The pancreas functions uniquely as both an endocrine and an exocrine gland. The gland's exocrine role consists of excretion of digestive enzymes into the duodenum via the pancreatic duct. Pancreatic endocrine function consists of the secretion of two principal hormones—insulin and glucagon—into the bloodstream. Insulin and glucagon are involved primarily with the regulation of blood glucose. Insulin also plays a role in protein and lipid metabolism and is important in several aspects of growth and development. Problems with the production and function of insulin cause a fairly common and clinically significant disease known as diabetes mellitus.
This chapter reviews the normal physiological roles of the pancreatic hormones and describes the pathogenesis and treatment of diabetes mellitus (DM). Diabetes mellitus has many sequelae that influence patients' neuromuscular and cardiovascular functioning, and patients with this condition often undergo physical rehabilitation for problems related to these issues. Consequently, the nature of DM and its pharmacotherapeutic treatment are important to physical therapists and occupational therapists.
STRUCTURE AND FUNCTION OF THE ENDOCRINE PANCREAS
The cellular composition of the pancreas has been described in great detail in other sources.1–3 The relevance of pancreatic structure to its endocrine function is reviewed briefly here. The bulk of the gland consists of acinar cells that synthesize and release pancreatic digestive enzymes (thereby providing the exocrine function). Interspersed within the acinar tissues are smaller clumps of tissue known as the islets of Langerhans. These islets contain cells that synthesize and secrete pancreatic hormones, thus constituting the endocrine portion of the gland.
The pancreatic islets consist of at least five primary cell types: alpha (A) cells, which produce glucagon; beta (B) cells, which produce insulin; delta (D) cells, which produce somatostatin; (F) cells, which produce pancreatic polypeptide; and epsilon (E) cells, which produce ghrelin.4 As previously mentioned, this discussion will focus on the functions of insulin and glucagon. The exact physiological roles of the other pancreatic hormones are not entirely clear. Somatostatin, for example, is a polypeptide hormone that appears to affect several physiological systems, including the regulation of gastrointestinal (GI) absorption and motility. This hormone may inhibit the release of glucagon and insulin.5 Somatostatin is also produced in other tissues, including the brain and GI tract, and it may affect many other neuroendocrine responses.6,7
Likewise, pancreatic ghrelin inhibits insulin release from beta cells, but ghrelin is also produced in the stomach and other tissues. This peptide helps regulate GI function, lipid metabolism, cardiovascular function, growth hormone release, and several other physiological functions that continue to be investigated.8 The effects of the pancreatic polypeptide released from pancreatic F cells remain to be fully determined.4 Future studies are needed to further clarify the physiological effects of somatostatin, ghrelin, and pancreatic polypeptide.