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Introduction
Human energy requirements are met predominately by glucose. Cellular plasma membrane's permeability to glucose varies according to type of tissue. Glucose transporters control diffusion of glucose into the cell. These transporters are specific to each type of tissue. Hormonal and neural devices control homeostasis of blood glucose levels. At least eight hormones secreted by various endocrine glands play a role in blood glucose levels.
Insulin (secreted by the pancreas) is the chief glucose regulatory hormone. Insulin is synthesized by beta cells (ß-cells) located in the islets of Langerhans. Insulin decreases blood glucose levels though increased glycogenesis and the transport of glucose into muscle, liver and adipose tissue. Muscle, liver and adipose cells require activation by insulin at insulin receptors in order to facilitate glucose transport into the cell. Neural tissue and erythrocytes do not require insulin for glucose utilization. Once glucose has entered the cell, it may be oxidized for energy (glycolysis) or stored (glycogenesis) in the muscle or liver (See Figure 1). (3,6)
Figure 1: Glucose Metabolism
Also located in the islets of Langerhans are alpha cells. Alpha cells play a role in controlling blood glucose by producing glucagon. Unlike insulin, which acts to lower blood glucose levels, glucagon acts to increase blood glucose level by accelerating glycogenolysis (See Figure 2). (3,6)
Figure 2: Normal Response to Fasting
Other blood glucose regulating hormones (shown in Table 1) that act to raise glucose concentrations include: epinephrine, growth hormone (GH), Adrenocorticotropic hormone (ACTH) and glucocorticoids.
Table 1: Hormones Regulating Blood Glucose (3,6)
HORMONE
 |
ACTION(S)
|
RESULT ON BLOOD GLUCOSE
|
| INSULIN |
- Helps glucose enter cells
- Stimulates glycogenesis
- Stimulates glucose anabolism
|
Lowers |
| GLUCAGON |
- Stimulates glycogenolysis
|
Raises |
| EPINEPHRINE |
- Stimulates glycogenolysis
|
Raises |
| GROWTH HORMONE |
- Stimulates catabolism of fats
- Decreases carbohydrate utilization
|
Raises |
| ACTH |
- Stimulates secretion of glucocorticoids
|
Raises |
| GLUCOCORTICOIDS |
- Mobilization of protein
- Stimulates gluconeogenesis
- Increases insulin resistance
|
Raises |
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Stress on Blood
