Determination of insulin sensitivity: methodological considerations.

During the past decade, it became obvious that in contrast to defective insulin secretion in type I diabetes, defective insulin action (insulin resistance) is the most pertinent feature of type II diabetes. In addition, it has been known for a long time that obesity and insulin resistance are closely linked. Recently, hypertension also has been shown to often coincide with insulin resistance, although any causal relationships are still hypothetical. Last, several widely used pharmacological drugs such as diuretics, adrenergic blockers, and angiotensin-converting enzyme inhibitors may influence insulin sensitivity. Therefore, growing interest has emerged to most accurately measure insulin sensitivity. Although considerable knowledge has accumulated as to the actual mechanisms of insulin-dependent glucose transport, the signal transduction pathway of insulin remains poorly understood. When insulin sensitivity is measured, it is the overall glucose uptake that is quantified under controlled conditions. Other actions of insulin, such as the transport of ions, (e.g., sodium and potassium), synthesis of insulin-like growth factor-binding proteins, translocation of transporter proteins, and regulation of enzyme activities, are much more difficult to quantify. Of the many approaches used to quantify insulin action, the euglycemic hyperinsulinemic clamp technique has emerged as the most reliable tool, fulfilling clinical and scientific demands equally. In combination with tracer methodology and calorimetry, a detailed view into the quantitative aspects of insulin action at different target cells is possible. Whether insulin resistance extends to other known actions of insulin in addition to those on glucose metabolism remains open to debate.