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The development and regulation of the endocrine pancreas
EDITOR'S C O L U M N
The development and regulation of the endocrine pancreas IN THE MORE T H A N 5 0 YEARS s i n c e Banting a n d B e s t
showed that diabetes mellitus is an insulin-deficient state which can be treated by injections of insulin, our understanding of the pathophysiology of this condition has increased both in depth and complexity. In addition to the insulin-secreting beta cell, the islets of Langerhans contain at least three other cell types that secrete specific hormones. The alpha cell secretes glucagon, which stimulates hepatic glycogenolysis and gluconeogenesis; the delta cell secretes somatostatin, which is a potent inhibitor of insulin and glucagon release. '-~ Recently, a fourth hormone whose function(s) is still unknown, human pancreatic polypeptide, has been localized to a specific cell type within the islets of Langerhans. 5 ~ Some of these hormones are present in extrainsular tissues (e.g., glucagon in gastric fundus, somatostatin widely distributed in the central nervous system and the gastrointestinal tract). Islet hormonal secretion is modulated by a variety of integrated control mechanisms that include substrate levels, extrapancreatic hormones, the autonomic nervous system, 7 and intra-islet regulatory mechanisms? These intra-islet regulatory mechanisms are mediated humorally as well as through direct cell-cell communication. Studies have shown: (1) that exogenous glucagon stimulates the release of both insulin and somatostatin, ~.... (2) that somatostatin inhibits the secretion of both insulin and glucagon, 1~ (3) that insulin infusion suppresses release of glucagonr and (4) that glucagon levels are elevated in insulin-deficient diabetic patients, suggesting that beta cell deficiency results in uncontrolled release of glucagon. la Evidence for direct cell-cell interaction is derived from morphologic studies, showing the presence of gap junctions between islet cells. 1~These junctions provide for ionic and small molecular communication between cells. That gap junctions may be involved in intercell hormonal translocation is most improbable, because of the recent demonstration that the gap junction pore size in Chironomus salivary gland excludes molecules of 1,900 daltons or greater. 1~
518
The Journal o f P E D I A T R I CS Vol. 91, No. 3, pp. 518-520
The apparent organization of normal islets with intricate juxtaposition of the alpha, beta, delta, and pancreatic polypeptide cells indicates an exquisite degree of interaction among these various cell types to achieve hormonal homeostasis. Therefore, it is not surprising that disruption or disorganization of the islets during fetal development may result in abnormal hormonal regulation. Idiopathic hypoglycemia of infancy and transient neonatal diabetes mellitus may be examples of such architectural dysmaturation. Another example of islet disorganization of the acquired variety is juvenile-onset diabetes mellitus. The characteristic pancreatic pathology in juvenile diabetes mellitus is the absence of beta cells and hyperplasia of alpha, delta, and pancreatic polypeptide cells? ~ Glucagon levels are elevated in these patients, See related articles, p. 395.
and while exogenous insulin therapy leads to some reduction in plasma glucagon, these levels are rarely lowered to the range found in normal patientsY-" ':~ "' 18 Grajwer and associates 1" have shown that diabetic patients who maintain residual beta cell function, as evidenced by secretion of C-peptide in levels greater than 2.0 ng/ml, have better diabetic Control, possibly owing to modulation of the alpha cells by the remaining beta cells. Whether the hyperglucagonemia of diabetes is secondary to the alpha cell hyperplasia or to lack of regulation of these cells by the beta cells is still unclear. In idiopathic hypoglycemia of infancy, pancreatic pathology is variable and includes hyperplasia of the islets and nesidioblastosis, i.e., the presence of beta cells and small nests of other endocrine cells arising from the pancreatic ducts. The islets are frequently bizarre in shape and variable in size and contain localized clumps of one or another o f the endocrine cell types. There are individual alpha, beta, delta, and pancreatic polypeptide cells scattered throughout the exocrine parenchyma. These infants have severe hypoglycemia, serum insulin levels
Volume 91 Number 3
which are within the normal fasting range, albeit inappropriately elevated for the blood glucose levels, and increased glucagon levels. These infants clearly represent a hyperinsulin state as evidenced by their large size, which reflects the peripheral somatotrophic effects of insulin. The beta cells in these infants demonstrate exquisite sensitivity to infusion of somatostatin by suppression of serum insulin and a concomitant rise in blood glucose. ~..... Plasma glucagon levels, on the other hand, are not suppressed even at somatostatin infusion rates of 20 ~g/kg/hour. Infants with transient neonatal diabetes are characteristically of low birth weight. The pancreatic pathology in this disorder has not been well characterized. Schwartzmann and associates 2'-' reviewed 23 cases and found no uniformity in pancreatic pathology. The most common finding was atrophy of the pancreas with decreased size and number of islets. In a number of cases no pancreatic abnormality was found, and in others hypertrophy of islets was noted. Since these infants are now successfully treated with insulin, no characterization of islet cell types with modern techniques is now available. The study of Sodoyez-Goflhux and Sodoyez reported in this issue of ThE JOURNAL is of interest because of the rarity of transient neonatal diabetes. These authors found no measurable immunoreactive insulin in the portal blood, but the glucagon level was elevated; infusion of somatostatin failed to suppress glucagon secretion. Thus, this infant shares many of the common findings in infants with hypoglycemia due to nesidioblastosis. In each condition the infants may undergo spontaneous remission or improvement. Each condition is associated with either low or totally absent immunoreactive insulin, elevated and occasionally paradoxical glucagon responses, and failure of somatostatin to suppress glucagon secretion. The failure of somatostatin to suppress glucagon in these infants suggests that the alpha cell response may well be a normal finding in infancy and not a manifestation of either condition. This view is supported by the findings of Grajwer and associates -':~ and Sperling and associates '-'4 that the glucagon surge at birth in newborn lambs cannot be blocked by somatostatin, although at one day of age the alpha cells will be suppressed by somatostatin. This effect is consistent with the greater relative maturity of the lamb at birth as compared to that of the human infant. The similarity in findings in these two conditions raises questions about the role and function of the newly recognized pancreatic polypeptide cell. What does this peptide do? Is it a hormone? How is it regulated? Does it have a role in the pathophysiology of either condition? Recently, Schweisthal and associates '-'~'described the on-
Editor's column
5 19
togeny of four cell types in fetal rat islets. In addition to the alpha cell (appearing on the thirteenth fetal day), the beta cell (appearing by the fourteenth fetal day), and the delta cell (appearing by the seventeenth fetal day), these authors describe the appearance of a distinct cell ("fourth cell type") on the fifteenth fetal day. From the author's description and published electron micrographs, this cell is almost certainly the pancreatic polypeptide cell. These cells were found to be surprisingly abundant in the fetal islet, being nearly as numerous as the alpha and beta cells at 19 days of fetal age when they reach their peak numbers. After birth they were seen less frequently, and in the adult rat pancreas were relatively few in number in each islet. In our studies of fetal, neonatal, and adult pancreas, we find a similar age-related frequency of appearance of the pancreatic polypeptide cell." Although the function of this islet peptide is not known, we recently showed significant hepatic extraction of pancreatic polypeptide in infants with idiopathic hypoglycemia; the portal/peripheral ratio ranged from 2 to 10, suggesting that pancreatic polypeptide may have an hepatic site of action, z~ It is now clear that the complexity of the architectural arrangement and composition of the islets provides a hitherto unappreciated basis for understanding developmental and acquired pathophysiology of these islets. A better understanding of the function of this complex organ (the islets of Langerhans) is now feasible with the availability of radioimmunoassays to simultaneously measure all four islet peptides (insulin, glucagon, somatostatin, and pancreatic polypeptide). ~7 Immunocytochemical localization of these peptides, on the other hand, makes it possible to determine islet composition and structure and provide clarification of structure-function relationships in normal and disease states. Investigations of similarities and contrasts between hypoglycemia of infancy with nesidioblastosis, transient neonatal diabetes, and juvenile onset diabetes mellitus, will inevitably lead to a better understanding of each of these conditions as well as of the normal fetal and neonatal development of the islets of Langerhans. Harry J. Hirsch, M.D. Sherry W. Loo, M.D. Kenneth H. Gabbay, M.D. Cell Biology Laboratory Diabetes Unit, Endocrine Division Department of Medicine Children's Hospital Medical Center Department of Pediatrics Harvard Medical School Boston, MA 02115
520
Editor's column
REFERENCES
1. Hokfelt T, Efendic S, Hellerstrom C, et al: Cellular localization ofsomatostatin in endocrine-like ceils and neurons of the rat with special references to the Al-cells of the pancreatic islets and to the hypothalamus, Acta Endocrinologica Suppl 200:5, 1975. 2. Dubois MP: Immunoreactive somatostatin is present in discrete cells of endocrine pancreas, Proc Natl Acad Sci 72:1340, 1975. 3. Arimura A, Soto H, Dupont A, et al: Somatostatin: Abundance of immunoreactive hormone in rat stomach and pancreas, Science 189:1007, 1975. 4. Goldsmith PC, Rose JC, Arimura A, et al: Ultrastructural localization of somatostatin in pancreatic islets of the rat, Endocrinology 97:1061, 1975. 5. Larsson LI, Sundler F, and Hakanson R: Immunohistochemical localization of human pancreatic polypeptide (HPP) to a population of islet cells, Cell Tissue Res 156:167, 1975. 6. Bergstrom BH, Loo S, Hirsch HJ, et al: Ultrastructural localization of pancreatic polypeptide in human pancreas, J Clin Endocrinol Metab 44:795,1977. 7. Woods SC, and Porte D Jr: Neural control of the endocrine pancreas, Physiol Rev 54:596, 1974. 8. Orci L, and Unger RH: Functional subdivision of islets of Langerhans and possible role of D cells, Lancet 2:1243, 1975. 9. Samols E, Marri G, and Marks V: Promotion of insulin secretion by glucagon, Lancet 2:415, 1965. 10. Weir GC, Samols E, Ramseur R, et al: Influence of glucose and glucagon upon somatostatin secretion from the isolated perfused canine pancreas, Clin Res 25:403A, 1977. 11. Mortimer CH, Tunbridge WMG, Cart D, et al: Effects of growth hormone release-inhibitory hormone on circulating glucagon, insulin, and growth hormone in normal, diabetic, acromegalic, and hypopituitary patients, Lancet 1:697, 1974. 12. Raskin P, Fujita Y, and Unger RH: Effects of insulinglucose infusions on plasma glucagon levels in fasting diabetics and non-diabetics, J Clin Invest 56:1132, 1975. 13. Buchanan KD, and McCaroll AM: Abnormalities of glucagon metabolism in untreated diabetes mellitus, Lancet 2:1394, 1972.
The Journal of Pediatrics September 1977
14. Orci L: A portrait of the pancreatic B-cell, Diabetologia 10:163, 1974. 15. Simpson I, Rose B, and Loewenstein WR: Size limit of molecules permeating the junctional membrane channels, Science 195:294, 1977. 16. Gepts W, DeMey J, and Marichal-Pipeleers M: Hyperplasia of"pancreatic polypeptide" cells in the pancreas of juvenile diabetics, Diabetologia 13:27, 1977. 17. Dobbs R, Sakurai H, Sasaki H, et al: Glucagon: Role in the hyperglycemia of diabetes mellitus, Science 187:544, 1975. 18. Unger RH: Diabetes and the alpha cell, Diabetes 25:136, 1976. 19. Grajwer LA, Pildes RS, Horwitz DL, and Rubenstein AH: Control of juvenile diabetes mellitus and its relationship to endogenous insulin secretion as measured by C-peptide immUnoreactivity, J PEDIATR 90:42, 1977. 20. Hirsch HJ, Loo S, Evans N, Crigler JF, Jr., et al: Investigation of nesidioblastosis using somatostatin, Pediatr Res 10:410, 1976. 21. Hirsch HJ, Loo S, Evans N, et al: Hypoglycemia of infancy and nesidioblastosis: studies with somatostatin, N Engl J Med (in press). 2 2 . Schwartzman J, Crusius ME, and Beirne DP: Diabetes mellitus in infants under one year of age, Am J Dis Child 74:587, 1947. 23. Grajwer LA, Sperling MA, Sack J, et al: Possible mechanisms and significance of the neonatal surge in glucagon secretion: studies in newborn lambs, Pediatr Res (in press). 24. Sperling MA, Grajwer LA, Leake RD, et al: Effects of somatostatin (SRIF) infusion on glucose homeostasis in newborn Iambs: evidence for a significant role of glucagon, Pediatr Res (in press). 25. Schweisthal MR, Frost CC, and Brinn JE: Ontogeny of four cell types in fetal rat islets using histochemical techniques, Acta Diabetol Lat 13:30, 1976. 26. Loo SW, Hirsch HJ, and Gabbay KH: Human pancreatic polypeptide: portal-peripheral gradient in man, Diabetes 26:407, 1977. 27. Loo SW, Weir GC, Samols E, et al: Biphasic stimulation of pancreatic polypeptide and somatostatin by arginine from the isolated perfused canine pancreas, program, Fifty-ninth Annual Meeting, The Endocrine Society, p A-29, 1977.
The development and regulation of the endocrine pancreas IN THE MORE T H A N 5 0 YEARS s i n c e Banting a n d B e s t
showed that diabetes mellitus is an insulin-deficient state which can be treated by injections of insulin, our understanding of the pathophysiology of this condition has increased both in depth and complexity. In addition to the insulin-secreting beta cell, the islets of Langerhans contain at least three other cell types that secrete specific hormones. The alpha cell secretes glucagon, which stimulates hepatic glycogenolysis and gluconeogenesis; the delta cell secretes somatostatin, which is a potent inhibitor of insulin and glucagon release. '-~ Recently, a fourth hormone whose function(s) is still unknown, human pancreatic polypeptide, has been localized to a specific cell type within the islets of Langerhans. 5 ~ Some of these hormones are present in extrainsular tissues (e.g., glucagon in gastric fundus, somatostatin widely distributed in the central nervous system and the gastrointestinal tract). Islet hormonal secretion is modulated by a variety of integrated control mechanisms that include substrate levels, extrapancreatic hormones, the autonomic nervous system, 7 and intra-islet regulatory mechanisms? These intra-islet regulatory mechanisms are mediated humorally as well as through direct cell-cell communication. Studies have shown: (1) that exogenous glucagon stimulates the release of both insulin and somatostatin, ~.... (2) that somatostatin inhibits the secretion of both insulin and glucagon, 1~ (3) that insulin infusion suppresses release of glucagonr and (4) that glucagon levels are elevated in insulin-deficient diabetic patients, suggesting that beta cell deficiency results in uncontrolled release of glucagon. la Evidence for direct cell-cell interaction is derived from morphologic studies, showing the presence of gap junctions between islet cells. 1~These junctions provide for ionic and small molecular communication between cells. That gap junctions may be involved in intercell hormonal translocation is most improbable, because of the recent demonstration that the gap junction pore size in Chironomus salivary gland excludes molecules of 1,900 daltons or greater. 1~
518
The Journal o f P E D I A T R I CS Vol. 91, No. 3, pp. 518-520
The apparent organization of normal islets with intricate juxtaposition of the alpha, beta, delta, and pancreatic polypeptide cells indicates an exquisite degree of interaction among these various cell types to achieve hormonal homeostasis. Therefore, it is not surprising that disruption or disorganization of the islets during fetal development may result in abnormal hormonal regulation. Idiopathic hypoglycemia of infancy and transient neonatal diabetes mellitus may be examples of such architectural dysmaturation. Another example of islet disorganization of the acquired variety is juvenile-onset diabetes mellitus. The characteristic pancreatic pathology in juvenile diabetes mellitus is the absence of beta cells and hyperplasia of alpha, delta, and pancreatic polypeptide cells? ~ Glucagon levels are elevated in these patients, See related articles, p. 395.
and while exogenous insulin therapy leads to some reduction in plasma glucagon, these levels are rarely lowered to the range found in normal patientsY-" ':~ "' 18 Grajwer and associates 1" have shown that diabetic patients who maintain residual beta cell function, as evidenced by secretion of C-peptide in levels greater than 2.0 ng/ml, have better diabetic Control, possibly owing to modulation of the alpha cells by the remaining beta cells. Whether the hyperglucagonemia of diabetes is secondary to the alpha cell hyperplasia or to lack of regulation of these cells by the beta cells is still unclear. In idiopathic hypoglycemia of infancy, pancreatic pathology is variable and includes hyperplasia of the islets and nesidioblastosis, i.e., the presence of beta cells and small nests of other endocrine cells arising from the pancreatic ducts. The islets are frequently bizarre in shape and variable in size and contain localized clumps of one or another o f the endocrine cell types. There are individual alpha, beta, delta, and pancreatic polypeptide cells scattered throughout the exocrine parenchyma. These infants have severe hypoglycemia, serum insulin levels
Volume 91 Number 3
which are within the normal fasting range, albeit inappropriately elevated for the blood glucose levels, and increased glucagon levels. These infants clearly represent a hyperinsulin state as evidenced by their large size, which reflects the peripheral somatotrophic effects of insulin. The beta cells in these infants demonstrate exquisite sensitivity to infusion of somatostatin by suppression of serum insulin and a concomitant rise in blood glucose. ~..... Plasma glucagon levels, on the other hand, are not suppressed even at somatostatin infusion rates of 20 ~g/kg/hour. Infants with transient neonatal diabetes are characteristically of low birth weight. The pancreatic pathology in this disorder has not been well characterized. Schwartzmann and associates 2'-' reviewed 23 cases and found no uniformity in pancreatic pathology. The most common finding was atrophy of the pancreas with decreased size and number of islets. In a number of cases no pancreatic abnormality was found, and in others hypertrophy of islets was noted. Since these infants are now successfully treated with insulin, no characterization of islet cell types with modern techniques is now available. The study of Sodoyez-Goflhux and Sodoyez reported in this issue of ThE JOURNAL is of interest because of the rarity of transient neonatal diabetes. These authors found no measurable immunoreactive insulin in the portal blood, but the glucagon level was elevated; infusion of somatostatin failed to suppress glucagon secretion. Thus, this infant shares many of the common findings in infants with hypoglycemia due to nesidioblastosis. In each condition the infants may undergo spontaneous remission or improvement. Each condition is associated with either low or totally absent immunoreactive insulin, elevated and occasionally paradoxical glucagon responses, and failure of somatostatin to suppress glucagon secretion. The failure of somatostatin to suppress glucagon in these infants suggests that the alpha cell response may well be a normal finding in infancy and not a manifestation of either condition. This view is supported by the findings of Grajwer and associates -':~ and Sperling and associates '-'4 that the glucagon surge at birth in newborn lambs cannot be blocked by somatostatin, although at one day of age the alpha cells will be suppressed by somatostatin. This effect is consistent with the greater relative maturity of the lamb at birth as compared to that of the human infant. The similarity in findings in these two conditions raises questions about the role and function of the newly recognized pancreatic polypeptide cell. What does this peptide do? Is it a hormone? How is it regulated? Does it have a role in the pathophysiology of either condition? Recently, Schweisthal and associates '-'~'described the on-
Editor's column
5 19
togeny of four cell types in fetal rat islets. In addition to the alpha cell (appearing on the thirteenth fetal day), the beta cell (appearing by the fourteenth fetal day), and the delta cell (appearing by the seventeenth fetal day), these authors describe the appearance of a distinct cell ("fourth cell type") on the fifteenth fetal day. From the author's description and published electron micrographs, this cell is almost certainly the pancreatic polypeptide cell. These cells were found to be surprisingly abundant in the fetal islet, being nearly as numerous as the alpha and beta cells at 19 days of fetal age when they reach their peak numbers. After birth they were seen less frequently, and in the adult rat pancreas were relatively few in number in each islet. In our studies of fetal, neonatal, and adult pancreas, we find a similar age-related frequency of appearance of the pancreatic polypeptide cell." Although the function of this islet peptide is not known, we recently showed significant hepatic extraction of pancreatic polypeptide in infants with idiopathic hypoglycemia; the portal/peripheral ratio ranged from 2 to 10, suggesting that pancreatic polypeptide may have an hepatic site of action, z~ It is now clear that the complexity of the architectural arrangement and composition of the islets provides a hitherto unappreciated basis for understanding developmental and acquired pathophysiology of these islets. A better understanding of the function of this complex organ (the islets of Langerhans) is now feasible with the availability of radioimmunoassays to simultaneously measure all four islet peptides (insulin, glucagon, somatostatin, and pancreatic polypeptide). ~7 Immunocytochemical localization of these peptides, on the other hand, makes it possible to determine islet composition and structure and provide clarification of structure-function relationships in normal and disease states. Investigations of similarities and contrasts between hypoglycemia of infancy with nesidioblastosis, transient neonatal diabetes, and juvenile onset diabetes mellitus, will inevitably lead to a better understanding of each of these conditions as well as of the normal fetal and neonatal development of the islets of Langerhans. Harry J. Hirsch, M.D. Sherry W. Loo, M.D. Kenneth H. Gabbay, M.D. Cell Biology Laboratory Diabetes Unit, Endocrine Division Department of Medicine Children's Hospital Medical Center Department of Pediatrics Harvard Medical School Boston, MA 02115
520
Editor's column
REFERENCES
1. Hokfelt T, Efendic S, Hellerstrom C, et al: Cellular localization ofsomatostatin in endocrine-like ceils and neurons of the rat with special references to the Al-cells of the pancreatic islets and to the hypothalamus, Acta Endocrinologica Suppl 200:5, 1975. 2. Dubois MP: Immunoreactive somatostatin is present in discrete cells of endocrine pancreas, Proc Natl Acad Sci 72:1340, 1975. 3. Arimura A, Soto H, Dupont A, et al: Somatostatin: Abundance of immunoreactive hormone in rat stomach and pancreas, Science 189:1007, 1975. 4. Goldsmith PC, Rose JC, Arimura A, et al: Ultrastructural localization of somatostatin in pancreatic islets of the rat, Endocrinology 97:1061, 1975. 5. Larsson LI, Sundler F, and Hakanson R: Immunohistochemical localization of human pancreatic polypeptide (HPP) to a population of islet cells, Cell Tissue Res 156:167, 1975. 6. Bergstrom BH, Loo S, Hirsch HJ, et al: Ultrastructural localization of pancreatic polypeptide in human pancreas, J Clin Endocrinol Metab 44:795,1977. 7. Woods SC, and Porte D Jr: Neural control of the endocrine pancreas, Physiol Rev 54:596, 1974. 8. Orci L, and Unger RH: Functional subdivision of islets of Langerhans and possible role of D cells, Lancet 2:1243, 1975. 9. Samols E, Marri G, and Marks V: Promotion of insulin secretion by glucagon, Lancet 2:415, 1965. 10. Weir GC, Samols E, Ramseur R, et al: Influence of glucose and glucagon upon somatostatin secretion from the isolated perfused canine pancreas, Clin Res 25:403A, 1977. 11. Mortimer CH, Tunbridge WMG, Cart D, et al: Effects of growth hormone release-inhibitory hormone on circulating glucagon, insulin, and growth hormone in normal, diabetic, acromegalic, and hypopituitary patients, Lancet 1:697, 1974. 12. Raskin P, Fujita Y, and Unger RH: Effects of insulinglucose infusions on plasma glucagon levels in fasting diabetics and non-diabetics, J Clin Invest 56:1132, 1975. 13. Buchanan KD, and McCaroll AM: Abnormalities of glucagon metabolism in untreated diabetes mellitus, Lancet 2:1394, 1972.
The Journal of Pediatrics September 1977
14. Orci L: A portrait of the pancreatic B-cell, Diabetologia 10:163, 1974. 15. Simpson I, Rose B, and Loewenstein WR: Size limit of molecules permeating the junctional membrane channels, Science 195:294, 1977. 16. Gepts W, DeMey J, and Marichal-Pipeleers M: Hyperplasia of"pancreatic polypeptide" cells in the pancreas of juvenile diabetics, Diabetologia 13:27, 1977. 17. Dobbs R, Sakurai H, Sasaki H, et al: Glucagon: Role in the hyperglycemia of diabetes mellitus, Science 187:544, 1975. 18. Unger RH: Diabetes and the alpha cell, Diabetes 25:136, 1976. 19. Grajwer LA, Pildes RS, Horwitz DL, and Rubenstein AH: Control of juvenile diabetes mellitus and its relationship to endogenous insulin secretion as measured by C-peptide immUnoreactivity, J PEDIATR 90:42, 1977. 20. Hirsch HJ, Loo S, Evans N, Crigler JF, Jr., et al: Investigation of nesidioblastosis using somatostatin, Pediatr Res 10:410, 1976. 21. Hirsch HJ, Loo S, Evans N, et al: Hypoglycemia of infancy and nesidioblastosis: studies with somatostatin, N Engl J Med (in press). 2 2 . Schwartzman J, Crusius ME, and Beirne DP: Diabetes mellitus in infants under one year of age, Am J Dis Child 74:587, 1947. 23. Grajwer LA, Sperling MA, Sack J, et al: Possible mechanisms and significance of the neonatal surge in glucagon secretion: studies in newborn lambs, Pediatr Res (in press). 24. Sperling MA, Grajwer LA, Leake RD, et al: Effects of somatostatin (SRIF) infusion on glucose homeostasis in newborn Iambs: evidence for a significant role of glucagon, Pediatr Res (in press). 25. Schweisthal MR, Frost CC, and Brinn JE: Ontogeny of four cell types in fetal rat islets using histochemical techniques, Acta Diabetol Lat 13:30, 1976. 26. Loo SW, Hirsch HJ, and Gabbay KH: Human pancreatic polypeptide: portal-peripheral gradient in man, Diabetes 26:407, 1977. 27. Loo SW, Weir GC, Samols E, et al: Biphasic stimulation of pancreatic polypeptide and somatostatin by arginine from the isolated perfused canine pancreas, program, Fifty-ninth Annual Meeting, The Endocrine Society, p A-29, 1977.