Digitalis-like factor response to hyperinsulinemia in human pregnancy, a model of insulin resistance

Insulin resistance is strongly associated with hypertension and is postulated to participate in the elevation of blood pressure, although the mechanisms involved are not understood. Recently, we reported that acute increases in plasma insulin levels in normal subjects resulted in increased serum levels of a sodium pump inhibitor, termed the digitalis-like factor (DLF), which has been implicated in both experimental and essential human hypertension. This study looked at the DLF response to hyperinsulinemia, achieved by an oral glucose tolerance test (OGTT), in the setting of a naturally occurring and self-resolving state of human insulin resistance, during third-trimester pregnancy. This model allowed us the further opportunity to compare the DLF response to insulin in the same subjects postpartum, after resolution of their insulin resistance. Administration of an OGTT during pregnancy and postpartum in the same subjects elicited a comparable serum glucose response but a significantly greater insulin response during third-trimester pregnancy, consistent with diminished insulin sensitivity (integrated insulin response during pregnancy: 1611+/-236 vs postpartum: 685+/-101 pmol/l, P=0.004). The time courses of the glucose and insulin responses were identical whether women were pregnant or not. Plasma free fatty acids fell significantly and to a comparable degree during pregnancy and postpartum, but the response was slower during pregnancy. DLF levels increased in response to oral glucose in both pregnant and nonpregnant states. The response was more rapid during pregnancy than after. These findings showed that the increment of insulin induced by oral glucose during pregnancy caused a more rapid rise in circulating DLF levels than it did during the nonpregnant state. At the same time, the response of circulating fatty acids to glucose is retarded during pregnancy. This suggests that the insulin resistance of pregnancy impairs insulin's influence on intermediary metabolism but not its influence on DLF. As a vasoactive substance, DLF might contribute to the hypertension characteristic of insulin-resistant states.