D-glucose uptake by a rat liver plasma membrane preparation

Prior research has shown that glucose enters liver cells through mechanisms involving both passive and facilitated transport. Earlier studies on liver slices and perfused liver suggested the presence of a carrier-mediated system alongside simple diffusion. However, the exact contribution of membrane-bound proteins to glucose uptake remains unclear. This gap motivated further investigation into the plasma membrane's role in glucose transport. The literature highlights the need to distinguish between diffusion and facilitated transport at the membrane level. No prior work had resolved the interplay between these mechanisms in isolated plasma membranes. The binding capacity of membrane proteins has been studied, but their specific role in glucose transport remains uncertain. This uncertainty drives the need for experiments using labeled glucose analogues and filtration techniques.

The study aimed to clarify the mechanisms of glucose uptake in rat liver plasma membranes. Researchers sought to determine whether glucose transport occurs via simple diffusion, facilitated diffusion, or a combination of both. The motivation stemmed from unresolved questions about the role of membrane proteins in this process. The filtration technique allowed for precise measurement of glucose analogue uptake. The study also aimed to test the hypothesis that artificial disruption of membranes influences transport mechanisms. Binding studies were included to assess specific protein interactions with glucose. The goal was to isolate Tris-soluble membrane proteins and compare their binding capacity to unsonicated membranes. This approach aimed to provide evidence for a carrier-mediated transport system.

The study used plasma membranes isolated from rat liver via a modified Neville method. Labeled glucose analogues were employed in a filtration technique to measure uptake. The method allowed for the detection of linearity and stereospecificity in uptake patterns. Temperature dependence and counterflow phenomena were observed to identify facilitated diffusion. Phloretin inhibition was used to confirm the presence of a carrier system. Sonication of membranes enabled binding studies with Tris-soluble proteins. Isolated proteins were compared to unsonicated membranes for binding capacity differences. These methods aimed to distinguish between diffusion and facilitated transport mechanisms.

D-glucose uptake exhibited features of both simple and facilitated diffusion. Uptake was linear and lacked stereospecificity, indicating simple diffusion. Temperature dependence and phloretin inhibition confirmed facilitated diffusion. The counterflow phenomenon supported the presence of a carrier system. Binding studies showed higher capacity in Tris-soluble proteins than in unsonicated membranes. These findings suggest a carrier-mediated transport system. The results align with earlier studies on liver slices and perfused liver. The presence of artificial disruption in membranes allowed for free diffusion alongside carrier activity.

The authors propose that glucose uptake in rat liver plasma membranes involves both diffusion and carrier-mediated transport. The presence of a carrier system was supported by temperature dependence and phloretin inhibition. Binding studies revealed specific sites for glucose in membrane proteins. These findings confirm earlier observations in liver slices and perfused liver. The study suggests that artificial membrane disruption allows free diffusion. The authors conclude that both mechanisms coexist in glucose transport. No prior work had resolved the interplay between these mechanisms in isolated membranes. The evidence supports a dual mechanism for glucose uptake in liver plasma membranes.