Regulation of Metabolism 697B. Vitamins and elements serve primarily as cofactors and
coenzymes.
- Vitamins are divided into those that are fat-soluble (A,
D, E, and K) and those that are water-soluble. - Many water-soluble vitamins are needed for the activity
of the enzymes involved in cell respiration. - The fat-soluble vitamins A and D have specific functions
but share similar mechanisms of action, activating
nuclear receptors and regulating genetic expression.
19.2 Regulation of Energy Metabolism 669
A. The body tissues can use circulating energy substrates,
including glucose, fatty acids, ketone bodies, lactic acid,
amino acid, and others, for cell respiration.
- Different organs have different preferred energy sources.
- Circulating energy substrates can be obtained from food
or from the energy reserves of glycogen, fat, and protein
in the body.
B. Eating behavior is regulated, at least in part, by the
hypothalamus. - Lesions of the ventromedial area of the hypothalamus
produce hyperphagia, whereas lesions of the lateral
hypothalamus produce hypophagia. - A variety of neurotransmitters have been implicated
in the control of eating behavior. These include the
endorphins, norepinephrine, serotonin, cholecystokinin,
and neuropeptide Y.
C. Adipose cells, or adipocytes, are both the targets of
hormonal regulation and themselves endocrine in nature. - In children, circulating saturated fatty acids promote cell
division and differentiation of new adipocytes. This activity
involves the bonding of fatty acid and prostaglandin
ligands with a nuclear receptor known as PPAR g. - Adipocytes secrete leptin, which regulates food intake
and metabolism, and TNF a , which may help to regulate
the sensitivity of skeletal muscles to insulin.
D. The control of energy balance in the body is regulated by the
anabolic and catabolic effects of a variety of hormones.
19.3 Energy Regulation by the Pancreatic
Islets 677
A. A rise in plasma glucose concentration stimulates insulin
and inhibits glucagon secretion.
- Amino acids stimulate the secretion of both insulin and
glucagon. - Insulin secretion is also stimulated by parasympathetic
innervation of the islets and by the action of intestinal
hormones such as gastric inhibitory peptide (GIP).
B. During the intestinal absorption of a meal, insulin promotes the
uptake of blood glucose into skeletal muscle and other tissues. - This lowers the blood glucose concentration and increases
the energy reserves of glycogen, fat, and protein. - Skeletal muscles are the major organs that remove blood
glucose in response to insulin stimulation.
C. During periods of fasting, insulin secretion decreases and
glucagon secretion increases. - Glucagon stimulates glycogenolysis in the liver,
gluconeogenesis, lipolysis, and ketogenesis.
2. These effects help to maintain adequate levels of blood
glucose for the brain and provide alternate energy
sources for other organs.19.4 Diabetes Mellitus and Hypoglycemia 681
A. Diabetes mellitus and reactive hypoglycemia represent
disorders of the islets of Langerhans.
1. Type 1 diabetes mellitus occurs when the beta cells are
destroyed; the resulting lack of insulin and excessive
glucagon secretion produce the symptoms of this disease.
2. Type 2 diabetes mellitus occurs as a result of a relative
tissue insensitivity to insulin and inadequate insulin
secretion; this condition is aggravated by obesity and
improved by exercise.
3. Reactive hypoglycemia occurs when the islets secrete
excessive amounts of insulin in response to a rise in
blood glucose concentration.19.5 Metabolic Regulation by Adrenal Hormones,
Thyroxine, and Growth Hormone 686
A. The adrenal hormones involved in energy regulation include
epinephrine from the adrenal medulla and glucocorticoids
(mainly hydrocortisone) from the adrenal cortex.
1. The effects of epinephrine are similar to those of
glucagon. Epinephrine stimulates glycogenolysis and
lipolysis, and activates increased metabolism of brown fat.
2. Glucocorticoids promote the breakdown of muscle
protein and the conversion of amino acids to glucose in
the liver.
B. Thyroxine stimulates the rate of cell respiration in almost all
cells in the body.
1. Thyroxine sets the basal metabolic rate (BMR), which
is the rate at which energy (and oxygen) is consumed by
the body under resting conditions.
2. Thyroxine also promotes protein synthesis and is needed
for proper body growth and development, particularly of
the central nervous system.
C. The secretion of growth hormone is regulated by releasing
and inhibiting hormones from the hypothalamus.
1. The secretion of growth hormone is stimulated by a protein
meal and by a fall in glucose, as occurs during fasting.
2. Growth hormone stimulates catabolism of lipids and
inhibits glucose utilization.
3. Growth hormone also stimulates protein synthesis, and
thus promotes body growth.
4. The anabolic effects of growth hormone, including the
stimulation of bone growth in childhood, are produced
indirectly via polypeptides called insulin-like growth
factors, or somatomedins.19.6 Regulation of Calcium and Phosphate
Balance 690
A. Bone contains calcium and phosphate in the form of
hydroxyapatite crystals. This serves as a reserve supply of
calcium and phosphate for the blood.
1. The formation and resorption of bone are produced by
the action of osteoblasts and osteoclasts, respectively.