Heterogeneous responses to an intravenous glucose load

Abstract

The aim of this study was to determine how the insulin sensitive enzymes pyruvate dehydrogenase (PDH) complex and glycogen synthase (GS) of different tissues respond to an endogenous pulse of insulin elicited by an intravenous infusion of glucose. An infusion of glucose (0.5 g/kg) into conscious, unrestrained animals via an indwelling cannula rapidly elevated plasma insulin concentration (to approx. 600 μU/ml after 10 min). The animals were sacrificed at selected time points after the commencement of infusion. Samples of heart, red quadriceps muscle, white adipose tissue (WAT) and brown adipose tissue (BAT) were excised and assayed for PDH complex and GS activities. The glucose infusion elicited a rapid (<5–10 min) increase in both PDH complex and GS activities in heart, BAT and WAT. The maximum rise in the activity of PDH and GS above basal were (respectively) 2- and 8-fold for heart, 5.5- and 5-fold for BAT, and 3.5- and 4-fold for WAT. The return of PDH complex activity to basal values was also very rapid (occurring over the next 20 min). The glucose infusion also stimulated GS activity in red quadriceps muscle but was, however without effect on PDH complex activity in this tissue. We conclude that although insulin stimulates PDH and GS with the same time course and magnitude in many insulin sensitive tissues, the time course and magnitude of insulin stimulation of these enzymes can vary between tissues. These results may mean that the stimulation of PDH complex and GS by insulin occurs via different receptor-effector pathways.