We are currently characterizing the abnormalities for regulation of human muscle glycogen synthesis in insulin-resistant subjects. In insulin- resistant subjects, fasting glycogen synthase phosphatase and phosphorylase phosphatase activities are reduced and fail to show the peak insulin stimulation observed for insulin-sensitive subjects at 10-20 minutes. Using specific inhibitors the abnormal enzyme activities were identified as a type-1 phosphatase (PP-1) in human muscle from insulin- resistant subjects. The abnormally low fasting PP-1 activity in insulin- resistant subjects persisted following trypsin treatment, suggesting that inhibitors 1 and 2 (characterized regulators of PP-1) are not important determinants of the abnormal phosphatase activity. Western blots indicate an increased concentration of catalytic subunit for PP-1 in the muscle from insulin-resistant subjects. These results suggest that the intrinsic activity or regulation of the catalytic subunit is abnormal in insulin- resistant subjects. All insulin resistant subjects showed Mn activation of PP-1 in the absence of azide. 6 of 10 insulin sensitive subjects, however, required azide in order to see Mn activation of PP-1. The azide appears to reverse the effects of an inhibitor of Mn activation which as been localized in the glycogen-microsomal subcellular fraction of Mn- resistant (insulin sensitive) subjects. These results suggest that an azide sensitive structure in the glycogen microsomal fraction of muscle is responsible for the abnormal PP-1 activity in insulin resistant subjects. A second isoform of PP-1 has been identified in human muscle. Characterization of this isoform may explain the activity differences observed for insulin resistant and sensitive subjects. Compared to insulin sensitive subjects, subjects with NIDDM have elevated fasting PP-1 activity which, following insulin infusion, not only fails to increase but shows a significant decrease. Four weeks of insulin therapy reduces the elevated fasting PP-1 activity in NIDDM subjects and prevents their insulin mediated decrease in enzyme activity.
