Adipose tissue fatty acid mobilisation

Figure 3.23 A sequence of processes explaining the role of glucokinase in the liver and fi-cells in regulation of the blood glucose concentration. The increase in the plasma insulin increases glucose uptake by muscle and decreases fatty acid mobilisation from adipose tissue which lowers the plasma fatty acid level which also increases glucose uptake (Chapter 12).

The ketone bodies are provided from fatty acids mobilised from adipose tissue. 

Note that we have invoked lipase action several times but we have referred each time not to lipase, but to a lipase. This is because it is a different lipase each time. They all carry out the same chemical job, but they have to work in different places under different conditions - the intestinal one is a secreted enzyme working free in the intestinal juice, whereas the lipase at the surface of adipose tissue is a membrane-bound enzyme. Similarly, the lipases of adipose tissues are tailored for their separate jobs by being structured to respond to different physiological regulatory signals. Thus, the lipase involved in fatty acid mobilisation, for instance, is known as hormone-sensitive lipase . 

Fatty acids are released from adipose tissue into the bloodstream, from where they can be taken up and used by aerobic tissues, with the exception of brain and the intestine. In addition, an increase in the plasma fatty acid concentration is one factor that increases the rate of fatty acid oxidation by tissues. Flence, an increase in the mobilisation of fatty acid from adipose tissue is an immediate signal for tissues such as muscle, heart and kidney cortex to increase... 

Stimulation of the sympathetic nervous system releases noradrenaline which stimulates mobilisation of fatty acids in adipose tissue. The latter stimulate the activity of the uncoupling protein. 

In the ebb phase, there is increased activity of the sympathetic nervous system and increased plasma levels of adrenaline and glucocorticoids but a decreased level of insulin. This results in mobilisation of glycogen in the liver and triacylglycerol in adipose tissue, so that the levels of two major fuels in the blood, glucose and long-chain fatty acids, are increased. This is, effectively, the stress response to trauma. These changes continue and are extended into the flow phase as the immune cells are activated and secrete the proinflammatory cytokines that further stimulate the mobilisation of fuel stores (Table 18.2). Thus the sequence is trauma increased endocrine hormone levels increased immune response increased levels of cytokines metabolic responses. 

The increased mobilisation of fatty acids from adipose tissue raises the plasma concentration, which increases the rate of fat oxidation by muscle. It also releases some essential fatty acids from the store in the triacylglycerol in adipose tissue. These are required for formation of new membranes in proliferating cells and those involved in repairing the wound (e.g. fibroblasts) (Chapters 11 and 21 Figure 21.22). 

Mobilisation of triacylglycerol is due to the increased rate of lipolysis. Adipose tissue triacylglycerol contains some essential fatty acids (linoleic and linolenic acids) and these are mobilised along with the non-essential fatty acids. The... 

The fed state is also an opportunity to lay down storage triglycerides in adipose tissue. Insulin therefore switches on lipoprotein lipase, which downloads fatty acids from chylomicrons it switches on the enzymes that resynthesise triglyceride inside the adipocytes it switches off hormone-sensitive lipase, the one that releases fatty acids into the blood when the stores are mobilised. 

During fasting or starvation, the catabohc hormones glucagon and cortisol are secreted from the a-ceUs of the pancreas and the adrenal cortex, respectively. In response to severe stress or danger, adrenaline is secreted by the adrenal medulla. These hormones stimulate hormone-sensitive lipase that mobilises fatty acids and glycerol from white adipose tissue. 

Mobilisation of fatty acids from adipose tissue... 

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