Optimal absorption of macronutrients requires digestion by enzymatic hydrolysis to yield mono- mers and oligomers that can be taken up by enterocytes in the small intestine (see chapter
“ Overview ” under the part “Gastrointestinal tract”). Pancreatic enzymes are released into the duodenum to facilitate carbohydrate digestion (amylase), protein digestion (trypsin, chymotryp- sin, carboxypolypeptidase, elastase) and lipid digestion (lipases and cholesterol esterase) [ 24 ].
To prevent degradation and harm to the pancre- atic tissue, exocrine cells manufacture, store, and secrete digestive enzymes as proenzymes (inac- tive precursors) that are subsequently activated in the small intestine [ 1 , 24 ]. Exocrine function is critical to nutrient absorption, and this is high- lighted by the detrimental malabsorption that occurs in cases of exocrine insuffi ciency and decreased enzymatic output [ 24 ].
Glucose and other energy sources are absorbed after a meal (fed state) and released into the bloodstream and stimulate β cells to release insu- lin, the anabolic hormone that promotes the uptake and storage of glucose, lipids, and amino
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161
acids (Fig. 2a ). Insulin acts to promote the syn- thesis of glycogen (the storage form of glucose found in the muscle and liver, glycogenesis) as well as suppress glycogenolysis (glycogen break- down). Additionally, fatty acid synthesis (lipo- genesis) and protein synthesis are increased by insulin, while lipid breakdown (lipolysis) and protein breakdown are inhibited (Fig. 2a ) [ 4 ].
Conversely, the catabolic hormone glucagon counter-regulates insulin by stimulating the mobilization of fuels, particularly glucose, dur- ing states of low fuel availability (fasting/exer- cise, Fig. 2b ) [ 27 ]. Glucagon elevates blood glucose levels by stimulating hepatic glucose output via the breakdown of glycogen (glycoge- nolysis) as well as promoting the conversion of other carbon sources (pyruvate, lactate, glycerol,
and glucogenic amino acids) into glucose (gluco- neogenesis). Unlike the liver, the glucose yielded from glycogen breakdown in the muscle is used only within the skeletal muscle. Additionally, glucagon also decreases processes that remove glucose from the bloodstream, including glyco- genesis and glycolysis (Fig. 2b ) [ 28 ].
Final Remarks
Both the endocrine and exocrine compart- ments of the pancreas are vital to proper diges- tion, absorption, and assimilation of nutrients.
The exocrine compartment (acinar, duct, and centroacinar cells) releases digestive enzymes and bicarbonate fl uid to aid in the intestinal
Insulin Fed Glucose Lipids AAs
Pancreatic β-cells
Glucagon Fasted Glucose Lipids AAs
Energy
source Response to
Insulin Main target
tissue Glucose
Lipid
Protein
Uptake Glycogenesis Glycogenolysis Uptake Lipogenesis Lipolysis Uptake Synthesis Breakdown
Pancreatic α-cells
Muscle, adipose Muscle, liver Muscle, liver Liver, adipose Liver, adipose Liver, adipose Muscle Muscle Muscle
Glucose
Lipid
Protein
Output Glycogenesis Glycogenolysis Output Lipogenesis Lipolysis Uptake Synthesis Breakdown
Liver Muscle, liver Muscle, liver Adipose Adipose Adipose Muscle Muscle Muscle Energy
Source Response to
Glucagon Main Target Tissue
a b
Fig. 2 How pancreatic metabolites affect other tissues.
( a ): The anabolic hormone insulin is secreted by β cells when nutrient levels are high (fed state) to promote the uptake and storage of energy sources. ( b ): The catabolic
hormone glucagon released by α-cells counter-regulates insulin by stimulating the mobilization of fuels, particu- larly glucose, during states of low fuel availability (fast- ing/exercise). AAs amino acids
Overview
162
breakdown of lipids, carbohydrates, and proteins.
As these nutrients are absorbed, the endocrine cells of the islet (α-, β-, δ-, and pancreatic poly- peptide cells) release hormones that affect how cells use and store these macronutrients. Insulin release by β-cells controls the uptake and stor- age of nutrients following a meal, while gluca- gon release by α-cells promotes mobilization of energy stores during times when food intake is low. The importance of proper pancreatic func- tion is highlighted by conditions that can occur in cases of exocrine insuffi ciency (nutrient malab- sorption) and impaired endocrine function (dia- betes, see chapter “ Diabetes mellitus ”).
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A.E. Folias and M. Hebrok
E. Lammert, M. Zeeb (eds.), Metabolism of Human Diseases, 163 DOI 10.1007/978-3-7091-0715-7_26, © Springer-Verlag Wien 2014