Effects of insulin on glucose transport and glucose transporters in rat heart

Abstract

The effect of insulin on glucose transport and glucose transporters was studied in perfused rat heart. Glucose transport was measured by the efflux of labelled 3-O-methylglucose from hearts preloaded with this hexose. Insulin stimulated 3-O-methylglucose transport by: (a) doubling the maximal velocity (Vmax); (b) decreasing the Kd from 6.9 to 2.7 mM; (c) increasing the Hill coefficient toward 3-O-methylglucose from 1.9 to 3.1; (d) increasing the efficiency of the transport process (k constant). Glucose transporters in enriched plasma and microsomal membranes from heart were quantified by the [3H]cytochalasin-B-binding assay. When added to normal hearts, insulin produced the following changes in the glucose transporters: (a) it increased the translocation of transporters from an intracellular pool to the plasma membranes; (b) it increased (from 1.6 to 2.7) the Hill coefficient of the transporters translocated into the plasma membranes toward cytochalasin B, suggesting the existence of a positive co-operativity among the transporters appearing in these membranes; (c) it increased the affinity of the transporters (and hence, possibly, of glucose) for cytochalasin B. The data provide evidence that the stimulatory effect of insulin on glucose transport may be due not to the sole translocation of intracellular glucose transporters to the plasma membrane, but to changes in the functional properties thereof.

Dysregulation of glucose transport and transporters in perfused hearts of genetically obese (fa/fa) rats

“…It should be recalled that in normal rat hearts, insulin has been shown to increase the Hill coefficient of plasma membranes glucose transporters creased plasma membrane glucose transporters intrinsic activity [12,13]. tin obese rat hearts and as shown in the present study (Table 2), these properties were altered: insulin paradoxically decreased the Hill coefficient and failed to increase the affinity (Ka) values of plasma membrane glucose transporters.…”

“…As binding curves obtained with either 10 or 5 different ligand concentrations (40-500 nmol/l) resulted in identical results [12], the present experiments were made with only five ligand concentrations, using a modified technique described and validated previously [11][12][13]19]. As also described previously [11][12][13]191, binding of labelled cytochalasin was carried out either in the presence of 250 mmol/l sucrose (i.e.…”

“…at the same period of the same year. However, as many questions were initially posed regarding normal animals and published elsewhere [12,13], comparisons between lean and obese animals, particularly when dealing with glucose transporters number and characteristics were made by referring to the previously published papers on lean rats.…”

“…Furthermore, insulin has been shown [8][9][10] to favour the translocation of microsomal glucose transporters to the plasma membrane, thereby stimulating glucose uptake. Recently, our laboratories have further shown that, in normal brown adipose tissue [11] or perfused hearts from normal rats [12] insulin does not only favour the translocation of glucose transporters, but increases the "intrinsic activity" of the transporters once inserted in the plasma membrane. This was evidenced by the observation of two insulin-induced changes in glucose transport: an apparent increase in the affinity of the transporter for glucose and the occurrence of positive cooperativity [12].…”

“…Recently, our laboratories have further shown that, in normal brown adipose tissue [11] or perfused hearts from normal rats [12] insulin does not only favour the translocation of glucose transporters, but increases the "intrinsic activity" of the transporters once inserted in the plasma membrane. This was evidenced by the observation of two insulin-induced changes in glucose transport: an apparent increase in the affinity of the transporter for glucose and the occurrence of positive cooperativity [12]. This was reflected by changes in the properties of the glucose transporter toward cytochalasin B (increase in the affinity and cooperativity).…”