In virtually all mammalian cells, most if not all glucose uptake from the blood is achieved by members of the GLUT family of molecules.1 The different members of this family have different characteristics with regard to affinity, preferred substrate (glucose or another sugar), directionality (preference for uptake, export, or a concentration-dependent process), and responsiveness to hormonal stimuli, which appear to be based on the host tissue's need for glucose. Thus, the characteristics of glucose transport in a specific tissue can be estimated by determining which GLUT molecules populate the outer membranes of its cells.
Of the more common GLUT molecules, insulinindependent transporters can be widely distributed (eg, GLUT-1 molecules) or concentrated on tissues with important roles in glucose synthesis or regulation (eg, GLUT-2 molecules located on pancreatic beta cells, hepatocytes, and renal tubular cells) or high glucose requirements (eg, GLUT-3 molecules located on nervous tissue).1 Because these molecules represent constitutive uptake of glucose to provide basal requirements, their numbers and activity are fairly constant in these tissues. Their absence or inactivity would likely result in severe tissue dysfunction before or simultaneous to any noticeable effect on glucose clearance.
The insulin-dependent GLUT-4 molecule is most common on tissues with glucose requirements that vary considerably with activity (eg, skeletal muscle) or those in which the rate of glucose uptake is altered considerably in response to availability (eg, skeletal muscle and adipose tissue).1 Except in instances of overt obesity, skeletal muscle is a greater user of systemically available glucose, compared with adipose tissue.2 Thus, slow glucose clearance can often be linked directly or indirectly to slow glucose uptake by skeletal muscle.
The rate of glucose clearance in camelids is slower than rates in many other domestic mammals.3–8 This may be attributable in part to a smaller insulin response to exogenous glucose and in part to peripheral insulin resistance in camelids. The smaller insulin response could potentially be a result of a lack of insulin-producing beta islet cells, a lack of GLUT molecules on these cells (a function ascribed to GLUT-2 in other species), a defect in intracellular signaling, or some other defect in intracellular insulin synthesis or release. Peripheral insulin resistance could be attributable to a lack or failure of insulin receptors or a lack of insulin-sensitive GLUT-4 molecules, among other causes. Slow glucose clearance may be contributory to some of the hyperglycemic conditions detected in camelids, including hyperosmolar disorder,9 stress hyperglycemia,10 and persistent hyperglycemia11 (a diabetes mellitus–like condition).
Some of these aspects of glucose metabolism have been investigated in camels, which in evolutionary terms are the closest relatives to New World camelids.
The density and distribution of beta islet cells within the pancreas of camels appear to be similar to those of other animals,12 except that the pancreas of camels has additional isolated insulin-immunoreactive cells that are not associated with the islets.13 Also, camels appear to have insulin14 and an insulin receptor15 with morphologic and functional characteristics similar to those of other mammalian species. To our knowledge, the microscopic anatomic features of the pancreas have not been examined in New World camelids and the distribution of GLUT molecules has not been assessed in any species of camelid.
Tissue distribution of GLUT molecules can be examined via immunohistochemistry, western blot analysis, or identification of mRNA. Although camelidspecific anti-GLUT antibodies and probes are not available for these investigations, GLUT molecules in most species have relative homology and cross-reactions between species in immunologic and cDNA-based assays are common.1 Compared with other methods of GLUT detection, immunohistochemistry has the advantages that it only detects the transporter molecule itself, rather than untranslated mRNA (which may be present in tissues that do not express the transporter protein),16 and can be performed on fixed tissues. The purpose of the study reported here was to describe the microanatomic features of pancreatic islets and the immunohistochemical distribution of GLUT molecules in various tissues of New World camelids. By doing so, it was our intention to attempt to clarify the mechanisms of slow glucose clearance in camelids. We hypothesized that New World camelids would have pancreatic microanatomic features similar to those of camels and other species, with a lack of GLUT-2 molecules on islet cells (resulting in their poor insulin response to hyperglycemia) and a lack of GLUT-4 molecules on skeletal muscle cells (resulting in their peripheral insulin resistance).
Endo/Blocker, Biomeda, Foster City, Calif.
10X automation buffer, Biomeda, Foster City, Calif.
Antigen demasker reagent, Biomeda, Foster City, Calif.
Pronase reagent, Biomeda, Foster City, Calif.
Mouse/rat glucose transporter 1 antibody, Alpha Diagnostic International, San Antonio, Tex.
Rat glucose transporter 2 antibody, Alpha Diagnostic International, San Antonio, Tex.
Mouse glucose transporter 3 antibody, Alpha Diagnostic International, San Antonio, Tex.
Mouse glucose transporter 4 antibody, Alpha Diagnostic International, San Antonio, Tex.
Vector Universal Elite ABC kit, Vector Laboratories, Burlingame, Calif.
Gould GW, Holman GD. The glucose transporter family: structure, function and tissue specific expression. Biochem J 1993;295:329–341.
Dahlborn K, Benlamlih S, Wallsten C, et al.Glucose regulation in the camel. In: Allen WR, Higgins AJ, Mayhew IG, eds. Proceedings of the 1st International Camel Conference. Newmarket, UK: R&D Publications, 1992;414–415.
Elmahdi B, Sallmann HP, Fuhrmann H, et al.Comparative aspects of glucose tolerance in camels, sheep, and ponies. Comp Biochem Physiol A Physiol 1997;118:147–151.
Ommaya AK, Atwater I, Yañez A, et al.Lama glama (the South American camelid, llama): a unique model for evaluation of xenogenic islet transplants in a cerebral spinal fluid driven artificial organ. Transplant Proc 1995;27:3304–3307.
Arraya AV, Atwater I, Navia MA, et al.Evaluation of insulin resistance in two kinds of South American camelids: llamas and alpacas. Comp Med 2000;50:490–494.
Cebra CK. Hyperglycemia, hypernatremia, and hyperosmolarity in 6 neonatal llamas and alpacas. J Am Vet Med Assoc 2000;217:1701–1704.
Stehman SM, Morris LI, Weisensel L, et al.Case report: picornavirus infection associated with abortion and adult onset diabetes mellitus in a herd of llamas, in Proceedings. 40th Annu Meet Am Assoc Vet Lab Diagn 1997;43.
Khatim MS, Gumaa KA, Petersson B, et al.The structure and hormone content of the endocrine pancreas of the one-humped camel (Camelus dromedarius). Anat Anz 1985;159:181–186.
Adeghate E. Immunohistochemical identification of pancreatic hormones, neuropeptides and cytoskeletal proteins in pancreas of the camel (Camelus dromedarius). J Morphol 1997;231:185–193.
Abdel-Wahab MF, Abdel-Moneim Hussein M, Abdo MS. Preparation and purification of 131I-labelled insulin from the pancreas of domestic animals. Endokrinologie 1968;53:128–135.
al-Attas OS. Comparative studies on the major features of insulin receptors in mammalian and non-mammalian liver membranes. Comp Biochem Physiol B 1989;93:125–133.
Zhao FQ, Glimm DR, Kennelly JJ. Distribution of mam-malian facilitative glucose transporter messenger RNA in bovine tissues. Int J Biochem 1993;25:1897–1903.
Fowler ME. Digestive system. In: Medicine and surgery of South American camelids. 2nd ed. Ames, Iowa: Iowa State University Press, 1998;327–328.
Hazelwood RL. Embryology and anatomical organization of the vertebrate pancreas. In: The endocrine pancreas. Englewood Cliffs, NJ: Prentice-Hall Inc, 1989;9–15.
Xu RJ, Wang T, Zang SH. Functional structure and growth of the pancreas in postnatal growing animals. In: Pierzynowski SG, Zabielski R, eds. Biology of the pancreas in growing animals. New York: Elsevier Press, 1999;15–25.
Jackson HD, VanDewark SD, VanVleet JF. Blood chemical and pancreatic histologic alterations in alloxan-diabetic ewes and their fetuses. Am J Vet Res 1970;31:1577–1587.
Guillam MT, Hümmler E, Schaerer E, et al.Early diabetes and abnormal postnatal pancreatic islet development in mice lacking Glut-2. Nat Genet 1997;17:327–330.
Zisman A, Peroni OD, Abel ED, et al.Targeted disruption of the glucose transporter 4 selectively in muscle causes insulin resistance and glucose intolerance. Nat Med 2000;6:924–928.
Graziotti GH, Rios CM, Rivero JL. Evidence for three fast myosin heavy chain isoforms in type II skeletal muscle fibers in the adult llama (Lama glama). J Histochem Cytochem 2001;49:1033–1044.
Smerdu V, Karsch-Mizrachi I, Campione M, et al.Type IIx myosin heavy chain transcripts are expressed in type IIb fibers of human skeletal muscle. Am J Physiol 1994;267:C1723–C1728.
Talmadge RJ, Grossman EJ, Roy RR. Myosin heavy chain composition of adult feline (Felis catus) limb and diaphragm muscles. J Exp Zool 1996;275:413–420.
Tanabe R, Muroya S, Chikuni K. Sequencing of the 2a, 2x, and slow isoforms of the bovine myosin heavy chain and the different expression among muscles. Mamm Genome 1998;9:1056–1058.