Medical Physiology, 3rd Edition

CHAPTER 51. The Endocrine Pancreas

Eugene J. Barrett

The islets of Langerhans are endocrine and paracrine tissue

The pancreas contains two types of glands: (1) exocrine glands, which secrete digestive enzymes (see p. 882) and image (see pp. 885–886) into the intestinal lumen; and (2) endocrine glands, called the islets of Langerhans. The islets are spread throughout the pancreas and in aggregate comprise only 1% to 2% of its tissue mass.

The normal human pancreas contains between 500,000 and several million islets. Islets can be oval or spherical and measure between 50 and 300 µm in diameter. Islets contain at least four types of secretory cells—α cells, β cells, δ cells, and F cells—in addition to various vascular and neural elements (Fig. 51-1 and Table 51-1). β cells secrete insulin, proinsulin, C peptide, and a newly described protein, amylin (or IAPP for islet amyloid polypeptide). β cells are the most numerous type of secretory cell within the islets; they are located throughout the islet but are particularly numerous in the center. α cells principally secrete glucagon, δ cells secrete somatostatin, and F cells (also called pancreatic polypeptide cells) secrete pancreatic polypeptide.

image

FIGURE 51-1 Islet of Langerhans. The distribution of cell types is representative of islets from ~90% of the human pancreas, which arises embryologically from the dorsal pancreatic bud. In the other islets (not shown), F cells dominate.

TABLE 51-1

Products of Pancreatic Islet Cells

CELL TYPE

PRODUCT

α

Glucagon

β

Insulin
Proinsulin
C peptide
Amylin

δ

Somatostatin

F

Pancreatic polypeptide

The islets are richly perfused (blood flow per gram of tissue is >5 times that of the myocardium) and receive both sympathetic and parasympathetic innervation. These cells also can communicate with each other and influence each other's secretion. We can group these communication links into three categories:

1. Humoral communication. The blood supply of the islet courses outward from the center of the islet toward the periphery, carrying glucose and other secretagogues. In the rat—and less strikingly in humans—β cells are more abundant in the center of the islet, whereas α and δ cells are more abundant in the periphery. Cells within a given islet can influence the secretion of other cells as the blood supply courses outward through the islet carrying the secreted hormonal product of each cell type with it. For example, glucagon is a potent insulin secretagogue, insulin modestly inhibits glucagon release, and somatostatin potently inhibits the secretion of both insulin and glucagon (as well as the secretion of growth hormone and other non-islet hormones).

2. Cell-cell communication. Both gap and tight junctional structures connect islet cells with one another. Cells within an islet communicate via gap junctions, which may be important for the regulation of both insulin and glucagon secretion.

3. Neural communication. Both the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS) regulate islet secretion. Cholinergic stimulation augments insulin secretion. Adrenergic stimulation can have either a stimulatory or inhibitory effect, depending on whether β-adrenergic (stimulatory) or α-adrenergic (inhibitory) stimulation dominates (see p. 1033). imageN51-1

N51-1

Antagonistic Effects of α- and β-Adrenergic Receptors on Insulin Secretion

Contributed by Emile Boulpaep, Walter Boron

On page 1033, we noted that the general rule for α- and β-adrenergic receptors—first noted by Raymond Ahlquist—is that α activation leads to stimulation, whereas β activation leads to inhibition. The pattern in pancreatic islets is just the opposite, as noted in the text.

These three communication mechanisms allow for a tight control over the synthesis and secretion of islet hormones.

Insulin

Glucagon

Somatostatin

References