Page 432 - Anatomy and Physiology of Farm Animals, 8th Edition

Nutrition and Metabolism / 417

as very low density lipoproteins (VLDLs). event bringing about the changes in endo-
crine secretions that orchestrate the meta-
Lipoproteins are particles that contain
VetBooks.ir lipids, cholesterol, and proteins in various bolic changes during the postabsorptive
state. Two major endocrine changes are a
ratios. VLDLs are so named because their
lipid content is high relative to their protein gradual drop in insulin secretion and a rise
content. Because lipids are less dense than in the release of glucagon. Recall that
protein, the density of VLDL particles is increases in blood glucose stimulate insu-
quite low. lin release from β‐cells, whereas decreases
Recall that chylomicrons are also circu- in blood glucose stimulate glucagon release
lating lipoproteins, but the triglycerides in from α‐cells in pancreatic islets.
these lipoproteins were absorbed from the During the absorptive period, when
intestinal tract. As chylomicrons and glucose and amino acids were being
VLDLs circulate throughout the body, they absorbed into the blood from the intestinal
encounter lipoprotein lipase, an enzyme tract, insulin stimulated the synthesis of
bound to endothelial cells that acts on their glycogen for glucose storage, proteins, and
triglycerides to release free fatty acids. lipids (from any excess glucose and amino
When the triglycerides are released within acids). As insulin levels decrease, its stimu-
adipose tissue, the free fatty acids are avail- latory effect on these synthetic (anabolic)
able to adipose cells for the resynthesis and processes is lost, and this is a major factor
storage of lipids as triglycerides. In other in changing the overall metabolic balance
organs, such as skeletal muscle, cells use the from anabolism to catabolism.
free fatty acids for energy. The synthesis Glucagon stimulates the breakdown of
and storage of triglycerides in adipose tis- glycogen (glycogenolysis) in the liver to
sue is stimulated by insulin, which is typi- provide glucose that the liver can release
cally elevated during the absorptive period. into the blood. Glycogenolysis is the initial
After losing triglycerides by the process by which the liver derives glucose
action of lipoprotein lipase, some VLDLs to add to the blood, but later the liver also
undergo changes in the circulation and releases glucose formed by gluconeogene-
become a different type of lipoprotein, sis, which is also stimulated by glucagon.
low‐density lipoprotein (LDL). LDLs The amino acids used for gluconeogenesis
contain a great deal of cholesterol, and are derived from the catabolism of body
cells throughout the body receive cho- protein.
lesterol from the blood by the endocyto- The maintenance of a minimal or fast-
sis of LDLs. Cholesterol is a necessary ing blood glucose level during this period
component of cell membranes, and all has primary importance to neuronal func-
cells need some cholesterol. However, tion. Neurons do not have the metabolic
abnormal increases in LDL levels are processes to permit them to use fatty acids
associated with an increased risk of car- for energy, so they need a ready supply of
diovascular disease in humans. Recall glucose for cell energy. During fasting, the
that the original VLDLs were produced catabolism of adipose tissue increases the
in the liver, so much of the cholesterol in supply of fatty acids, which are used by
the blood is produced by the liver. cells other than neurons for energy. The
increased use of fatty acids by other cells
reduces the overall need for glucose and
Postabsorptive State: Catabolism conserves it for use by neurons. This con-
servation of glucose is termed glucose
After a meal has been digested and sparing. The liver also metabolizes the cir-
absorbed, blood glucose concentration culating fatty acids to produce ketones,
gradually decreases as glucose is used for another cellular energy substrate. The
energy throughout the body (Fig. 22‐2). ketones produced by the liver include ace-
This drop in blood glucose is the primary tone, acetoacetate, and β‐hydroxybutyrate.