Insulin-resistant Na+ pump activity in adipocytes from obese humans.

Basal and maximally insulin-stimulated Na+-pump activity was measured in adipocytes from subjects with normal glucose tolerance over a range of body mass indexes (BMI). In a comparison of 13 lean (BMI less than 25) vs. 15 extremely obese (BMI greater than 40) subjects basal activities per unit surface area were similar, but the maximally insulin-stimulated activity was significantly reduced in the extremely obese group [9.2 +/- 0.6 vs. 12.1 +/- 1.0 (min X dam2)-1, P less than 0.05]. The mean percent insulin stimulation of the Na+ pump above basal activity was 48 +/- 7% for the lean compared with 14 +/- 2% for the extremely obese group (P less than 0.001). A similar relationship was observed in these subjects for glucose transport where basal activities per unit surface area again were similar but the maximally insulin-stimulated transport was reduced in the extremely obese subjects (2.2 +/- 0.3 vs 5.1 +/- 0.6 attol/um2 X s, P less than 0.001). These results indicate that alterations in Na+-pump activity may be a manifestation of the insulin-resistant state that could contribute to the development of obesity via decreased cellular thermogenesis.