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J. Biochem, 1987, Vol. 101, No. 4 977-985
© 1987 Japanese Biochemical Society

other

Effect of Insulin on the Glucose Utilization in Isolated Cardiac Myocytes from Adult Rat

Yukikazu SAEKI*, Atsunori KASHIWAGI** and Noriyoshi UEHARA*,1

*Central Research Laboratory
**The Department of Internal Medicine, Shiga University of Medical Science Ohtsu, Shiga 520–21

1Present address: Unitika Hospital, Uji, Kyoto 611.

The acute effects of insulin on glucose utilization in isolated rat quiescent cardiac myocytes were studied. Insulin (80 nM) increased the rate of glucose clearance by 2–3 times in the presence of glucose ranging from 0.3 µM to 5.5 mM. Glucose transport, which was measured in terms of both D-glucose uptake in the presence of 0.3 µM D-glucose and initial rate of uptake of 3-O-methylglucose, was stimulated 3-fold in the presence of insulin. At higher glucose concentrations (> 100 µM), a decrease in glucose clearance rate due to a shift of the rate-limiting step from glucose transport to a post-transport step in the pathway of glucose metabolism was observed. At the physiological concentration of glucose (5.5 mM), about 73 % of glucose was metabolized into lactate, about 10% was oxidized into CO2 and the rest (17%) remained inside the cells. The pentose phosphate pathway did not contribute to the glucose metabolism in these cells. Insulin (80 nM) significantly increased the uptake of glucose (112%), and the conversions of glucose into lactate (16%), glycogen (64%), and triglyceride (18%), but not into CO2 (3%). Insulin transiently increased the percentage of I-form of glycogen synthase by 16% above basal, but did not affect the percentage of a-form of glycogen phosphorylase. The content of glucose 6-phosphate in the cells was increased by 46% above the basal value in the presence of insulin. These results indicate that insulin has different acute stimulatory effects on various steps in the metabolic pathway of glucose in isolated quiescent cardiac myocytes. Insulin has a marked influence on the steps of glucose transport and glycogen synthesis, of which the latter is stimulated not only by the large increase in glucose 6-phosphate content but also by the small increase in the percentage of I-form of glycogen synthase.


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