Glucose is removed from the blood into the extracellular space by passive diffusion and then into the intracellular space via membrane glucose transporter proteins. Because the glucose concentration in most cells is barely detectable, the glucose concentration gradient from outside to inside a cell (—90 mg/dl) strongly favors inward diffusion. However, since glucose is soluble in water but not fat,
glucose cannot cross the lipid bilayer cell membrane. To allow this, a water "pore" spanning the membrane is required. This barrier to diffusion presents an opportunity to regulate glucose uptake.
Glucose transport into cells is permitted by glucose transporter proteins, which span the membrane and provide the aqueous pore through which glucose can pass (5). Distinct forms of these proteins allow for differential regulation of glucose uptake in different tissues. For example, in muscle and fat tissue the glucose-4-transporter protein subtype is predominant. Following exposure to insulin, the glucose-4-transporter proteins rapidly move to the cell surface, allowing glucose transport into the cell. This action by insulin to stimulate glucose uptake in tissues with insulin-sensitive glucose-4-transporter proteins defines muscle and fat as so-called insulin-sensitive tissues.
However, even in insulin-sensitive tissues, insulin levels are not the only factor that regulates glucose uptake. Exercise also facilitates glucose transporter availability at the muscle cell membrane, increasing the rate of glucose uptake
(6). Also, the glucose concentration per se regulates the rate of glucose uptake
(7). At any given insulin level, glucose uptake increases with increasing glucose concentrations (so-called mass action of glucose). The action of glucose to promote glucose uptake results in increased rates of glucose uptake in diabetic patients, with hyperglycemia versus nondiabetic controls, provided that at least some insulin is present (8). This increased rate of glucose uptake matches the increased rate of hepatic glucose release that has caused the hyperglycemia, with the net effect that the blood glucose concentration remains stable but high.
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Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...