Muscles sugar transport into

Sugar Transport into Muscle during Rest and Exercise... 

An important action of insulin on muscle and adipose tissue is to stimulate transport of glucose into the cell (7) and we thought it interesting to see whether the action of insulin on sugar transport could be caused by changes in the cellular level of cyclic AMP. Adipose tissue rather than muscle was used in this study because in preliminary experiments we could not... 

Yet another way to alter the effective activity of an enzyme is to change the accessibility of its substrate. The hexokinase of muscle cannot act on glucose until the sugar enters the myocyte from the blood, and the rate at which it enters depends on the activity of glucose transporters in the plasma membrane. Within cells, membrane-bounded compartments segregate certain enzymes and enzyme systems, and the transport of substrate into these compartments may be the limiting factor in enzyme action. 

For analysis of glucose uptake in muscle it is convenient to divide the process into three steps. The first step is transfer of sugar from the capillary to the cell surface second, transport of sugar through the cell membrane and third, phosphorylation of glucose within the cell. Each step limits uptake to some extent under all conditions, but the degree of limitation imposed by an individual step is variable under various experimental conditions. 

Aggravation of disorders of metabolism—When people obtain their nourishment via the digestive tract the absorbed nutrients are conveyed by the portal system of blood vessels to the liver, where excess sugars are converted into fats and glycogen (a complex carbohydrate that is stored in the liver and muscles), fats are combined with proteins for safe transport in the blood, and amino acids are converted into proteins and waste products such as urea. 

Active transport mechanisms have been observed for a large number of substances. A variety of ions are transported actively, for example, Na+, K+, Ca++, and even H+ ions (for the production of gastric juice). Among organic substances, the transport of sugars is particularly important (the flow of glucose into cells), as well as that of amino acids and other low molecular weight substan< es which arc reabsorbed by the kidney tubules. In several specialized tissues, active transport is a prerequisite for their function, especially in nerves (cf. Chapt. XXIII-7). Also in the muscle, excitability cannot be maintained if active transport has come to a standstill (Chapt. XXIII-6). 

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