Pancreatic endocrine function in cystic fibrosis A n t o i n e t t e Moran, MD, Peter Diem, MD, D a v i d J. Klein, MD, PhD, M i c h a e l D. Levitt, MD, a n d R. Paul Robertson, MD From the Diabetes Center and the Endocrine Divisions,Departments of Pediatrics and Medicine, Universityof Minnesota, Minneapolis To characterize p a n c r e a t i c endocrine secretion and to e x a m i n e interrelationships among alterations in alpha, beta, and p a n c r e a t i c p o l y p e p t i d e cell function in patients with cystic fibrosis (CF), we studied 19 patients with exocrine insufficiency (EXO), including 9 receiving insulin therapy (EXO-IT); 10 patients with no exocrine insufficiency (NEXO); and 10 normal control subjects. First-phase Cp e p t i d e response to intravenously administered glucose was significantly impaired in CF patients with exocrine insufficiency (EXO-IT = 0.02 • 0.01; EXO = 0.11 • 0.02; NEXO = 0 . 2 5 _ 0.05; control subjects = 0.30 • 0.04 nmol/L). Lowering fasting glucose levels with exogenous insulin administration in EXO-IT did not improve beta cell responsivity to glucose. The C-peptide response to arginine was less impaired (EXO-IT = 0.12 _+ 0.02; EXO = 0.15 • 0.02; NEXO = 0.23 • 0.06; control subjects = 0.28 • 0.04 nmol/L). Alpha cell function, measured as peak g l u c a g o n secretion in response to h y p o g l y c e m i a , was diminished in EXO but not NEXO (EXO-IT = 21 • 10; EXO = 62 • 19; NEXO = 123 • 29; control subjects = 109 • 12 ng/L). Despite diminished g l u c a g o n response, EXO patients recovered normally from h y p o g l y c e m i a . Peak p a n c r e a t i c p o l y p e p t i d e response to h y p o g l y c e m i a distinguished CF patients with exocrlne insufficiency from those without exocrine insufficiency (EXO-IT = 3 • 2; EXO = 3 • I; NEXO = 226 • 68; control subjects = 273 • 100 pmol/L). Thus CF patients with exocrine disease have less alpha, beta, and p a n c r e a t i c p o l y p e p t i d e cell function than CF patients without exocrine disease. These d a t a suggest either that exocrine disease causes endocrine dysfunction in CF or that a c o m m o n pathogenic process simultaneously and i n d e p e n d e n t l y impairs exocrine and endocrine function. (J PEDIATR1 9 9 1 ; 1 1 8 : 7 1 5 - 2 3 )
The basic defect in cystic fibrosis appears to be abnormal phosphorylation-dependent activation of the chloride ion channel. I The primary pathologic finding is inspissated ductular mucus, which leads to progressive obstructive damage to the lungs, reproductive system, and pancreas. Up to 95% of CF patients have exocrine pancreatic insuffiSupported by grants (R01-DK-39994 and M01-RR-00400) from the National Institutes of Health and a grant from the Cystic Fibrosis Foundation. Submitted for publication Aug. 20, 1990; accepted Dec. 12, 1990. Reprint requests: Antoinette Moran, MD, Department of Pediatrics, Box 391, University of Minnesota Health Science Center, 516 Delaware St., Minneapolis, MN 55455, 9/20/27290
ciency,2, 3 and approximately 50% of the CF population has impaired glucose tolerance. 46 Overt diabetes has been reported in 6% to 10% of CF patients27 and may be associated with a significantly increased mortality rate for pulmonary disease. 7 However, despite the significant number of affected patients, the pathogenesis of defective pancreatic endocrine function in CF is poorly understood. Fibrosis of the exocrine pancreas has been proposed to lead to either disorganization or ischemic destruction of the islets of Langerhans.4' 8-10 However, an intrinsic abnormality in pancreatic endocrine cells has also been suggested, both because the primary defect in CF is cellular and because autonomic nervous system regulation (which is important for pancreatic hormone secretion) is abnormal in CF. 1114
715
7 16
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BMI CF EXO EXO-IT IVGTT NEXO OGTT PP
Body mass index Cystic fibrosis Exocrine-insufficient patients with cystic fibrosis Insulin-treated, exocrine-insufficient patients with cystic fibrosis Intravenous glucose tolerance test Exocrine-sufficient patients with cystic fibrosis Oral glucose tolerance test Pancreatic polypeptide
Normal beta cell function is characterized by biphasic C-peptide and insulin secretion. The first-phase response to intravenously administered glucose and other agonists occurs 2 to 5 minutes after stimulation 15, 16 and is a sensitive index of beta cell integrity. ~7 The counterregulatory response to insulin-induced hypoglycemia is a traditional method of assessing both alpha and pancreatic polypeptide cell function. 18, 19These functions have not been adequately assessed in patients with CF. Moreover, no studies using any index of islet function have considered either insulin-treated CF patients or those without exocrine disease as distinct patient populations; although approximately 7% of C F patients receive insulin therapy, their ability to counterregulate hypoglycemia has not been examined. To address these issues, we examined the hormonal responses to orally administered glucose, intravenously administered glucose and arginine, and insulin-induced hypoglycemia in four groups of young adult subjects: CF patients with exocrine insufficiency, such patients who were receiving insulin treatment for hypoglycemia, C F patients with no exocrine insufficiency, and normal control subjects. We believe that such information should be useful to pediatricians because they frequently care for adult CF patients and because our observations may be generalizable to childhood CF. METHODS
Patients. The diagnosis of CF was based on an elevated sweat chloride concentration obtained on at least two occasions by quantitative pilocarpine iontophoresis. Exocrine status had been determined previously by fecal fat measurement and was confirmed at the time of the study by pancreatic isoamylase levels as previously described) ~ 21 Body mass index was calculated as weight (in kilograms) divided by height (in meters) squared. Study groups included 19 C F patients with exocrine insufficiency, 10 of whom had not had an abnormal hemoglobin Ale or an elevated random glucose level (EXO) whereas 9 were receiving insulin therapy to control hyperglycemia (EXO-IT); 10 CF patients with no exocrine insufficiency who had not had an abnormal hemoglobin Ale or elevated random glucose level (NEXO); and 10 healthy control subjects matched with the patients for age, gender, and BMI (Table). Patients
The Journal of Pediatrics May 1991
were recruited from the University of Minnesota Cystic Fibrosis Center, which actively follows approximately 360 patients. Glucose levels are routinely measured at 3-month intervals, and glycosylated hemoglobin determinations are performed annually. Because of limited numbers in the CF population, all N E X O and all insulin-treated CF patients followed at the University of Minnesota between the ages of 18 and 30 years were recruited. Non-insulin-treated EXO patients who matched the NEXO and EXO-IT patients for gender, weight, and BMI were randomly selected from the clinic. Patients treated with steroids and those who had been acutely ill during the 2 months before the study were excluded. Insulin therapy had been initiated in EXO-IT patients during hospitalization for pulmonary infection, at which times fasting glucose levels were >7.8 m m o l / L (140 mg/dl) and at least two random glucose values were >11.2 mmol/L (200 mg/dl). These patients had been treated with insulin for 6 months to 11 years (mean 4.1 years) and were receiving an average insulin dose of 0.5 U / kg per day. All the insulin-treated patients were exocrine insufficient. Approval for these studies was obtained from the University of Minnesota Committee for the Use of Human Subjects in Research; informed consent was obtained from all subjects. Oral glucose tolerance test. Subjects were admitted to the University of Minnesota Clinical Research Center after an overnight fast. All patients had been eating a high-calorie, high-carbohydrate diet. An indwelling catheter was inserted for blood sampling. Glucose, 1.75 gm/kg up to a maximum of 75 gm, was given orally for 3 minutes. Samples for glucose and C-peptide levels were drawn 5 minutes before glucose administration, at the time of administration, and at 30, 60, 90, 120, and 180 minutes afterward. Insulin therapy was withheld for 2 days before the OGTT was performed in the EXO-IT patients. Intravenous glucose tolerance test and arginine pulse. On a separate admission, indwelling catheters were inserted in both antecubital veins, one for blood sampling and the other for glucose infusion. Beta cell responses to a maximum intravenous dose of glucose 22 and to a maximum dose of arginine 23 were evaluated as follows: 20 gm of glucose was injected intravenously for 20 seconds, and glucose and C-peptide levels were measured at - 1 0 , - 5 , 0, 1, 3, 4, 5, 10, 15, 20, 25, 30, 45, and 60 minutes (time 0 being the moment just before glucose injection). Two hours after the glucose dose, 5 gm of arginine was given intravenously for 20 seconds. Samples for measurement of glucose, C-peptide, and glucagon levels were drawn at - 1 0 , - 5 , 0, 2, 3, 4, 5, 7, 10, 15, and 30 minutes. Insulin therapy was withheld from EXO-IT patients for 1 week before admission. After the initial IVGTT and arginine test, these patients were given intravenous insulin infusions at rates adjusted to maintain circulating glucose
Volume 118 Number 5
Pancreatic endocrine function in cystic fibrosis
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Table. Characteristics and baseline hormone levels of study subjects
Control subjects NEXO EXO EXO-IT
Age
Gender (F/M)
24 _+ 1 26 • 3 25 + 3 22 ___2
5/5 5/5 4/6 6/3
BMI (kg/m 2) 21 23 22 20
• • • •
1 1 1 1
Hgb A4e (%) 5.0 5.3 5.2 6.7
• 0.4 _+ 0.1 • 0.1 _+ 0.4*
FBG (mmol/L) 4.9 4.9 5.1 6.1
_+ 0.1 • 0.1 • 0.2 • 0.4*
Basal C-pept (nmol/k) 0.17 • 0.14 • 0.15 • 0.14 •
0.01 0.02 0,04 0.03
Basal Olgn (ng/L) 148 • 118 • 119 • 84 •
23 35 26 10
Basal PP (pmol/L) 13• 16• 8_+1" 8+_1"
Isoamylase (U/L) 54 121 5 7
• • • •
42 49 2* 7*
Hgb Ale, HemoglobinAte;FBG,fastingbloodglucose;C-pept, C-peptide;Glgn,glucagon. Insulinwas withheldfor i weekbeforetestingin EXO-ITpatients.Data are expressed as mean +_SD. *p
The basic defect in cystic fibrosis appears to be abnormal phosphorylation-dependent activation of the chloride ion channel. I The primary pathologic finding is inspissated ductular mucus, which leads to progressive obstructive damage to the lungs, reproductive system, and pancreas. Up to 95% of CF patients have exocrine pancreatic insuffiSupported by grants (R01-DK-39994 and M01-RR-00400) from the National Institutes of Health and a grant from the Cystic Fibrosis Foundation. Submitted for publication Aug. 20, 1990; accepted Dec. 12, 1990. Reprint requests: Antoinette Moran, MD, Department of Pediatrics, Box 391, University of Minnesota Health Science Center, 516 Delaware St., Minneapolis, MN 55455, 9/20/27290
ciency,2, 3 and approximately 50% of the CF population has impaired glucose tolerance. 46 Overt diabetes has been reported in 6% to 10% of CF patients27 and may be associated with a significantly increased mortality rate for pulmonary disease. 7 However, despite the significant number of affected patients, the pathogenesis of defective pancreatic endocrine function in CF is poorly understood. Fibrosis of the exocrine pancreas has been proposed to lead to either disorganization or ischemic destruction of the islets of Langerhans.4' 8-10 However, an intrinsic abnormality in pancreatic endocrine cells has also been suggested, both because the primary defect in CF is cellular and because autonomic nervous system regulation (which is important for pancreatic hormone secretion) is abnormal in CF. 1114
715
7 16
Moran et al.
BMI CF EXO EXO-IT IVGTT NEXO OGTT PP
Body mass index Cystic fibrosis Exocrine-insufficient patients with cystic fibrosis Insulin-treated, exocrine-insufficient patients with cystic fibrosis Intravenous glucose tolerance test Exocrine-sufficient patients with cystic fibrosis Oral glucose tolerance test Pancreatic polypeptide
Normal beta cell function is characterized by biphasic C-peptide and insulin secretion. The first-phase response to intravenously administered glucose and other agonists occurs 2 to 5 minutes after stimulation 15, 16 and is a sensitive index of beta cell integrity. ~7 The counterregulatory response to insulin-induced hypoglycemia is a traditional method of assessing both alpha and pancreatic polypeptide cell function. 18, 19These functions have not been adequately assessed in patients with CF. Moreover, no studies using any index of islet function have considered either insulin-treated CF patients or those without exocrine disease as distinct patient populations; although approximately 7% of C F patients receive insulin therapy, their ability to counterregulate hypoglycemia has not been examined. To address these issues, we examined the hormonal responses to orally administered glucose, intravenously administered glucose and arginine, and insulin-induced hypoglycemia in four groups of young adult subjects: CF patients with exocrine insufficiency, such patients who were receiving insulin treatment for hypoglycemia, C F patients with no exocrine insufficiency, and normal control subjects. We believe that such information should be useful to pediatricians because they frequently care for adult CF patients and because our observations may be generalizable to childhood CF. METHODS
Patients. The diagnosis of CF was based on an elevated sweat chloride concentration obtained on at least two occasions by quantitative pilocarpine iontophoresis. Exocrine status had been determined previously by fecal fat measurement and was confirmed at the time of the study by pancreatic isoamylase levels as previously described) ~ 21 Body mass index was calculated as weight (in kilograms) divided by height (in meters) squared. Study groups included 19 C F patients with exocrine insufficiency, 10 of whom had not had an abnormal hemoglobin Ale or an elevated random glucose level (EXO) whereas 9 were receiving insulin therapy to control hyperglycemia (EXO-IT); 10 CF patients with no exocrine insufficiency who had not had an abnormal hemoglobin Ale or elevated random glucose level (NEXO); and 10 healthy control subjects matched with the patients for age, gender, and BMI (Table). Patients
The Journal of Pediatrics May 1991
were recruited from the University of Minnesota Cystic Fibrosis Center, which actively follows approximately 360 patients. Glucose levels are routinely measured at 3-month intervals, and glycosylated hemoglobin determinations are performed annually. Because of limited numbers in the CF population, all N E X O and all insulin-treated CF patients followed at the University of Minnesota between the ages of 18 and 30 years were recruited. Non-insulin-treated EXO patients who matched the NEXO and EXO-IT patients for gender, weight, and BMI were randomly selected from the clinic. Patients treated with steroids and those who had been acutely ill during the 2 months before the study were excluded. Insulin therapy had been initiated in EXO-IT patients during hospitalization for pulmonary infection, at which times fasting glucose levels were >7.8 m m o l / L (140 mg/dl) and at least two random glucose values were >11.2 mmol/L (200 mg/dl). These patients had been treated with insulin for 6 months to 11 years (mean 4.1 years) and were receiving an average insulin dose of 0.5 U / kg per day. All the insulin-treated patients were exocrine insufficient. Approval for these studies was obtained from the University of Minnesota Committee for the Use of Human Subjects in Research; informed consent was obtained from all subjects. Oral glucose tolerance test. Subjects were admitted to the University of Minnesota Clinical Research Center after an overnight fast. All patients had been eating a high-calorie, high-carbohydrate diet. An indwelling catheter was inserted for blood sampling. Glucose, 1.75 gm/kg up to a maximum of 75 gm, was given orally for 3 minutes. Samples for glucose and C-peptide levels were drawn 5 minutes before glucose administration, at the time of administration, and at 30, 60, 90, 120, and 180 minutes afterward. Insulin therapy was withheld for 2 days before the OGTT was performed in the EXO-IT patients. Intravenous glucose tolerance test and arginine pulse. On a separate admission, indwelling catheters were inserted in both antecubital veins, one for blood sampling and the other for glucose infusion. Beta cell responses to a maximum intravenous dose of glucose 22 and to a maximum dose of arginine 23 were evaluated as follows: 20 gm of glucose was injected intravenously for 20 seconds, and glucose and C-peptide levels were measured at - 1 0 , - 5 , 0, 1, 3, 4, 5, 10, 15, 20, 25, 30, 45, and 60 minutes (time 0 being the moment just before glucose injection). Two hours after the glucose dose, 5 gm of arginine was given intravenously for 20 seconds. Samples for measurement of glucose, C-peptide, and glucagon levels were drawn at - 1 0 , - 5 , 0, 2, 3, 4, 5, 7, 10, 15, and 30 minutes. Insulin therapy was withheld from EXO-IT patients for 1 week before admission. After the initial IVGTT and arginine test, these patients were given intravenous insulin infusions at rates adjusted to maintain circulating glucose
Volume 118 Number 5
Pancreatic endocrine function in cystic fibrosis
7 17
Table. Characteristics and baseline hormone levels of study subjects
Control subjects NEXO EXO EXO-IT
Age
Gender (F/M)
24 _+ 1 26 • 3 25 + 3 22 ___2
5/5 5/5 4/6 6/3
BMI (kg/m 2) 21 23 22 20
• • • •
1 1 1 1
Hgb A4e (%) 5.0 5.3 5.2 6.7
• 0.4 _+ 0.1 • 0.1 _+ 0.4*
FBG (mmol/L) 4.9 4.9 5.1 6.1
_+ 0.1 • 0.1 • 0.2 • 0.4*
Basal C-pept (nmol/k) 0.17 • 0.14 • 0.15 • 0.14 •
0.01 0.02 0,04 0.03
Basal Olgn (ng/L) 148 • 118 • 119 • 84 •
23 35 26 10
Basal PP (pmol/L) 13• 16• 8_+1" 8+_1"
Isoamylase (U/L) 54 121 5 7
• • • •
42 49 2* 7*
Hgb Ale, HemoglobinAte;FBG,fastingbloodglucose;C-pept, C-peptide;Glgn,glucagon. Insulinwas withheldfor i weekbeforetestingin EXO-ITpatients.Data are expressed as mean +_SD. *p
