Biology Reference
In-Depth Information
mechanisms by which the liver regulates blood glucose homeostasis are
glycogenesis (storage and uptake of glucose), glycogenolysis (synthesis of
glucose from glycogen), and gluconeogenesis (synthesis of glucose from
nonglycogen sources;
Nordlie et al., 1999
). One of the principle substrates
for gluconeogenesis is amino acids (
Mallette, Exton, & Park, 1969
)
and
Pozefsky, Marliss, & Cahill, 1970
)
. The kidney also produces glucose pri-
marily via gluconeogenesis and appears to be as important as the liver in this
respect. Glutamine, lactate, and glycerol have been suggested to be impor-
tant
renal gluconeogenic precursors
Glucose uptake in peripheral skeletal muscle and adipose sugar is facili-
tated through the action of insulin. Insulin stimulates the translocation of the
GLUT-4 glucose transporter to the muscle plasma membrane and facilitates
the diffusion of glucose into the muscle. Insulin plays a very central role in
glucose homeostasis. It is a small globular protein that is secreted from the
b
-cells in the pancreatic islets of Langerhans. A small quantity is also pro-
duced in the brain (
Gerozissis, 2008
)
. The daily output of insulin by the pan-
creas is approximately 40-50 units (15-20% of pancreatic insulin stores;
Keim, Levin, & Havel, 2006
)
and is secreted in response to macronutrient
metabolites (
Woods, Lutz, Geary, & Langhans, 2006
)
and incretin hor-
Nauck, 2006
)
. While carbohydrate is the most potent insulin secretagogue,
proteins have also been shown to stimulate it, although not fat (
Teff &
Kapadia, 2008
). The majority of glucose absorption in the postabsorptive
state occurs in noninsulin-dependent tissue while the bulk of the glucose
in the postprandial state occurs in the insulin-dependent muscles (
Kelley
et al., 1988
)
. Therefore, insulin plays an important role in maintaining
postprandial glucose homeostasis. Insulin also suppresses endogenous
glucose production
(
Moore et al., 2003
). Conversely, glucagon which is
produced in the
a
-cells of the pancreas stimulates the conversion of glycogen
to glucose during hypoglycemic conditions. It further safeguards against
hypoglycemia by regulating gluconeogenesis
Although dietary proteins and amino acids appear to have little direct
effect on blood glucose (
Nuttall & Gannon, 1991
)
, they influence glucose
homeostasis through their effects on insulin and glucagon secretion. Proteins
and some amino acids, in particular, have been shown to stimulate the secre-
tion of both insulin and glucagon. They have also been shown to
