glycogen synthase.

Inhibition of gluconeogenic enzymes.

Delayed (hours)

Increase in the production of lipogenic enzymes.

Insulin

Box 3.2

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Effects of insulin on: Adipose tissue

Increased glucose entry. Increased fatty acid synthesis.

Increased glycerolphosphate synthesis. Increased triglyceride deposition. Activation of lipoprotein lipase. Inhibition of hormone-sensitive lipase. Increased potassium uptake.

Muscle

Increased glucose entry.

Increased glycogen synthesis.

Increased amino acid uptake.

Increased protein synthesis.

Decreased protein catabolism.

Decreased release of gluconeogenic amino acids.

Increased ketone uptake.

Increased potassium uptake.

Cell growth.

Liver

Decreased ketogenesis. Increased protein synthesis. Increased lipid synthesis.

Decreased glucose output due to decreased gluconeogenesis and increased glycogen synthesis.

Glucagon

The actions of glycogen are exactly the opposite to that of insulin i.e. they are all catabolic. Its main target organ is the liver where it stimulates hepatic glucose output by breaking down glycogen stores (glycogenolysis) and by stimulating glucose synthesis by increasing the conversion of gluconeogenic amino acids into glucose (gluconeogenesis).

It also stimulates the uptake of amino acids for the process of gluconeogenesis and stimulates fatty acid oxidation and ketone

Type 1 diabetes mellitus

formation, an alternative source of fuel for the brain when

28glucose is not available.

Insulin receptors

Cells which are insulin sensitive have a specific insulin receptor on their outer cell surface. Insulin binds to this and then triggers its effects. Not all tissues are insulin sensitive.The brain is the most unresponsive; other non-responsive organs are the intestine and kidney.

Glucose transport

Glucose enters cells by a process of facilitated diffusion, or in the kidneys and intestine, by secondary active transport with sodium. Insulin facilitates glucose entry by increasing the number of glucose transporters in the cell membrane.

Clearly any interference with insulin receptors and/or glucose transport could impair the action on insulin.

Other influences on blood glucose regulation

Insulin is the only hormone that lowers blood glucose concentrations. Corticosteroids, adrenaline, growth hormone and glucagon can all increase blood glucose levels.Thus glucose homeostasis is dependent on striking a right balance between the effects of insulin and the effects of the hormones which raise blood glucose levels.

Type 1 diabetes management

Diet and lifestyle modification are the cornerstones of management in both type 1 and type 2 diabetes.

In type 1 diabetes, dietary recommendations should be matched to a person’s overall lifestyle including occupation. For instance, the energy needs of someone doing heavy manual work are much greater than those of a sedentary worker.

Type 1 diabetes management

The diet must also meet the macro and micro nutritional

needs of the individual in order to achieve normoglycaemia and 29 reduce the risk of cardiovascular disease.

It is customary to reduce the intake of saturated fat, to provide no more than 30% of energy requirements and to replace this by monoand polyunsaturated fats.

Carbohydrates which rapidly raise the blood glucose levels after their ingestion are discouraged and replaced by those with a lower glycaemic index such as whole grain foods, legumes, fruit and nuts.

Fish oils, which are rich in n-3 fatty acids, lower triglyceride levels (which tend to be raised in people with diabetes) so two or three servings of oily fish weekly are beneficial.

Alcohol has a protective effect on cardiovascular disease, if taken in moderate amounts, by raising high density lipoprotein (HDL) cholesterol (the protective cholesterol), decreasing coagulation factors and enhancing insulin sensitivity, so light to moderate drinking should not be discouraged.

Practical food recommendations

●Quench thirst with water or non-sugary drinks.

●Have regular meals.

●Avoid fried and very sugary foods.

●Eat plenty of legumes, whole grain foods or brown rice as part of main meals.They have a low glycaemic

index (GI).

●Limit the consumption of starchy foods such as mashed potatoes and white bread.

●Five portions of fruit and vegetables each day.

●For snacks, avoid biscuits or confectionary — use fruit and nuts.

●Limit the intake of red meat, eggs, liver and high fat dairy products. Instead, use lean meats, poultry (without skin) and low fat dairy products.

●Cook and fry with natural vegetable oils.

●Use soft instead of hard margarine.

●Do not over eat.

Type 1 diabetes mellitus

Smoking

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This should be strongly discouraged. It is a major risk factor for cardiovascular disease (CVD) and lung cancer. Stopping smoking is as effective as lowering cholesterol and treating hypertension in reducing CVD.

Exercise

Regular exercise should be encouraged. It improves glycaemic control and lowers long-term morbidity and mortality. Patients should carry measures to combat hypoglycaemia should this occur.

The use of insulin

Type 1 diabetes is a disease of insulin deficiency, and insulin replacement is necessary for survival. Insulin is normally given by a subcutaneous injection.

In non-diabetic people, blood glucose levels are maintained within a fairly narrow range by a combination of endogenous insulin and regulatory hormones.

The current approach to the use of insulin in people with type 1 diabetes is to recreate this physiological state as closely as possible.

To do this there are a plethora of insulin preparations available but basically they can be divided into three types, based on their rapidity and duration of action (see Table 3.1).

Analogue insulins

Analogue insulins have been modified to make them closely resemble naturally occurring human insulin and to make achieving glucose control closely resemble the normal physiological state. The word ‘analogue’ conveys the meaning of similar to, in this case similar to human insulin, but not an exact replicate. Like human insulins they are genetically engineered.There are rapid

Type 1 diabetes management

Table 3.1 Some commonly used insulin preparations

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Time of action (hours)

and long-acting analogue insulins and there are mixtures of the two. Examples of rapid-acting analogue insulins are:

●Insulin Lispro (Humalog).

●Insulin Aspart (Novo Rapid).

●Insulin Glulisine (Aspiara).

Long-acting analogues include:

●Insulin Glargine (Lantus).

●Insulin Determir (Levemir).

The trade names are given in brackets.

Type 1 diabetes mellitus

Currently there are three analogue mixtures which are: 32 Humalog Mix 25, Humalog Mix 50 and Novo Mix 30.

The rapid-acting analogue insulins take effect quicker than human or animal short-acting insulins.The latter are usually taken 15–30 minutes before the meal whereas the analogues can be taken just before or during a meal.This is very useful if a person wants to have greater flexibility in choosing meal times.

Long-acting analogues are generally taken once a day at bed time and tend to have a relatively smooth action lasting between 16 and 24 hours.

Both rapidand long-acting analogue insulins are well suited for basal bolus regimens of insulin delivery (see below). Analogue insulins are not superior to other forms of insulin. Their benefit is that they offer greater flexibility.

Regimes

The two most commonly used insulin regimes are:

●an evening basal bolus (long-acting insulin) with injections of short-acting insulin before each main meal;

●injections of a mixture of short and intermediate insulin, the first before breakfast, the second before the evening meal.

Injection sites

These include the:

●abdomen;

●the outer aspect of the thigh;

●the upper outer buttock.

Injection devices

There are a range of injection devices all designed to make giving the injection easier and less painful.

Type 1 diabetes management

Side effects of insulin therapy

effect is hypoglycaemia.

Other rarer side effects

Lipohypertrophy — fatty bumps at injection sites.These are most likely to occur when insulin is constantly injected into the same site. Rotation of sites helps avoid this.

Some weight gain is inevitable as insulin is primarily an anabolic hormone.

Lipoatrophy — hollowed out areas mainly due to immune responses to impurities in insulin. Rare with newer, more pure insulins.

Glucose monitoring

At the start of insulin therapy, people with diabetes are also taught how to monitor their blood glucose levels.

The blood glucose test is normally prepared from a drop of capillary blood obtained from the side of a fingertip using a modern spring-loaded device.

Tests are normally done before meals and before going to bed. However, if before-meal tests are reasonable but the HbA1C is raised, it is then usual practice to add post-prandial monitoring about 90 minutes after a meal.The frequency

of glucose monitoring should be largely determined by the person with diabetes hopefully taking into account professional advice.

Target values for blood glucose levels

These are: 4.0–7.0 m.mols/l before meals and 6.0–9.0 m.mols/l after meals.