Insulin and Oral Hypoglycaemic Agents

Summary

A number of metabolic pathways are open to glucose-6-phosphate following its conversion via glucose from ingested carbohydrates. Examination of these pathways shows that close interrelationships exist between all food constituents — carbohydrate, fat and protein. Insulin, in concert with other hormones, growth hormone, cortisol and catecholamines, affects all these pathways — either directly or indirectly — and serves a major integrative role in the intermediary metabolism of muscle and adipose tissue as well as having important effects in the liver.

Factors regulating insulin secretion are many and varied, again reflecting the extensive metabolic role of insulin. Secretion of insulin facilitates storage of food; low levels of secretion permitting mobilisation of stores in periods of fasting.

Although there is general agreement that the biochemical disturbance of diabetes mellitus is due to insulin deficiency, there is lack of agreement on the genesis of this deficiency. Neither is there clear understanding of the mode of action of oral hypoglycaemic agents.

The sulphonylureas have been shown in acute experiments to have pancreatic β-cell stimulating effects which are reflected by a rise in plasma insulin in vivo, or in the medium in vitro. There is, however, some uncertainty as to this effect with long-term administration. A diminution in hepatic glucose output has been shown, but it is uncertain whether this is the result of the insulin secretion induced by the sulphonylurea, or a direct hepatic effect of the drug. A synergistic effect of sulphonylureas on the action of insulin has been presented as evidence for a second effect of this group of drugs on disordered carbohydrate metabolism — a direct peripheral tissue effect.

The biguanides have the unusual property of inducing hypoglycaemia only in the diabetic. The possible sites of the action, evidence for each of which has been well documented, are an increase in anaerobic glycolysis (along the glycolytic pathway) resulting in increased peripheral glucose uptake; increased peripheral glucose utilisation in the presence of insulin; and impaired absorption of glucose from the gut.

A spectrum of characteristics of hypoglycaemic activity has been built into a succession of insulin preparations by modifying the speed of onset, time of maximum effect and duration of action of crystalline animal insulin. Thus for a given clinical situation it is possible in most cases to ‘tailor make’ the insulin to individual needs.

Variations in the chemical structure of the sulphonylurea and biguanide oral hypoglycaemic agents has lead to a variety of drugs with different potencies, pharmacokinetic properties and characteristics of hypoglycaemic activity. Most of the compounds are metabolised in the liver and excreted in the urine; but others, like chlorpropamide and metformin, are excreted unchanged in the urine. Because chlorpropamide is excreted slowly (plasma half-life 36 hours) it need only be given as a single daily dose. Some maturity onset diabetics — particularly those who require a small dose — may be controlled with a single daily dose of the other sulphonylureas (plasma half-life about 4 to 7 hours), but a number require 2 doses daily for adequate control. Although the biguanides phenformin and metformin have a short plasma half-life (about 3 hours), their duration of action can be prolonged by inclusion in a sustained release capsule or tablet.