Reduction of glucose and insulin concentrations during in vitro incubation of human whole blood at different temperatures

Background: Incubation of human whole blood at body temperature has been used in numerous studies without addition of glucose and insulin. The purpose of the study was to examine glucose concentration changes during in vitro incubation of human whole blood at different temperatures, and whether it was affected by addition of insulin and bacterial endotoxin. We also wanted to quantify changes in endogenous insulin concentrations during incubation for six hours at 37 oC. Methods: Young, healthy and fasting males donated whole blood. Glucose concentrations were compared at baseline and after six hours incubation at 37 oC, 22 oC or 0 oC, in aliquots with and without addition of insulin and bacterial endotoxin. Glucose data are presented as mean (± 1 standard deviation). Endogenous insulin concentrations in aliquots without addition were measured at baseline and after six hours incubation at 37 oC, data are shown as median (interquartile range). Results: Glucose concentration at baseline was 5.3 (± 0.6) mmol/L. After incubation for six hours at different temperatures, the glucose level at 37 oC was 1.0 (± 0.5) mmol/L (p<0.01), at 22 oC 3.0 (± 0.6) mmol/L (p<0.01), and at 0 oC 5.4 (± 0.7) mmol/L (p=0.95). The decline in glucose concentration seemed to be independent of addition of insulin and bacterial endotoxin. Endogenous insulin levels decreased from baseline 48 (36-94) pmol/L to 23 (18-27) pmol/L (p=0.03) during six hours incubation at 37 oC. Conclusions: Glucose concentration was markedly reduced during in vitro incubation of whole blood from healthy volunteers for six hours at 37 oC and 22 oC, but was maintained at 0 oC. Endogenous insulin level after six hours incubation of whole blood at 37 oC was more than halved compared to baseline. During in vitro studies of glucose and/or insulin effects lasting for hours, measures must be undertaken to maintain stable glucose and/or insulin concentrations.