Somatostatin modulation of glucagon secretion and its importance in human glucose homeostasis

Somatostatin Modulation of Glucagon Secretion and Its Importance in Human Glucose Homeostasis John E. Gerich

T

HE ABILITY of somatostatin to inhibit glucagon and insulin release from the pancreas without directly affecting substrate metabolism’-” has provided investigators with a potent tool for studying the physiological and pathological interaction of insulin and glucagon in man. The presence of somatostatin itself in D cells of human pancreatic islets4 and in those of other species”-” suggests that local release of somatostatin may regulate glucagon and insulin secretion and, thus, that somatostatin may also indirectly be an important determinant of substrate homeostasis. Finally, since glucagon”’ and growth hormone” both appear to have adverse effects in human diabetes, the ability of somatostatin to suppress release of these hormones may find therapeutic application in the treatment of diabetes. IMPACT

OF SOMATOSTATIN

ON THE STUDY OF HUMAN

CARBOHYDRATE

HOMEOSTASIS

Role of Glucagon in Diabetes Mellitus Prior to the discovery of somatostatin, the role of glucagon relative to that of insulin in human carbohydrate metabolism was mainly inferential. Despite demonstration that glucagon had potent actions on hepatic glucose production in vitro,‘* the physiological and pathological implications of this were unclear, since administration of glucagon did not generally result in sustained hyperglycemia or diabetes.‘” Moreover, combined glucagon and insulin deficiency (which was presumed to occur after total pancreatectomy) seemed to result in the same manifestations of isolated insulin deficiency. ” To some extent, recent studies”p22 have provided explanations for these findings consistent with an important role for glucagon in both normal human glucose metabolism and the aberrations of it found in diabetes mellitus. With the availability of synthetic somatostatin, it became possible to study in man the effects of variations of glucagon or insulin levels independent of changes in the other hormone, and also the effects of unequivocal deficiency of either or both hormones. When somatostatin is infused into normal man, a combined deficiency of both insulin and glucagon is produced. This results in a transient fall of circulating glucose levels followed by the gradual development of mild hyperglycemia.2”,“4 Studies in dogs (Fig. 1) have shown that during infusion of somatostatin an initial fall in hepatic glucose production exceeds an initial decrease in peripheral glucose utilization; later, hepatic glucose production returns to presomatostatin levels, while restoration of peripheral glucose utilization lags be-

From the Diabetes and Metabolism Research Laboratory. Endocrine Research Unit, Mayo Clinic. and the Departments of Medicine and Physiology. Mayo MedicalSchool, Rochester. Minn. Supported in part by the Mayo Foundation and by USPHS Grant AM20411-01. Address reprint requests to Dr. J. E. Gerieh, Diabetes and Metabolism Research Laboratory. Endocrine Research Unit. Mayo Clinic, Rochester, Minn. 55901. o 1978 by Grune & Stratton. Inc. 00260495/78/2713-0029$01.00/0 Metabolism, Vol. 27. No. 9. Suppl. 1 (September). 1978

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150 r

NOTSIGNIFICANTLY DIFFERENT

30

2.5 GLUCOSE 2.0 TURNOVER w/kg/min 1.5 ,I-, _ 0

60

120 I80

240 300 360

MINUTES

Fig. 1. Plasma glucose levels and glucose production and utilization rates during prolonged suppression of glucagon and insulin release by somatostatin in normal conscious dogs. (From Sherwin et al: Proc Nat1 Acad Sci USA 74~34%352,1977l

hind. The net result oT these discrepancies between glucose production and utilization is mild hyperglycemia. It is important to point out that, despite virtually complete inhibition of insulin secretion, hepatic production of glucose does not exceed normal values. Thus, these observations and the results of recent studies in totally pancreatectomized humans,?’ whose plasma lacked immunoreactive glucagon, indicate that, while hyperglucagonemia is not essential to the development of fasting hyperglycemia in man, glucagon nevertheless influences the degree of diabetic hyperglycemia. This has been further demonstrated in other studies”” using somatostatin (Fig. 2): Insulin-dependent, ketosis-prone human diabetics were maintained euglycemic by means of overnight infusions of regular insulin and then were abruptly withINSULIN

MEAN

+ SEW N=?

250 SOMATOSTATIN

150

50

L

‘T I -2

I

. I

I

I

I

0

4

8

12

16

20

HOURS Fig. 2. Comparison of fasting hyperglycemia developing after acute withdrawal of insulin from juvenile-onset. insulin-dependent diabetics in presence of hyperglucagonemia and during suppression of glucagon secretion by somatostatin. (From Gerich et al: N Engl J Med 292:985-989, 1975)

MODULATION

AND

IMPORTANCE

OF GLUCAGON

INSULIN

----_-___ SOMATOSTATIN

SALINE

y_-

01 -2’

1285

SECRETION

____------___ - - - -

500

w/h

I

I

I

I

I

I

I

0

2

4

6

8

10

12

HOURS Fig. 3. Effect of suppression of prolonged hyperglucagonemia hyperglycemia in juvenile-onset, insulin-dependent diabetics.

by somatostatin

on fasting

drawn from that insulin; plasma glucose rapidly rose to levels approximating 300 mg/lOO ml within 10 hr. When the same subjects were withdrawn from insulin during an infusion of somatostatin (thus producing combined growth hormone and glucagon deficiency) plasma glucose levels rose only to 150 mg/lOO ml after 18 hr. That these results were due to glucagon deficiency rather than the growth hormone deficiency induced by the somatostatin infusion is seen from the fact that infusion of physiological quantities of glucagon reproduced the hyperglycemia found during withdrawal of insulin in the absence of somatostatin, whereas infusion of growth hormone did not. y7 These results also show that glucagon can cause prolonged hyperglycemia during insulin deficiency even though prolonged hyperglucagonemia may cause only transient stimulation of hepatic glucose overproduction. In most studies, diminished responsiveness of the liver to glucagon occurs after a 2-3-hr exposure to glucagon. To determine whether hyperglucagonemia in this time frame could contribute to fasting hyperglycemia, we infused somatostatin for 6 hr following a period (6 hr) of acute withdrawal from insulin in insulin-dependent diabetic subjects. Presumably, if hyperglucagonemia had only a transient effect on hepatic glucose production, no change in plasma glucose would be observed. As can be seen in Fig. 3, despite prolonged hyperglucagonemia for 6 hr, the subsequent inhibition of glucagon secretion by somatostatin produced a progressive decrease in circulating glucose levels to approximately 150 mg/lOO ml. These results and those recently reported by Bomboy et al.,” indicate that persistent hyperglucagonemia still exerts an effect on hepatic glucose production. Role of Glucagon as a Counterregulatory

Hormone

Observations that glucagon can antagonize the effects of insulin on hepatic glucose production,” that hypoglycemia is a potent stimulus for glucagon secretion,“’ and that plasma glucose levels fall during inhibition of glucagon secretion suggest that glucagon may function physiologically as an acute glucose counterregulatory hormone. Recent studies with somatostatin have provided further evidence in support of this concept. 2g Infusion of somatostatin along with insulin (0.05 U/kg, i.v.

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J.W. 33~1 ADRENALX.d

INSULIN 0.05 U/Kg

I

-30

0 *

E GERICH

30 .

60 *

1 SO 120 I50 1

1

MINUTES

Fig. 4. Effect of glucagon deficiency on msulin-induced hypoglycemia in a glucocorticoid-treated adrenalectomized subject.

bolus) to normal subjects was found to enhance the hypoglycemic action of insulin and to prolong the recovery of plasma glucose to normoglycemia, which infusion of growth hormone did not correct. 29These results suggest that the rise in plasma glucagon during insulin-induced hypoglycemia functions to counteract the hypoglycemia action of insulin and help restore normoglycemia. In these studies, however, it was not possible to fully evaluate the contribution of glucagon, since enhanced plasma catecholamine responses were observed during the infusion of somatostatin. This might have masked the full impact of glucagon deficiency on recovery of plasma glucose levels. To circumvent this problem, similar studies were carried out in glucocorticoid-replaced adrenalectomized subjects. As shown in Fig. 4, the inability to release adrenomedullary catecholamines had no apparent effect itself on plasma glucose recovery from insulin-induced hypoglycemia. When somatostatin was infused to inhibit glucagon secretion under similar experimental conditions, not only was the degree of insulin-induced hypoglycemia enhanced but no glucose recovery was seen. The results of these studies with somatostatin indicate that the acute secretion of glucagon plays an important role in man as a counterregulatory hormone; they also indicate that adrenomedullary catecholamines function as counterregulatory hormones. SOMATOSTATIN

SECRETION-AN

AS

A

REGULATOR

ADDITIONAL

OF

INSULIN

MODULATOR

AND

GLUCAGON

OF CARBOHYDRATE

HOMEOSTASIS

Since somatostatin is present within pancreatic islets of various species,‘-* including mana alteration of insulin and glucagon secretion by somatostatin could be an important factor influencing carbohydrate homeostasis. This topic is discussed in detail elsewhere in these proceedings by Dr. Unger. D cells normally constitute about 10% of the total islet cell mass and in some species appear to be asymmetrically distributed in islets so as to be more closely associated with glusecreted somatocagon-producing alpha cells. 3o This suggests that endogenously statin may preferentially affect glucagon secretion. The fact that arginine,“’ a secretagogue of glucagon, and glucagon itselfZi2 both stimulate the release of pancreatic somatostatin suggests the presence of a feedback loop involving pancreatic A and D cells. Also, somatostatin may mediate the suppressive effects of glucose on glucagon secretion, since glucose has recently been reported to

MODULATION

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OF GLUCAGON

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stimulate release of pancreatic somatostatin.33.34 Perhaps the most convincing evidence to date for a preferential effect of somatostatin on glucagon secretion has been obtained through the use of antisomatostatin antiserum.35 Incubation of isolated pancreatic islets in the presence of antiserum against somatostatin resulted in augmented glucagon secretion but no effect on insulin release. In human juvenile-onset diabetes mellitus” and in experimentally induced diabetes in animals,4,8 hyperplasia of pancreatic D cells and increased somatostatin content within islets has been found. The relationship between these findings and the enhanced glucagon secretion in diabetes remains unclear, since in obese hyperglycemic mice who have hyperglucagonemia as well as hyperinsulinemia, a decreased pancreatic somatostatin content has been found.’ Nevertheless, it is clear that aberrations of D cell function may influence glucose metabolism in man. This is exemplified by two recent reports of pancreatic D cell tumors producing excessive somatostatin secretion:3”.37 In both instances, the patients had mild glucose intolerance associated with diminished or inappropriately low insulin and glucagon secretion-a condition similar to that seen in normal subjects receiving prolonged infusions of somatostatin. In one of the patients,“‘j removal of the “somatostatinoma” resulted in complete remission of the diabetes. POTENTIAL THERAPEUTIC

USE OF SOMATOSTATIN

IN DIABETES

MELLITUS

At the present time, there is increasing evidence that the chronic complications of diabetes are probably related to control of hyperglycemia.“’ Unfortunately, insulin therapy presently available has not been uniformly successful in restoring carbohydrate homeostasis sufficiently close to normal so as to prevent these complications. This has led to attempts to develop improved methods of insulin replenishment (artificial pancreas and transplantation of pancreas). The recognition that excess glucagon’” as well as insulin deficiency may be an important factor in the carbohydrate intolerance of diabetes suggests that suppression of glucagon (and growth hormone) secretion may be advantageous. This has prompted investigations of the potential therapeutic use of somatostatin as an adjunct to insulin in the treatment of human diabetes. Short-term infusions of somatostatin have been shown to diminish postprandial hyperglycemia in insulin-dependent diabetic patients3”,1’1and to render exogenous insulin more effective.“g Since somatostatin has numerous effects on gastrointestinal function (see discussions elsewhere in these proceedings by Creutzfeldt and by Raptis and Schlegel), it is not clear at the present time whether this effect can be attributed solely to suppression of glucagon secretion. Nevertheless, a major question is whether an agent like somatostatin can provide advantage over the optimal use of insulin alone. Figure 5 shows results of experiments4’ designed to examine this. Seven insulin-requiring diabetic patients were admitted to a metabolic ward, placed on a dietary regimen consisting of five feedings, and were treated with mixtures of NPH and regular insulin twice a day. During the 4-6-day equilibration period, insulin doses were individually titrated in each patient to a point considered optimal for diminishing postprandial hyperglycemia without causing hypoglycemia. Patients were then studied for 7 consecutive days with almost hourly blood sampling. During the first 2 days insulin doses were held constant. During the next 3 days, somatostatin was administered by continuous intravenous infusion, while insulin doses were

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_ DAILYINSULIN DDSE (NPH + REGULAR)

X-UNITS

/:INSULIN

I II I I I II I I I II I l/1 II I I I II I I I

7A.M. Fig. 5. betics.

I

I

7A.M.

7A.M.

Effect of B-day somatostatin

7A.M.

I

7A.M.

I

7A.M.

I

7A.M.

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7A.M.

infusion in diurnal serum glucose levels in insulin-requiring

dia-

gradually reduced by over 50% to avoid anticipated hypoglycemia.,‘!’ During the last 2 days, the same dose of insulin was used as during the last day of the somatostatin infusion, but this time without somatostatin. Diabetic control, as assessed by degree of postprandial hyperglycemia and its fluctuations throughout the day, was significantly better while patients were receiving less insulin (38 U) plus somatostatin compared to twice as much insulin (79 U) alone (which had been previously determined to represent an optimum insulin regimen). That these results were specifically due to somatostatin is indicated by the fact that diabetic control rapidly deteriorated when the somatostatin infusion was discontinued but the insulin dose was held constant. Results such as these, if fount consistently in a large number of patients, would indicate a definite potential for the use of an agent like somatostatin in the management of diabetes. The short half-life and diverse actions of somatostatin make such a use presently unfeasible. To date, effects to prolong the duration of somatostatin such as combination with protamine zinc have not succeeded in the extension of activity to a practical extent. Since somatostatin inhibits insulin secretion as well as glucagon secretion and might thus cause diabetic control to deteriorate in adult-onset diabetics relying on residual beta-cell function,” its use would be limited to insulin-requiring patients or would require the addition of insulin in those not already taking insulin. Recently, attempts to find a selective analog that primarily inhibited glucagon release have met with partial success. ‘A Whether such compounds will prove useful clinically in doses that permit dissociation of their effects on insulin and glucagon secretion remains to be determined.

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ACKNOWLEDGMENT The author is grateful for the collaboration of Drs. M. Lorenzi, N. Bohanon, E. Tsalikian, J. Karam, V. Schneider, J. Davis, T. A. Schultz, S. B. Lewis, D. Bier, and P. Cryer in the conduction of these experiments; for supply of somatostatin by Drs. R. Guillemin, J. Rivier, M. Brown, and W. Vale; and for the excellent technical assistanceof J. Hatteberg, W. Blanchard, L. Adam, and G. Gustafson. REFERENCES I. Gerich J, Lorenzi M, Hane S, et al: Evidence for a physiologic role of pancreatic glucagon in human glucose homeostasis: Studies with somatostatin. Metabolism 24:175-182, 1975 2. Chideckel E, Palmer J, Loerker D, et al: Somatostatin blockade of acute and chronic stimuli of the endocrine pancreas and the consequences of this blockade on glucose homeostasis. J Clin Invest 55:754-762, 1975 3. Cherrington A, Caldwell M, Dietz M, et al: The effect of somatostatin on glucose uptake and production by rat tissues in vitro. Diabetes 26: 740-748, 1977 4. Orci L, Baetens D, Rufener C, et al: Hypertrophy and hyperplasia of somatostatincontaining D-cells in diabetes. Proc Natl Acad Sci USA 73:1338-1342, 1976 5. Hokfelt T, Efendic S, Hellerstrom C, et al: Cellular localization of somatostatin in endocrine-like cells and neurons of the rat with special references to the A-cells of the pancreatic islets and to the hypothalamus. Acta Endocrinol (Kbh) 80 (Suppl200):1~41, 1975 6. Dubois M: lmmunoreactive somatostatin is present in discrete cells of the endocrine pancreas. Proc Natl Acad Sci USA 72:1340-1343,197s 7. Polak J, Grimelius L, Pearse A, et al: Growth-hormone release-inhibiting hormone in gastrointestinal and pancreatic D cells. Lancet 1:1220-1222.1975 8. Pate1 Y, Weir G: Increased somatostatin content of islets from streptozotocin-diabetic rats. Clin Endocrinol5:191~194, 1976 9. Pate1 Y, Orci L, Bankier A, et al: Decreased pancreatic somatostatin (SRIF) concentration in spontaneously diabetic mice. Endocrinology99:1415-1418, 1976 IO. Unger R: Diabetes and the alpha cell. Diabetes25:136-151, 1976 Il. Luft R, Guillemin R: Growth hormone and diabetes in man: Old concepts-new implications. Diabetes 23:783-787, 1974 12. Exton J, Park C: Interaction of insulin and glucagon in control of liver metabolism, in Steiner D, Frankel N (eds): Handbook of Physiology, vol I, section 7, Endocrinology. Williams & Wilkins, 1972, pp 437-455

13. Sherwin R, Fisher M, Hendler R, et al: Hyperglucagonemia and blood glucose regulation in normal, obese, and diabetic subjects. N Engl J Med 2941455~46 I, 1976 14. Barnes A, Bloom S: Pancreatectomized man: A model for diabetes without glucagon. Lancet 1:219~221, 1976 15. Samols E, Marri G, Marks V: Promotion of insulin secretion by glucagon. Lancet 2:415416, 1965 16. De Rubertis J. Craven P: Reduced sensitivity of hepatic adenylate cyclose-cyclic AMD system to glucagon during sustained hormonal stimulation. J Clin Invest 57:435-443, 1976 17. Bomboy J, Lewis S, Lacy W, et al: Transient stimulatory effect of sustained hyperglucagonemia on splanchnic glucose production in normal and diabetic man. Diabetes 26:177184, 1977 18. Wahren J, Felig P, Hagenfeldt L: Effect of protein ingestion on splanchnic and leg metabolism in normal man and in patients with diabetes mellitus. J Clin Invest 57:987-999, 1976 19. Palmer J, Werner P, Benson J, et al: Plasma-glucagon after pancreatectomy. Lancet l:l290, 1976 20. Matsuyama T, Foa P: Plasma glucose, insulin, pancreatic and enteroglucagon levels in normal and depancreatectomized dogs. Proc Sot Exp Biol Med 147:97- 102, 1974 21. Sasaki H, Rubalcava B, Baetens D, et al: Identification of glucagon in the gastrointestinal tract. J Clin Invest 56:135-145, 1975 22. Mashiter K, Hadring P, Chou M, et al: Persistent pancreatic glucagon but not insulin response to arginine in pancreatectomized dogs. Endocrinology 95:678-693, 1975 23. Lins P, Efendic S: Hyperglycemia induced by somatostatin in normal subjects. Horm Metab Res 8:497 498, 1976 24. Sherwin R, Hendler R, De Fronzo R, et al: Glucose homeostasis during prolonged suppression of insulin and glucagon by somatostatin. Proc Natl Acad Sci USA 74:348-352, 1977 25. Barnes A, Bloom S, Mashiter K, et al: Glucagon and ketogenesis: Studies in pancreatectomized man. Diabetologia 12:379, 1976

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26. Gerich J, !*orenzi M, Bier D, et al: Prevention of human diabetic ketoacidosis by somatostatin: Evidence for an essential role of glucagon. N Engl J Med 292:985 -989, 1975 27. Gerich J. Lorenzi M, Bier D, et al: Effects of physiologic levels of glucagon and growth hormone on human carbohydrate and lipid metabolism: Studies involving administration of exogenous hormone during suppression of endogenous hormone secretion with somatostatin. J Clin Invest 57:875 -884, 1976 28. Gerich J, Schneider V. Dippe S, et al: Characterization of the glucagon response to hypoglycemia in man. J Clin Endocrinol Metab 38:77 82. 1974 29. Gerich J. Davis J. I.orenzi M, et al: Interaction of catecholamines and glucagon in glucose counterregulation. Diabetes 26 (Suppl 2):384. 1977 30. Orci L, Unger R: Functional subdivision of the islets of Langerhans and possible role of D cells. Lancet 2:1243 1244, 1975 31. Patton G. Ipp E, Dobbs R, et al: Response of pancreatic immunoreactive somatostatin to arginine. Life Sci 19:1957 1960. t976 32. Patton G, Dobbs R, Orci L, et al: Stimulation of pancreatic immunoreactive somatostatin release by glucagon. Metabolism 25 (Suppl l).l499~ 1500,1976 33. Schauder P. McIntosh C, Arends J. et al: Somatostatin and insulin release from isolated rat pancreatic islets stimulated by glucose. FEBS Lett 6X:225- 227, 1976 34. Ipp E. Patton G. Dobbs R, et al: Endogenous immunoreactive somatostatin secre-

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GERlCH

1):359.

35. Barden N, Lavoie M, Alvardo-Urbina G. et al: A physiologic role of somatostatin in the control of insulin and glucagon secretion. Fed Proc 36:298. 1977 36. Ganda 0. Weir G, Soeldner J. et al: Somatostatinoma: A somatostatin containing tumor of the endocrine pancreas. N Engl J Med 296~963-967. 1977 37. Larsson L, Holst J. Kuhl C, et al: Pancreauc somatostatinoma: Clinical features and physrologic implications. Lancet 1:666-668. 1977 38. Gerich complications 85 91. 1977

JE: Diabetic of diabetes,

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39. Gerich J, Lorenzi M, Karam J, et al: Abnormal pancreatic glucagon secretion and postprandial hyperglycemia in diabetes mellitus. JAMA 234.159..I65 1975 40. Meissner C, Thum C, Beischer W, et al: Antidiabetic action of somatostatin-assessed by the artificial pancreas. Diabetes 24:988 -996, 1975 41. Gerrch J. Schultz T. Tsalkian E, et al: Clinical evaluation of somatostatin as a potential adjunct IO insulin in the management of diabetes mellitus. Diabetologia I3:537- 544. 1977 42. Tamborlane W, Sherwin R, Hendler R, et al: Metabolic effects of somatostatin in maturityonset diabetes. N Engl J Med 297:lSl 183, 1977 43. Brown M, Rivier J, Vale W: Somatostatin: Analogs with selected biological activities. Science 196:1467 1469, 1977