Evidence for functionally distinct glucose transporters in basal and insulin-stimulated adipocytes

Abstract

The activity and Km of glucose transport of rat adipocytes are quite variable in the basal state. This could be due to differing levels of highly saturable transport against a background of less saturable transport. Such heterogeneity could lead to differing conclusions as to the Km of basal cells compared to insulin-stimulated cells depending on the choice of substrate, the range of concentrations tested, and the rigor of data analysis. In the present work, we used a cell preparation which was stable and partially activated by constant agitation. We used a two-component model to fit the concentration dependence of D-glucose uptake. We defined two parallel pathways of glucose entry, a high-affinity/low-capacity pathway and a low-affinity/high-capacity pathway. Both pathways were stereospecific and were inhibited by cytochalasin B. The low-affinity pathway in basal cells had 97% of the total capacity (Vmax) with a high Km (greater than 50 mM). A second pathway had a very low Km (less than 1 mM) and only 3% of the total capacity, but contributed to 30-60% of glucose uptake at 8 mM glucose. In insulin-stimulated cells, a pathway with a Km of 4-5 mM dominated and contributed 85% of glucose transport. The low-affinity but not the very high affinity pathway persisted in stimulated cells, but its contribution was only 10-15% of transport at 8 mM glucose. These results suggest the presence of at least two functionally distinct transporters whose respective contributions can be characterized by nonlinear regression of data over a wide range of glucose concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport and metabolism of glucose in an insulin-secreting cell line, .beta.TC-1

“…This avoids the experimental bias introduced by differing substrate specificities among transporters. The problem with use of analogues is demonstrated by the rat white adipocyte, which provides an example of how the function of several transporters with different substrate specificities can complicate interpretation of regulatory effects on transport (Whitesell et al, 1989b). Glutl, Glut3, and Glut4 have been identified in adipose tissue (Kayano et al, 1990), but other species of glucose transporters may be present.…”

“…Because of these factors, the cells can exhibit one or more dominant kinetic characteristics depending on the conditions of prior incubation and hormonal stimulation. We tested transport in isolated adipocytes using methodology adapted for glucose as the substrate (Whitesell et al, 1989b). Under the specified conditions, a pattern of functional heterogeneity was found, probably from the activities of the distinct transporter species.…”

Pancreas Hormones- Inspired Method

“…Glucose enters the cell by facilitated diffusion mechanisms, which represent an example of regulated mass exchange across the cell membrane. Specialized families of membrane proteins called GLUT (i.e., GLUcose) Transporters [10]observed in a large variety of cell types [23], actively operate the removal of free glucose from the interstitial spaces and transport it inside the cytoplasm, with distinct affinities and maximal transport rates [24]. The absorption of glucose in the cells determines the actual concentration of blood glucose [25].…”

Insulin release at the molecular level: Metabolic-electrophysiological modeling of the pancreatic beta-cells

“…Actually, glucose enters the cell by facilitated diffusion mechanisms, which represent an example of regulated mass exchange across the cell membrane. Specialised families of membrane proteins called GLUT (i.e., GLUcose Transporters), observed in a large variety of cell types [22], actively operate the removal of free glucose from the interstitial spaces and transport it inside the cytoplasm, with distinct affinities and maximal transport rates. Following previously proposed descriptions [7], [17], we assume a single type of glucose transporter to be uniformly distributed over the entire cell membrane and to be characterized by the experimentally determined nonlinear properties of the GLUT-2 transporter [23].…”