Insulin responsiveness of superficial forearm tissues in type 2 (non-insulin dependent) diabetes

Contributions of insulin-resistance and insulin-secretory defects to the pathogenesis of type 2 diabetes mellitus

Controlled clinical trials have shown that optimal glycemic control can prevent the microvascular complications of type 2 diabetes mellitus; considerable epidemiological data suggest that this may also be true for macrovascular complications. However, this is frequently not achieved. Consequently, research efforts have been undertaken to better understand the pathophysiology of this disorder. It is now well recognized that 2 factors are involved: impaired beta-cell function and insulin resistance. Prospective studies of high-risk populations have shown insulin-resistance and/ or insulin-secretory defects before the onset of impaired glucose tolerance. Thus, there has been a long-standing debate whether an alteration in insulin sensitivity or in insulin secretion is the primary genetic factor. Most of the available evidence favors the view that type 2 diabetes is a heterogeneous disorder in which the major genetic factor is impaired beta-cell function and insulin resistance is the major acquired factor. Superimposition of insulin resistance on a beta cell that cannot appropriately compensate leads to deterioration in glucose tolerance. Therefore, clinicians managing type 2 diabetes must reduce insulin resistance and augment and/or replace beta-cell function.

The genetic basis of type 2 diabetes mellitus: impaired insulin secretion versus impaired insulin sensitivity

Despite the fact that it is the prevalent view that insulin resistance is the main genetic factor predisposing to development of type 2 diabetes, review of several lines of evidence in the literature indicates a lack of overwhelming support for this concept. In fact, the literature better supports the case of impaired insulin secretion being the initial and main genetic factor predisposing to type 2 diabetes, especially 1) the studies in people at high risk to subsequently develop type 2 diabetes (discordant monozygotic twins and women with previous gestational diabetes), 2) the studies demonstrating compete alleviation of insulin resistance with weight loss, and 3) the studies finding that people with type 2 diabetes or IGT can have impaired insulin secretion and no insulin resistance compared with well matched NGT subjects. The fact that insulin resistance may be largely an acquired problem in no way lessens its importance in the pathogenesis of type 2 diabetes. Life style changes (exercise, weight reduction) and pharmacological agents (e.g., biguanides and thiazolidendiones) that reduce insulin resistance or increase insulin sensitivity clearly have major beneficial effects (122, 144-146, 153-155).

Hepatic glucose extraction in normal and diabetic man

Fractional hepatic extraction of glucose was determined from the appearance in the systemic circulation of ingested 3-[3H]glucose. Using the glucose clamp technique, studies were done under steady-state conditions of basal glycemia and insulinemia, normoglycemia (0.8 mg/mL) and mild hyperinsulinemia (approximately 40 microU/mL), hyperglycemia (2 mg/mL-1) and hyperinsulinemia (approximately 100 microU/mL). Based on previous results in the dog, an oral glucose load of 2 g was used to label the portal vein glucose; this amount was chosen so as to minimize disturbance of the portal steady state but still avoid excessive loss during absorption. Additional subjects with hyperglycemia and hyperinsulinemia received an oral load of 50 g of glucose. Fractional extraction in normal subjects under near-basal conditions of glycemia and insulinemia was 19% in normal subjects and in patients with noninsulin-dependent diabetes mellitus (NIDDM) elevation of serum insulin, with or without hyperglycemia, which led to an average extraction rate of 32% of the ingested glucose. Absolute hepatic glucose uptake, calculated from the fractional extraction the plasma glucose concentration, and hepatic plasma flow accounted for 50% to 72% of total glucose use during the various steady states and following ingestion of 50 g of glucose. It is concluded that hepatic uptake or extraction, as opposed to net uptake, proceeds actively even when plasma glucose and insulin are within the normal basal range; it is increased in the presence of hyperinsulinemia, with or without hyperglycemia; and it is unaltered in NIDDM.