On the pathophysiology of late onset non-insulin dependent diabetes mellitus. Current controversies and new insights.

The development of late onset non-insulin dependent diabetes mellitus (NIDDM) is due to a complicated interplay between genes and environment on one side, and the interaction between metabolic defects in various tissues including the pancreatic beta cell (decreased insulin secretion), skeletal muscle (insulin resistance), liver (increased gluconeogenesis), adipose tissue (increased lipolysis) and possibly gut incretin hormones (defective glucagon like peptide 1 (GLP1) secretion) on the other side. Evidence for a genetic component includes the finding of a variety of metabolic defects in various tissues in non-diabetic subjects with a genetic predisposition to NIDDM, higher concordance rates for abnormal glucose tolerance including NIDDM in monozygotic compared with dizygotic twins, and the more recent demonstration of different NIDDM susceptibility genes at the sites of Insulin Receptor Substrate 1 (IRS1), the beta-3 adrenergic receptor, and the sulfonylurea receptor. However, the latter susceptibility genes only explain a minor proportion of NIDDM in the general population, and the quantitative extent to which genetic versus non-genetic factors contribute to NIDDM is presently unsolved. Environmental components include both an early intrauterine component associated with low birth weight, and later postnatal components including low physical activity, high fat diet, and the subsequent development of obesity and elevated plasma and tissue free fatty acid levels. Our finding of lower birth weights in monozygotic twins compared with their non-diabetic genetically identical co-twins excludes the possibility that the association between NIDDM and low birth weight as demonstrated in several studies may solely be explained by a coincidence between a certain gene causing both a low birth weight and an increased risk of NIDDM. Young first degree relatives of patients with NIDDM are characterized by hyperinsulinaemia and peripheral insulin resistance, which in turn may be explained by a decreased insulin activation of the enzyme glycogen synthase in skeletal muscle. Therefore, a defective skeletal muscle glycogen synthase activation may represent an early phenotypic expression of a genetic defect contributing to an increased risk of later development of NIDDM. However, elderly insulin resistant non-diabetic co-twins (64 years old) of twins with overt NIDDM does not--in contrast to their NIDDM co-twins--have a significantly decreased insulin activation of glycogen synthase in skeletal muscle. This demonstrates that the defective muscle glycogen synthase insulin activation has an apparent non-genetic component, and that this key defect of metabolism can be escaped or postponed even in non-diabetic subjects with a presumably 100% genetic predisposition to NIDDM. The insulin activation of glycogen synthase in skeletal muscle is compensated or apparently normalised in NIDDM patients when studied during their ambient fasting hyperglycaemia and a subsequent isoglycaemic (hyperglycaemic) physiologic insulin infusion. This indicates that the prevailing hyperglycaemia in NIDDM subjects compensates for the defective insulin activation of glycogen synthase present in those subjects when studied during eulycaemia. Our data and those of others also indicates that hyperglycaemia in NIDDM compensates for the defects in insulin secretion, the disproportionately elevated hepatic glucose production, and to some extent for the increased lipid oxidation and the decreased glucose oxidation present in NIDDM patients. Accordingly, NIDDM subjects exhibit all of those defects of metabolism when studied during "experimental decompensation" when the ambient hyperglycaemia is normalized by a prior and later withdrawn intravenous insulin infusion. However, shortly after the withdrawal of the intravenous insulin infusion, the plasma glucose concentration increased spontaneously in the NIDDM patients. (ABSTRACT TRUNCATED)