Forearm Glucose Uptake During the Oral Glucose Tolerance Test in Normal Subjects

Abstract

Total forearm glucose utilization (FGU) was determined during 100 gm. oral glucose tolerance tests (GTT) in twenty-five normal volunteers. In addition, the concomitant balance of insulin, growth hormone, and lactate across the forearm was studied in thirteen subjects. The increment in FGU during the three hours following glucose ingestion amounted to 74 mg./100 ml. forearm; it may be calculated that increased peripheral glucose utilization accounted for the disposal of 42 per cent of the 100 gm. load and that during the GTT some 58 gm. of extra glucose reached the peripheral circulation. Serum insulin concentrations in mixed venous (MY) blood rose steeply after glucose loading, while growth hormone levels were reduced. Additional observations in thirteen subjects showed that MV insulin levels remained significantly lower than corresponding arterialized venous (AV) concentrations between thirty and 150 minutes, suggesting that insulin was being continually removed bythe forearm tissues during this time. Significant AV-MV differences were not detected in growth hormone levels. Plasma lactate concentrations rose immediately after glucose loading, reaching a peak at sixty minutes and declining thereafter. The initial elevation was associated with lactate uptake by the forearm and the subsequent fall with lactate release, suggesting that peripheral lactate metabolism has little or no influence on the shape of the lactate response curve. It is suggested thatthis early rise in lactate concentrations is the result of increased hepatic lactate production and that the timing and height of the lactate peak reflect the pattern of enhanced hepatic glucoseutilization after oral glucose loading. Our results suggest that the disposition of a 100 gm. oral glucose load is accounted for mainly by hepatic glucose conservation rather than peripheral uptake and, therefore, that the former is the major determinant of the shape of the oral glucose tolerance curve.