HYPERTENSION, INSULIN, AND ATHEROGENESIS

Essential hypertension and non-insulin-dependent diabetes mellitus are both associated with hyperinsulinemia and it has been proposed that this might contribute to increased atherogenesis in these conditions. In hypertension, hyperinsulinemia probably reflects reduced insulin-stimulated glucose uptake, but the reason for this, and the contribution of hyperinsulinemia (or of resistance to insulin) to the development of hypertension and atheroma, remains unclear. As well as glucose uptake, insulin has important effects on other aspects of cell function; for example, the hormone is an important regulator of the expression and function of the major inhibitory guanine nucleotide binding protein G(i). In insulin deficiency, G(i) levels and function are greatly reduced and are restored by insulin treatment. We have examined whether in human hypertension or in animal models of hypertension there is evidence of abnormal regulation of this protein. Platelet membranes from humans and rat membranes from a range of tissues, including myocardium and vasculature, were studied. No alteration in G(i) levels or function was found in these studies, and there is no evidence that this aspect of insulin action on cell function is abnormal. Insulin is also involved in the regulation of cell growth, and in vascular smooth muscle cells there is evidence that this effect involves action of other growth factors, such as PDGF. If the growth regulatory actions of insulin are also unimpaired despite limitation of insulin-stimulated glucose uptake, chronic hyperinsulinemia could lead to increased vascular smooth muscle cell growth and contribute to development of atheroma.