Abnormal alpha cell function in human diabetes

Abnormal Alpha Cell Function in Human Diabetes The Response to Oral Protein

PHILIP RASKIN, ISMET AYDIN, TAKASHI

M.D. M.D.+

YAMAMOTO,

M.D.

ROGER H. UNGER, M.D. Dallas, Texas

From the Veterans Administration Hospital and Department of Internal Medicine, The University of Texas Southwestern Medical School, Dallas, Texas. This study was supported in part by VA Institutional Research Support Grant 549-8000-l; NIH Grant AM 02700, I-ROI-AM 18179, I-MOI-RR 0633 and NIH Contract NOl-AM-62219; 30 K Rabbit Fund, and The American Diabetes Association, North Texas Affiliate, Salk Institute-Texas Research Foundation; Pfizer Laboratories, New York, New York: Bristol Myers Co., New York, New York: Dr. Karl Thomae GmbH, Germany; Merck, Sharpe 8. Dohme, Rahway. New Jersey; Ciba-Geigy Corp., Summit, New Jersey; The Upjohn Co., Kalamazoo, Michigan; Eli Lilly 8 Co., Indianapolis, Indiana. Requests for reprints should be addressed to Dr. Philip Raskin, The University of Texas, Southwestern Medical School, 5323 Harry Hines Boulevard, Dallas, Texas 75235. Manuscript accepted January 11, 1978. Present address: Gulhane Askeri top Akademisi, 1 Dahiliye Klinigi, Etlik, Ankara, Turkey. l

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The response of immunoreactive glucagon (IRG) to a protein meal and the effects of hyperglycemia and/or hyperinsulinemia on this response were studied in patients with adult and juvenile-type diabetes and in nondiabetic subjects. In nondiabetic subjects, hyperglycemia induced by intravenously administered glucose abolished or reduced the protein-induced increase in IRG. In subjects with adult-type diabetes made normoglycemic by the overnight administration of insulin (1 U/hour), the IRG response to proteln was the same as when they were hyperglycemic, whereas in subjects with juvenile-type diabetes overnight insulin infusion restored the response to normal. In juvenile-type diabetic subjects, hyperglycemia induced during insulin infusion did not reduce the IRG response to protein, but the administration of additional insulin during hyperglycemia lowered it. In adult-type diabetic subjects, hyperglycemia during insulin infusion reduced the IRG response, but the administration of additional insulin did not lower it further. In both groups of diabetic subjects, the protein-induced Increase in IRG was accompanied by a significant 50 mg/dl increase in plasma glucose In 2 hours despite a constant infusion of insulin. It is concluded that in subjects with adult-type diabetes the IRG response to protein is uninfluenced by insulin whereas in those with juvenile-type diabetes the defect is virtually restored to normal by insulin. The increase in plasma glucose associated with the protein-induced increase in IRG despite a constant insulin infusion suggests that an increase in glucagon, when unaccompanied by an increase in insulin, contributes to the postprandial hyperglycemia of diabetes. In common forms of experimental and spontaneous diabetes, the plasma levels of immunoreactive glucagon (IRG) are high relative to the ambient glucose levels, and the patterns of A-cell responses differ qualitatively, and often quantitatively, from those in nondiabetic subjects [l-4]. It is not clear whether such differences are the simple consequence of insulin deficiency or whether defects independent of insulin lack may also be involved. For example, in subjects with juvenile-type diabetes insulin, at least in large doses, appears to restore to normal the abnormal IRG response to a carbohydrate meal [5], suggesting that it may be secondary to the insulin abnormality, whereas, in subjects with adult-type diabetes the same anomalous IRG response is less sensitive to the same doses of insulin. Similarly, the exaggerated IRG response to arginine is corrected by insulin in those with juvenile-type diabetes [6,7] but not in those with the maturity onset form of the disease [7].

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A-CELL FUNCTION IN DIASEIES~ RESPONSE TO PWlEfN-RASKIN

minutes before the protein meal was given. This group of 12 nondiibetic subjects included fiie from the other nondiabetic group. Six of the 12 subjects were male. Their ages ranged from 23 to 45 years. Weights ranged from 48 to 77 kg, 95 to 110 per cent of ideal body weight, and averaged 66 kg. The

Another important derangement of A-cell function in diabetes is the IRG response to a protein meal [a]. In normoglycemic nondiabetic subjects IRG levels increase during the absorption of amino acids, and thus prevent hypoglycemia [9,10] from aminogenic insulin secretion; however, when glucose is given during a protein meal, making protection against hypoglycemia unnecessary, the protein-induced increase in IRG is completely abolished; in fact, the suppression of IRG by hyperglycemia is maintained despite the ingestion of protein [a]. By contrast, in diabetic subjects with fasting hyperglycemia the ingestion of protein elicits an increase in IRG that is at least as great as that in normoglycemic nondiabetic subjects [a], suggesting that their protein-induced IRG response is autonomous of glycemic control. Such inappropriate secretion of glucagon by diabetic patients during a mixed meal could well be a contributing factor to their postprandial hyperglycemia.

nondiabetic subjects were, therefore, comparable with respect to age and weight to the patients with juven%e-type diabetes but not to the group with adult-type diabetes. Each diabetic subject was studied on five separate occa-

The present study was designed to gain insight into the mechanism of the apparent autonomy from glycemic control of the A-cell response to protein by determining whether or not it can be restored to or towards normal by simulation of nondiabetic patterns of insulin response that occur during hyperglycemia. MATERIALS

ET AL.

AND METHODS

Ten patients with ketoacidosis-prone juvenile-type diabetes, 10 with adult-type diabetes mellitus and two groups of 12 nondiabetic volunteer subjects were studied. Six of the IO patients with juvenile diabetes were male. Their ages ranged from 19 to 52 years and averaged 29 years. Weights ranged from 54 to 89 kg, 88 to 120 per cent of ideal body weight, and averaged 66 kg. None gave a history of pancreatitis. Seven of the 10 patients with adult-type diabetes were female. Their ages ranged from 48 to 57 years and averaged 51 years. Weights ranged from 60 to 162 kg, 108 to 179 per cent ideal body weight, and averaged 91 kg. Five of the 10 patients were obese (
sions. The sequence of the experimental protocols was randomly determined. On all test days after base line blood sampling, the subject ingested a meal consisting of 1 g of protein/kg of body weight in the form of waterpacked tuna fish with the water poured off. On one of the experimental days, the protein meal was accompanied by an infusion of saline solution as a control, beginning 20 minutes before the protein meal was given; on this day the patients were tested as outpatients. On the four other test days the diabetic patients were hospitalized in the Clinical Research Center at least 24 hours prior to study. In those patients being treated with depot insulin, regular insulin was substituted at least 24 hours before the start of an experiment. At 6 P.M., 14 hours before the experiment, subcutaneous insulin treatment was terminated and glucagon-free insulin was infused intravenously at a rate of 1 U/hour so as to maintain normcglycemia throughout the night and during the first 2 hours after the protein meal. In one of these four experimental protocols the protein meal was ingested duringthe 1 U/hour insulin infusion alone. On another experimental day, in addition to the overnight 1 U/hour infusionof insulin, hyperglycemia was induced by a glucose superinfusion For 140 minutes at a rate of 0.5 g/kg/hour beginning 20 minutes prior to the ingestion of the protein meal. On another experimental day, in addition to the overnight 1 U/hour infusion of insulin, hyperinsulinemia was induced by a superinfusion of insulin as a bolus of 0.03 U/kg at the beginning of the meal followed by 0.06 U/kg for 2 hours. Finally, on another experimental day, in addition to the overnight IU/hour infusion of insulin, hyperglycemia plus hyperinsulinemia were induced by superinfusingboth glucose at a rate of 0.5 g/kg/hour for 140 minutes beginning 20 minutes before the meal and insulin as a bolus of 0.06 U/kg of insulin at the time of the meal followed by a continuous superinfusion of insulin at a rate of 0.06 U/kg for 2 hours. All subjects were studied in a fasting state. Insulin and all other medications, including all oral hypoglycemic medications, were omitted on the morning of each test. Blood specimens were obtained via a lg-gauge butterfly needle inserted into an antecubital vein. Specimens were collected in chilled tubes containing 12 mg EDTA and 1 ml Trasylol@ (500 Kallikrein Inhibitor Units/ml of blood) and centrifuged promptly at 4%. Plasma was separated and stored at -2OOC until the time of the hormone assay. Glucagon was assayed by a modification [ 1 I] of the previously described radioimmunoassay using antiserum 30K [ 121. Insulin was measured by the Herbert modification [ 131 of the method of Yalow and Berson [14] in all patients without circulating insulin antibodies. Glucose was measured on a Beckman Glucose Analyzer by the glucose oxidase method. For comparison within groups the Student t-test for paired data was used. The t-test for two groups was employed for

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A-CELL FUNCTION IN DIASETES: RESPONSE TO PROTEIN-RASKIN ET AL.

300’ 250-

I /’

;

200-

;

150-

g

loo-

n

500

q::,~?-+

. -60

0

60 MINUTES

120

180

Figure 7. The plasma glucose, insulin and immunoreactive glucagon (IRG) response to a protein meal in 12 nondiabetic subjects during saline (0 - - - 0) or glucose infusion (@_._._. 0 0.5glkglhour; 0 - 0 l.Og/kglhour).

comparison between groups. Base line values were the mean of the three base line samples. The sum of the IRG increments represents the sum of the IRG increment at each of the sampling points above the base line samples. Samples were drawn at 10, 20, 30, 60, 90 and 120 minutes, and all were used in the calculation. A sum for each individual subject was calculated, and the mean of the individual sums was determined for each group. Informed consent of all subjects was obtained.

RESULTS Nondiabetic Subjects. The response of plasma IRG to protein during normoglycemia and hyperglycemia: Figure 1 shows the response to a protein meal in 12 nondiabetic subjects during normoglycemia. Plasma insulin levels rose from a base line mean of 9 f 1

pU/ml to a peak of 16 f 1 yU/ml at 60 minutes. The base line plasma IRG levels averaged 71 f 4 pg/ml and

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rose significantly (p < 0.05) at all points from 20 to 180 minutes after the ingestion of the protein meal, averaging 127 f 10 pg/ml at 120 minutes. The sum of IRG increments in each subject during the 2 hours following the meal averaged 224 f 24 pg/ml (Table I). When in the same 12 nondiabetic subjects hyperglycemia above 200 mg/dl was maintained by means of a glucose infusion of 1 g/kg/hour (Figure l), ingestion of the protein meal was associated with a rise in plasma insulin levels from 32 f 5 +/ml just before the meal to a peak of 181 f 8 @/ml 20 minutes later. The mean IRG levels, which had averaged 72 f 6 pg/ml prior to the glucose infusion and had declined to 55 f 4 pg/ml during the glucose infusion, did not rise above this level following the protein meal, remaining below 43 pg/ml throughout. When the glucose infusion was terminated at 120 minutes, IRG levels rose to a peak of 140 f 30 pg/ml within an hour. Thus, there was no protein-induced IRG increment during hyperglycemia in the nondiabetic subjects (Table I, column A). To determine if mild hyperglycemia would similarly block the protein-induced increase in IRG, glucose was infused at a rate of 0.5 g/kg/hour beginning 20 minutes before the protein meal in the second group of nondiabetic control subjects). As shown in Figure 1, mean glucose levels, which averaged 71 mg/dl before the infusion, were maintained above 100 mg/dl for most of the 2 hours, ranging between 113 and 99 mg/dl during the first 120 minutes. Mean insulin levels, which averaged 9 PUlml before the infusion, ranged between 22 and 70 PUlml during the infusion. IRG levels declined significantly below the inexplicably higher preinfusion base line level of 115 f 7 pg/ml to a level of 89 f 10 pg/ml just before the meal. Despite the very mild and transient hyperglycemia, mean IRG remained below 96 pg/ml during the first hour following the protein meal, returning thereafter to the original preinfusion level as plasma glucose receded to below 100 mg/dl during the last hour of the infusion. IRG never exceeded the prehyperglycemic base line, but the mean of the sums of the IRG increments above 89 f 10 pg/ml, the level after 20 minutes of glucose infusion and just before the ingestion of the protein level, averaged 57 f 26 pg/ml. However, most of this increase occurred during the last 60 minutes of the glucose infusion when the hyperglycemia had receded. Patients with Adult-Type Diabetes. The response of plasma IRG to protein during spontaneous fasting hyperglycemia: The IRG and insulin responses to the protein meal in spontaneously hyperglycemic patients with adult-onset diabetes whose fasting plasma glucose levels averaged 197 f 23 mg/dl are shown in Figure 2A. Plasma insulin, measured only in the seven patients without insulin antibodies, rose from a basal level of 14 f 3 pU/ml to 33 f 5 pU/ml at 90 minutes and was

A-CELL FUNCTION

TABLE I

IN DIABETES:

RESPONSE

TO PROTEIN-RASKH

33

AL.

Mean (i SEW Sum of Protein-induced IRG Increments in Diabetic and Nondiabetic Subjects

Experimental Protocol Saline infusion

A Nondiabelic Subjects (n = 12) ZAIRG pg/ml1120min.

B Patients with Adult-Onset Diabetes (n = 10) ZAIRG pglmlll20 min

+224 f 24

+450 f

C Patients with Juvenile Diabetes (n = 10) ZAIRGpg/ml/l20 min

124

+304 f

80

Insulin (1 U/hour)

+337 f 35+

+210 f 47

Insulin (1 U/hour) +0.03 U/kg bolus -IO.06 U/kg/2 hour

-t-478 f 79+

+194 f 50

Glucose infusion (1 .O gm/kg/hour)

-52 f 12+ (-151 f 349)

Glucose infusion (0.5 gmlkglhour)

57 f 26+ -112f57§)

.

Glucose infusion (0.5 gmlkglhour)

+I04

f 467

+I28

f 361

Insulin (1 (J/hour) $118 f 8211

Glucose infusion (0.5 gmlkglhour) Insulin (1 U/hour) +0.06 U/kg bolus -to.06 U/kg/2 hour

+36 f 221

= ZAIRG represents the sum of IRG increments from the base line samples at each of the sampling points. The samples were drawn at 10, 20, 30. 60, 90 and 120 minutes. All were used in the calculation of the ZAIRG. A sum for each individual subject is calculated from which the mean for each group was determined. + = p <0.05 diabetic subjects versus normoglycemic nondiabetic subjects. 1: = calculated from the value immediately before the protein meal. 5 = calculated from the prehyperglycemic base line value. n = p
statistically higher than that of the normoglycemic nondiabetic subjects at all time points (p X0.05). The rise was less than that in the mildly hyperglycemic nondiabetic subjects, and was far below that of the severely hyperglycemic nondiabetic subjects. Plasma IRG, which averaged 124 f 15 pg/ml in the base line period, increased despite hyperglycemia to a peak of’ 225 f 41 at 180 minutes and was significantly higher than in the normoglycemic nondiabetic subjects and both groups of hyperglycemic nondiabetic subjects at all time points. The sum of IRG increments for the 2 hours following the meal averaged 450 f 124 pg/ml (Table I, column B), not significantly different (p 0.05) from the 224 f 24 pg/ml value in the normoglycemic nondiabetic subjects but obviously greater than the suppressed values in both groups of nondiabetic subjects made hyperglycemic by a glucose infusion. However, the significance of differences between the nondiabetic subjects and the patients with adultonset diabetes is uncertain because of intergroup age and weight differences. The response of plasma IRG to protein during normoglycemia maintained by constant insulin infusion: To determine if the abnormal IRG response in these same patients with adult-onset diabetes could be corrected or improved by prior reduction of their hyper-

glycemia, an overnight insulin infusion was begun 14 hours before the protein meal was given and continued for 2 hours after its ingestion. As shown in Figure 2B, the mean base line plasma glucose was now normal. Plasma insulin levels averaged about 32 f 2 kU/ml before and after the protein meal, significantly above the insulin levels of the normoglycemic nondiabetic subjects (p <0.05). Base line IRG values were lower than without insulin, averaging 104 f (not significant) and increased to a peak of 197 f 26 pg/ml at 180 minutes, not significantly greater than those in the normoglycemic nondiabetic subjects (p <0.05). The sum of the plasma IRG increments for the 120 minutes of the insulin infusion averaged 337 f 35 pg/ml, not significantly different from the 450 f 124 pg/ml value without the insulin infusion (Table I, column B). Interestingly, plasma glucose increased from 90 f 6 mg/dl before the meal to 141 f 6 mg/dl 120 minutes later, a statistically significant difference from that in the nondiabetic subjects (p CO.05) (Figure 2B). Effect of induced hyperglycemia on the response of plasma IRG to protein during constant insulin Infusion: To determine the effect of induced hyperglycemia upon the response of plasma IRG to the protein meal in the patients with maturity onset diabetes made normoglycemic by an overnight infusion of insulin, a superinfu-

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A-CELL FUNCTION IN DIABETES: RESPONSE TO PROTEIN-RASKIN

1

SALINE

ET AL.

1

I

PROTEIN MEAL

14 h. INSULIN lU/ h. I.V. PROTEIN MEAL I

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Fl @we 2. The plasma glucose, insulin and immunoreactive glucagon (IRG) response to a protein mea in 10 subjects with adult-onsettype diabetesduring saline infusion (A) or constant 7 Uf hour insulin infusion 03 . - - - 0 diabetic subjects, 0 - - - 0 nondiabetic subjects (n = 12); = p < 0.05 diabetic versus nondiabeticsubjects; 0 = p < 0.05 versus base line. The nondiabeticsubjects did not receive the M-hour insulin infvsion. Because their weights and age were less than those with adult-onset diabetes, intergroup comparisons may be spurious and are made for purposes of reference only. l

sion of glucose was begun 20 minutes before the protein meal was given. Plasma glucose, which had averaged 79 f 4 mg/dl, was thereby maintained between 162 and 332 mg/dl during the meal (Figure 3A). Plasma insulin levels averaged 30 f 3 pU/ml during the hyperglycemia before the meal and rose further after the meal to 6 1 f 4 pU/ml at 120 minutes; these were significantly below those in the markedly hyperglycemic nondiabetic subjects (p <0.05) and approximately the same as those in the mildly hyperglycemic nondiabetic subjects. Yet, despite the hyperglycemia, IRG levels rose from the basal average of 108 f 14 pg/dl to a peak of 142 f 28 pg/ml at 90 minutes following the protein meal, significantly above the base line during the second hour only, and significantly below their normoglycemic values in Figure 26 at 30 and 90 minutes (p <0.05). The sum of IRG increments now averaged only 104 f 46 pg/ml (Table I, column B), significantly less than the 337

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pg/ml value noted in the absence of hyperglycemia (p <0.02). Obviously, this differed from the absent IRG response in the markedly hyperglycemic nondiabetic subjects but not from the 57 pg/ml mean sum of the increments in the mildly hyperglycemic nondiabetic subjects calculated from a base line during the glucose infusion. But, despite the improvement, all of their IRG values remained significantly above those of both groups of hyperglycemic nondiabetic subjects; again, however, age and weight differences obscure the significance of the intergroup comparisons. Effect of hyperglycemia and hyperinsulinemia upon the response of plasma IRG to protein during constant overnight insulin infusion: To determine the effect of hyperglycemia plus hyperinsulinemia on the IRG response of the patients with adult-type diabetes to the protein meal, a supplementary 0.06 U/kg body weight bolus injection of insulin followed by a 2-hour superin-

A-CELL FUNCTION IN DIABETES: RESPONSE TO PROTEIN-RASKIN ET AL.

)

kg/hlV

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PROTEIN MEAL

PROTEIN MEAL

6

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400.

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; 2 300. “X 3 r ZOOa-100O-

400 g200

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igure 3. The plasma glucose, insulin and immunoreactive glucagon (IRG) response to a protein meal in 70 subjects with adult-onset diabetes during induced hyperglycemia (A), hyperglycemia plus hyperinsulinemia (8) and hyperinsolinemia (C). 0 - - - 0 diabetic subject; 0 - - - 0 nondiabetic subjects (n = 12); = p < 0.05 diabetic versus nondiabetic subjects; 0 = p < 0.05 versus base line. l

fusion of 0.06 U/kg was added to the overnight insulin infusion and glucose superinfusion. The results are shown in Figure 3B. Mean plasma insulin was thereby increased to 297 f 88 &t/ml at 20 minutes and was maintained above the levels in the severely hyperglycemic nondiabetic subjects during the first hour after the meal (p <0.05) and the mildly hyperglycemic nondiabetic group at all time points (p <0.05), while mean glucose was maintained from 141 to 255 mg/dl. Despite the hyperglycemia and hyperinsulinemia, mean plasma IRG increased above the base line value of 123 f 16 pg/ml to a peak of 163 f 24 pg/ml at 90 minutes, but none of the values was significantly above the base line level. The sum of the IRG increments in these patients during the 120 minutes of hyperglycemia plus hyperinsulinemia averaged 118 f 82 pg/ml (Table I, column B), not below the 104 f 46 pg/ml value observed during hyperglycemia without hyperinsulinemia but, of course, significantly less than the 337 pg/ml increase noted when they were normoglycemic (p < 0.05). All the base line and post-protein IRG levels were obviously greater than the negative response of the severely hyperglycemic nondiabetic subjects, but not greater than the increase in the mildly hyperglycemic nondiabetic

subjects when calculated from a base line during the glucose infusion. Thus, supplementary insulin plus hyperglycemia had no greater effect on the IRG response to the protein meal than hyperglycemia without hyperinsulinemia. Effect of hyperinsulinemia without hyperglycemia on plasma IRG response to protein during constant overnight insulin infusion: To determine the effect of hyperinsulinemia without hyperglycemia the insulin superinfusion was given at the time of the protein meal but without the glucose superinfusion. The results are shown in Figure 3C. Insulin levels rose from 33 f 3 to 139 f 24 @/ml at 10 minutes and remained above 50 @j/ml for 120 minutes. Glucose levels fell to 76 f 5 mg/dl at 30 minutes and ranged between this and Ill mg/dl for the 120 minutes of the insulin infusion. Despite the hyperinsulinemia, mean IRG increased from 95 f 12 pg/ml to 224 f 27 pg/ml at 120 minutes after the protein meal and was significantly above the base line at all time points. These values were not significantly different from their corresponding values when they received the constant overnight insulin infusion alone or just the saline solution infusion and were significantly above those in the normoglycemic nondiabetic subjects

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A-CELL FUNCTION IN DIABETES: RESPONSE TOPROTEIN-RASKIN

ET AL.

f PROTEIN MEAL

ii

09 o=

l

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MIN%S

MINUTES

The plasma glucose and immunoreactive glucagon (IRG) response to a protein meal in IO Figure 4. subjects with juvenile-type diabetes during saline infusion (A), or constant 1 U/hour insulin infusion (B). 0 - - - 0 diabetic subjects; 0 - - - 0 nondiabetic subjects (n + 12); = p < 0.05 diabetic versus nondiabetic subjects; 0 = p < 0.05 versus base line. l

at almost all points (p <0.05). The sum of individual IRG increments averaged 478 f 79 pg/ml (Table I, column B), not below that of the same diabetic subjects with either the overnight insulin infusion or saline solution alone, and well above the value in normoglycemic nondiabetic subjects. Thus, supplementary insulin without glucose had no effect on the IRG response to protein in these diabetic subjects. Patients with Juvenile-Type Diabetes. The response of plasma IRG to protein during spontaneous fasting hyperglycemia: The IRG response to the protein meal of spontaneously hyperglycemic patients with juvenile diabetes and mean plasma glucose levels above 300 mg/dl is shown in Figure 4A. Plasma IRG averaged 119 f 4 pg/ml in the basal state and increased significantly to a peak of 212 f 38 pg/ml at 90 minutes (p <0.05). Despite the hyperglycemia, IRG levels were significantly above the levels of the normoglycemic nondiabetic subjects at all points, but the sum of IRG increments for 120 minutes after the meal averaged 304 f 80 pg/ml, not significantly different different from the 224 pg/ml value in normoglycemic nondiabetic subjects (Table I, column C). The response of plasma IRG to protein during normogiycemia maintained by constant insulin infusion: When these patients were made normoglycemic by constant overnight infusion of insulin (Figure 48) base

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line plasma IRG levels were significantly reduced (p
A-CELL FUNCTION IN DIABETES: RESPONSE TO PROTEIN-RASKIN

ET AL.

14hr INSULIN lU/h IV I INSULIN O.O6U/ kg +006U/kg I V GLUCOSE 0 5gm /kg/h I” _ PROTEIN MEAL

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180

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Figure 5. Theplasma glucose and immunoreactive g/wagon (RG) response to a protein meal in 10 s&~jects with juveni@y~ diabetes during induced hyperglycemti (A), hyperglycemia plus hyperinsulinemia (8) and hyperinsulinemia (C). 0 - - - 0 dkbetic subjects; 0 - - - 0 nondiabetic subjects (n = 72); l = p < 0.05 diabetic versus nondiabetic subjects; 0 = p < 0.05 versus base line.

pg/ml at 120 minutes. But the postprotein levels were statistically greater than the base line only during the second hour (p
pg/ml at the latter point. Although mean IRG levels were not statistically different from those observed during hyperglycemia alone at any time point, the sum of IRG increments during the 120 minutes after the meal averaged only 36 f 22 pg/ml (Table I, column C), significantly less tizn the mean incremental IRG of 210 f 47 pg/ml when thesc patients were normoglycemic (p <0.02) and that of 128 f 36 pg/ml when they were made hyperglycemic without hyperinsulinemia (p <0.05). Thus, in patients with juvenile-type diabetes made normoglycemic by constant insulin infusion, induced hyperglycemia plus hyperinsulinemia was more effective in reducing the IRG response to protein than hyperglycemia alone. However, the response was obviously still greater than the absent response in the markedly hyperglycemic nondiabetic subjects (p < O.OOl), but was about the same as that of the mildly hyperglycemic group when calculated from a base line during the glucose infusion. Effect of hyperlnsulinemia without hyperglycemia on plasma IRG response to protein during c-ant overnight Insulin lnfuslon: To determine if hyperinsulinemia alone would improve the IRG response to protein, these same patients, made normoglycemic by the overnight insulin infusion, were given the insulin superinfusion without a glucose superinfusion at the time of the protein meal (Figure 5C). Plasma glucose, which averaged 88 f 9 mg/dl in the basal state, declined to a nadir of 60 f 9 mg/dl at 30 minutes, but re-

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A-CELL FUNCTION IN DIASETES: RESPONSE TOPROTEIN-RASKIN ET AL.

turned to basal levels by 90 minutes. Mean plasma IRG, which average 67 f 11 pg/ml before the meal, increased significantly above the base line during the second hour only, reaching a peak of 121 f 22 pg/ml at 90 minutes (p <0.05). The mean sum of the IRG increments during the 2 hours of insulin administration was 194 f 50 pg/ml (Table I, column C), a value not significantly different from the 220 pg/ml value during constant insulin infusion without insulin superinfusion, and not different from the 128 pg/ml value during constant insulin plus induced hyperglycemia. It was significantly greater (p <0.02) than the mean incremental change of 36 f 22 pg/ml seen during hyperglycemia plus hyperinsulinemia. Thus, in the absence of hyperglycemia, hyperinsulinemia seemed to have no effect on plasma IRG response to a protein meal. COMMENTS In the present studies spontaneously hyperglycemic patients with juvenile and adult-type diabetes, both exhibited fasting and protein-induced hyperglucagonemia despite hyperglycemia, as previously reported [8]. In the group with juvenile diabetes, reduction of hyperglycemia to normal by overnight insulin infusion at a rate of 1 U/hour was associated with a significant reduction in both basal and postprotein IRG levels to the normal range. These results are similar to previous studies with arginine infusion by Gerich et al. [6] and by our own group [7]. Following restoration of glucose levels to normal by overnight insulin infusion, hyperglycemia induced by intravenously administered glucose lowered but did not abolish the increase in plasma IRG, in contrast to the suppressed IRG levels observed in nondiabetic subjects with induced hyperglycemia. When hyperglycemia and hyperinsulinemia were concomitantly induced in these subjects with juvenile diabetes, the increase in IRG was almost abolished, the sum of IRG increments averaging only 36 during the 2 hours following the protein meal, not unlike that in the minimally hyperglycemic nondiabetic subjects. That the improvement was a consequence of combined hyperglycemia and hyperinsulinemia is suggested by the fact that neither hyperglycemia alone nor hyperinsulinemia alone was as effective as the combination in abolishing protein-induced hyperglycemia. In other words, in those with juvenile diabetes, the exaggerated IRG response to a protein meal can be reduced to normal by an overnight insulin infusion in doses that produce normal basal glucose levels, and hyperglycemia alone plus hyperinsulinemia virtually abolish the IRG response to protein, although the IRG values remain significantly above those in hyperglycemic nondiabetic subjects. By contrast, in the maturity onset form of diabetes, neither the elevated basal and postprotein IRG levels

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nor the incremental IRG levels were significantly reduced by the 1 U/hour insulin infusion, despite the fact that fasting plasma glucose was reduced to normal and plasma insulin concentrations maintained significantly above those in the normoglycemic nondiabetic subjects. Nor did the 1 U/hour insulin infusion plus supplemental insulin, which induce marked hyperinsulinemia, reduce the response to protein. In other words, the administration of exogenous insulin had no effect on the IRG response to protein in these subjects with adult-onset diabetes whereas in the group with juvenile diabetes it did. But hyperglycemia alone, induced after prior normalization of basal glucose levels by overnight insulin infusion, did diminish the protein-induced increase in IRG, lowering the e-hour incremental value to 104 f 46 pg/ml, although it failed to suppress IRG levels below the base line as in nondiabetic subjects. And, in sharp contrast to these with juvenile-type diabetes, a combination of induced hyperglycemia and hyperinsulinemia did not reduce the IRG response to protein below that observed with induced hyperglycemia alone. The reasons for these differences in the A-cell response to insulin in the diabetic groups are not clear. In the subjects with juvenile diabetes it is possible that the absence of contact with B cells [ 151 and/or the “within islet” insulin deficiency [ 161 is responsible for the abnormal A-cell function and for the fact that repletion with exogenous insulin restores A-cell function towards normal. Most subjects with adult onset diabetes, on the other hand, probably do not lack B cells adjacent to A cells and do not have complete “within islet” insulin deficiency. Failure of exogenous insulin to correct the abnormality may indicate a totally different etiology-one not related to insulin lack. Since the nondiabetic control subjects in this study were matched more closely in age and weight to the subjects with juvenile onset diabetes than to the subjects with adult-onset diabetes, it is possible that the differences in A-cell function between the group with adult-onset diabetes and the nondiabetic control subjects is a reflection of age and weight differences. Marco et al. [ 171 have demonstrated both elevated basal IRG levels and arginine-induced IRG secretion in elderly (aged 65 to 90 years) as compared to young (aged 19 to 23 years) nondiabetic subjects, although Fedele et al. [ 181 found no correlation between fasting IRG levels or the IRG response to arginine in nondiabetic subjects ranging in age between 20 and 89 years. Schade and Eaton [ 191 found no difference in basal IRG levels between obese and normal weight subjects, and a decreased IRG response to arginine in obese subjects. It seems possible that the differences between the control groups and those with adult-type diabetes are a reflection of the older age and increased weight of the latter group.

A-CELL

The autonomy of the protein-induced IRG response from glycemic influence may have clinical significance in the management of diabetes. The inappropriate glucagon secretion occurring with protein-containing meals, irrespective of the level of hyperglycemia, may further increase hyperglycemia through unwanted enhancement of hepatic glucose production. That protein-induced secretion can induce hyperglycemia in both forms of diabetes is suggested by Figures 2B and 4B, in which plasma glucose levels rose during the protein-induced increase in IRG, despite the continuing infusion of insulin in doses sufficient to maintain fasting normoglycemia. These studies indicate that hyperglycemia plus hyperinsulinemia are required for maximal reduction of protein-induced IRG increments in those with juvenile diabetes, and that in those with adult-type

FLMCTIDN IN DIABETES: RESPONSE TO PROTElN-RASKIN ET AL.

diabetes hyperglycemia alone achieves maximal reduction. It seems clear from these results that postprandial increases in IRG may contribute to postprandjal hyperglycemia in both adult-type and juvenile-type diabetic subjects.

We wish to thank Margaret Bickham, Margaret Cason, Grace Chen, Loretta Clendenen, John Diffie, Helen Gibson, Virginia Harris, Sara Innis, Kay McCorkle, Daniel Sandlin and Lee Schmoker for their technical assistance; Susan Freeman and Carole Tower for secretarial assistance; and the staff and nurses of the Clinical Research Center at Parkland Memorial Hospital.

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