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0021-972x/96/$03.00/0 Journal of Clinical Endocrinology and Metabolism Copyright 0 1996 by The Endocrine Society Vol. 81, No. 2 Prmted in LT.7 A. Evidence for a Hypothalamic-Pituitary Versus Adrenal Cortical Effect of Glycemic Control on Counterregulatory Hormone Responses to Hypoglycemia in Insulin-Dependent Diabetes Mellitus* BRENDAN T. KINSLEY AND DONALD Joslin Diabetes Center, New England Deaconess Harvard Medical School, Boston, Massachusetts C. SIMONSON Hospital, 02115 Brigham ABSTRACT and Women’s Hospital, and adequate adrenocortical function, we also determined whether the blunted epinephrine response might result from the reduced cortisol secretion. Eleven of the control subjects underwent a second identical insulin clamp study during which metyrapone was administered to produce adrenal cortical blockade. Despite higher basal ACTH levels after metyrapone and sustained elevations in ACTH during hypoglycemia, the cortisol response was abolished during metyrapone treatment, indicating effective blockade. However, epinephrine responses did not differ during hypoglycemia with or without metyrapone treatment. We conclude that 1) ACTH, cortisol, and epinephrine responses during hypoglycemia are reduced in IDDM patients in strict glycemic control; 2) the lower cortisol response is correlated with reduced ACTH levels; and 3) in healthy subjects, the cortisol response to hypoglycemia is abolished by adrenocortical blockade with metyrapone, whereas the epinephrine response to hypoglycemia remains intact. These data suggest that central adaptations in hypothalamicpituitary responses to hypoglycemia rather than alterations in adrenal gland function per se underlie the reduced counterregulatory responses seen in IDDM subjects in strict glycemic control. (J Clin Endocrinol Metub 81: 684-691, 1996) The epinephrine and cortisol responses to hypoglycemia are reduced in insulin-dependent diabetes mellitus (IDDM) patients in strict glycemic control. However, it is not known whether these abnormalities are mediated at a central (hypothalamic-pituitary) or peripheral (adrenal) level. To examine this question, we measured counterregulatory hormone secretion during a 3-h hypoglycemic hyperinsulinemic clamp (12 pmol/kgmin) that lowered glucose from 5.0 to 2.2 mmol/L in steps of 0.55 mmol/L every 30 min in 13 well controlled IDDM subjects (hemoglobin A,, 7.8 ? 0.2%), 14 poorly controlled IDDM subjects (hemoglobin A,, 12.3 t- 1.5%), and 20 healthy volunteers. Basal levels of ACTH, cortisol, and epinephrine were similar in all 3 groups before hypoglycemia. At the nadir glucose level (2.2 mmol/L), ACTH, cortisol, and epinephrine levels were significantly lower in well controlled IDDM compared to healthy controls, and the glucose levels required for significant secretion of ACTH, cortisol, and epinephrine also were lower in well controlled IDDM compared to those in both poorly controlled IDDM and healthy volunteers (P < 0.05). During hypoglycemia, ACTH levels were significantly correlated with cortisol levels (r = 0.43; P < 0.05). Because adrenomedullary epinephrine synthesis is partially dependent on S TRICT GLYCEMIC control of insulin-dependent diabetes mellitus (IDDM) significantly reduces the incidence of diabetic complications (1, 2). This benefit of improved glycemic control is achieved at the expense of a 3-fold increasein the incidence of severe hypoglycemic events (1,3). Numerous studies of IDDM subjects in strict glycemic control have shown that these patients exhibit altered catecholamine, cortisol, and GH responsesto hypoglycemia (4-11). These subjects also exhibit reduced symptom perception of hypoglycemia and require lower glucose levels to activate counterregulatory hormones (11-16). Exposure to recurrent hypoglycemia is the most common mechanism by which these alterations occur in subjects with IDDM, as similar defects can be detected in subjectswith insulinomas (17-19) and can be induced in subjects with IDDM and in normal volunteers exposed to recurrent hypoglycemia (20-26). Thesedefects in counterregulation can be largely reversed by avoidance of hypoglycemia (27-32). The exact mechanism by which hypoglycemia induces these alterations in counterregulation remains uncertain. A central neural adaptation to hypoglycemia is the most likely mechanism. Animal studies (33-35) and a recent study in humans (36) suggest that the brain adapts to exposure to recurrent hypoglycemia by maintaining brain glucose uptake at lower blood glucose levels. As a result of the increased brain glucose uptake, intracerebral glucose levels are maintained, and there is less intracellular hypoglycemia. Thus, decreasedneuroglycopenic stressat a given level of systemic hypoglycemia could underlie the reduced counterregulatory hormone responsesin subjects with well controlled IDDM. However, few data exist on the effect of glycemic control on ACTH responsesto hypoglycemia (25,37). Although a central adaptation to hypoglycemia is probably involved in the altered epinephrine and cortisol responses(as well as other counterregulatory responses,e.g. GH) during strict glycemic control of IDDM, definitive evidence is lacking. A direct effect of glycemic control on adrenal cortical and medullary Received May 2, 1995. Revision received August 29, 1995. Accepted August 29, 1995. Address all correspondence and requests for reprints to: Dr. Brendan T. Kinsley, Section of Diabetes and Metabolism, Brigham and Women’s Hospital, 221 Longwood Avenue, Boston, Massachusetts 02115. * Presented in part at the 75th Annual Meeting of The Endocrine Society, Las Vegas, NV, June 1993. This work was supported in part by fellowship grants from the Juvenile Diabetes Foundation International (to B.T.K.), the Adler Foundation (to D.C.S.), and the NIH (DK-36836; Diabetes and Endocrinology Research Center at Joslin Diabetes Center). 684 HPA RESPONSE TO HYPOGLYCEMIA function cannot be excluded. Moreover, no study has addressed the possibility that an adaptation in the glucocorticoid-catecholamine interaction may play a role in the adaptations of epinephrine responses to hypoglycemia. We, therefore, designed the current study to examine whether 1) the ACTH response to hypoglycemia is affected by glycemic control in IDDM, and 2) a direct effect of hypoglycemia on the adrenal gland contributes to the reduced cortisol and epinephrine responses in well controlled IDDM subjects. Subjects and Methods Subjects Twenty-seven patients with IDDM and 20 healthy volunteers were studied. Thirteen patients were in good glycemic control (hemoglobin A,, <9.0%), and 14 were in poor glycemic control (hemoglobin A,, >ll%). The characteristics of the study subjects are shown in Table 1. None of the normal subjects had a personal or family history of adrenal disease or diabetes mellitus or were taking any medications known to interfere with pituitary or adrenal function. None of the diabetic subjects had clinical evidence of autonomic or peripheral neuropathy based on history or physical examination. Autonomic function was further assessed using the criteria of Ewing and Clarke (38) by measuring the heart rate variation during slow deep breathing at a rate of 5 breaths/min and the response to a Valsalva maneuver. Patients with more than 1 borderline result were excluded from the study. No patient had clinical or laboratory evidence of nephropathy or proliferative retinopathy. Voluntary written informed consent was obtained from each subject before the study, and the protocol was approved by the Joslin Diabetes Center committee on human studies. Procedures All studies were performed in the morning after an B- to 10-h overnight fast. Diabetic subjects received their usual dose of insulin on the evening before the study. Studies were postponed 1 week if the subject reported hypoglycemia (defined as symptoms or a measured plasma glucose, ~3.3 mmol/L) in the 24 h preceding the study. Subjects were instructed to perform home blood glucose monitoring at bedtime on the evening before the study and to eat extra food if the glucose value was 5.5 mmol/L or less to avoid possible asymptomatic nocturnal hypoglycemia. On the morning of each study day, a catheter was inserted into an antecubital vein of the nondominant hand for the administration of test substances, and a second catheter was placed retrogradely into a vein on the dorsum of the ipsilateral hand or wrist for blood sampling. The hand was placed in a heated box (70 C) to ensure arterialization of venous blood (39). IDDM subjects were given a variable infusion of insulin (0.6-1.8 pmol/kgmin) to maintain plasma glucose levels below TABLE 1. Demographic characteristics Healthy subjects No. Age (yr) Sex (MYiF) Body mass index (kg/m’) Diabetes duration (yr) HbAih Fasting plasma glucose (mmol/L) 20 28 -+ 5 1000 22 -t 2 5.1 * 0.2 Values are the mean i- SD. a P < 0.05 us. healthy subjects. b Normal range, 5.4-7.4%. ’ P < 0.001 between IDDM groups. of the study subjects IDDM patients Well controlled 13 28 k 5 815 23 2 2 12 5 7 7.8 t 0.9 8.4 k 4.4 IN IDDM 6.0 mmol/L before beginning the clamp, and three baseline blood samples were taken. A stepped hypoglycemic insulin clamp study was then performed in all uatients and controls. After the collection of baseline samples, a primed continuous infusion (12 pmol/kgmin) of crystalline human insulin (Eli Lilly Co., Indianapolis, IN) was begun and continued for 3 h. Plasma glucose levels were measured at 5-min intervals, and the glucose clamp technique (20,401 was used to produce a stepwise decline in the plasma glucose concentration from 5.0 to 4.4,3.9,3.3,2.8, and 2.2 mmol/L at 30-min intervals. During the final 10 min of each 30-min interval, plasma samples were obtained for measurement of epinephrine, norepinephrine, ACTH, cortisol, ll-deoxycortisol, and GH. To assess the effect of acute adrenal cortical blockade on catecholamine responses to hypoglycemia, 11 of the normal subjects underwent a second hypoglycemic insulin clamp study in random order, with at least 1 week between studies. At 2300 h on the night before 1 of the study days, subjects were given 2.5-3.0 g metyrapone (Metopirone, Ciba Pharmaceutical Co., Summit, NJ), orally, at a dose determined by their body weight (41). At the start of the clamp study at 0800 h on the following morning, subjects took an additional 0.5 g metyrapone to ensure adequate adrenal cortical blockade during the study. Dexamethasone (0.2 mg/h) was infused during the hypoglycemic study with metyrapone to prevent the potential cardiovascular and other systemic consequences of hypoglycemia during the period of adrenocortical insufficiency. Previous studies have shown that pretreatment with low dose dexamethasone does not affect the adrenal glucocorticoid response (42). Analyses Plasma glucose was measured at the bedside using the glucose oxidase method (Yellow Springs Instruments, Yellow Springs, OH). Total plasma insulin levels in the healthy volunteers were measured by a double antibody RIA (43). In IDDM subjects, free insulin was measured after separation with polyethylene glycol. Total glycosylated hemoglobin was measured by agar gel electrophoresis (44) with the GLYTRAC glycosylated hemoglobin set (Corning Medical, Palo Alto, CA) after removal of the labile component. Plasma epinephrine levels were determined by radioenzymatic assay (45). GH (46), cortisol (47), and lldeoxycortisol (48) were determined using standard RIA procedures (ICN Biomedicals, Costa Mesa, CA). ACTH was determined by immunoradiometric assay (49) (Nichols Laboratories, San Juan Capistrano, CA). Data are presented as the mean 2 SEM, except for demographic data (Table l), which are presented as the mean + SD. Comparisons between groups were assessed by Student’s t test or ANOVA with repeated measures as appropriate. For data that were not normally distributed, comparisons between groups were made using the Mann-Whitney U and Kruskall-Wallis tests. All statistical analyses were performed using the SYSTAT statistical software program (Evanston, IL). The glucose thresholds required for stimulation of release of each counterregulatory hormone were determined as the plasma glucose level at which the hormone achieved a sustained and physiologically significant increment above basal, as previously described (9). This predefined increment was 410 pmol/L above basal for epinephrine, 190 nmol/L above basal for cortisol, and 7 pg/L above basal for GH. As threshold values for ACTH had not previously been reported, we used the glucose level at which ACTH achieved a sustained increment greater than 2 SD above mean basal level for each subject. Poorly controlled 14 28 -+ 717 24 k 81-5 12.3 i10.7 k Results Plasma 7 3” 1.5” 5.2 glucose and insulin At the start of the experiment, the mean glucose level was 5.3 z 0.1 mmol/L in the healthy volunteers, 6.3 + 0.2 mmol/L in the group with well controlled diabetes, and 6.1 2 0.2 mmol/L in the group with poorly controlled diabetes (P < 0.01 between healthy controls and the two diabetic groups; Fig. 1). After the start of the clamp, glucose levels did not differ significantly between the groups, except during the final 30-min period, when mean glucose values were 2.4 -t 0.1 and 2.2 + 0.1 mmol/L, respectively, in the healthy vol- 686 KINSLEY A 30~ 8t 0 JCE&M*1996 Vol81 . No 2 AND SIMONSON Basal 0 30 60 so Time 120 150 180 Basal I 5 4.4 FIG. 1. Plasma glucose levels during hypoglycemic clamp studies in 20 healthy subjects (shaded bars), 13 subjects with well controlled IDDM (open bars), and 14 subjects with poorly controlled IDDM (solid bars). *, P < 0.01, healthy subjects us. the other two groups; #, P < 0.01, healthy subjects vs. well controlled IDDM. unteers and the well controlled diabetic group (P < 0.01 between groups). The nadir glucose level was 2.5 + 0.2 mmol/L in the poorly controlled IDDM group (P = NS between IDDM groups). The mean steady state total insulin concentration during the experiment was 826 + 54 pmol/L in the healthy volunteers, and the mean free insulin levels were 768 + 46 pmol/L in the well controlled diabetic group and 936 + 114 pmol/L in the poorly controlled patients (P = NS among groups). ACTH During the hypoglycemic study, mean basal ACTH levels were 2.6 rt 0.4 pmol/L in normal controls and 3.1 + 0.4 and 2.4 + 0.4 pmol/L in the well and poorly controlled diabetic groups (P = NS among groups). At the nadir glucose value, ACTH responseswere 27 + 4 pmol/L in the healthy controls and 15 + 3 and 20 + 5 pmol/L in the well and poorly controlled diabetic groups, respectively (P < 0.05 between the well controlled IDDM group and the healthy volunteers; Fig. 2A). Glucose thresholds for activation of the ACTH responsewere 2.9 t 0.1 mmol/L in the healthy controls and 2.7 2 0.2 and 3.7 IT 0.3 mmol/L in the well and poorly controlled IDDM groups, respectively (P < 0.01 between poorly controlled IDDM and the other two groups; Table 2). Cortisol Mean basal cortisol levels were 430 k 47 nmol/L in the healthy volunteers and 306 2 30 and 346 t 41 nmol/L in the well and poorly controlled diabetic groups, respectively (P = NS among groups; Fig. 2B). Cortisol levels at the glucose nadir were 715 t 61 nmol/L in the healthy controls and 499 ? 41 and 598 t 58 nmol/L in the well and poorly controlled patients, respectively (P < 0.05, healthy volunteers VS.well controlled IDDM; Fig. 28). Glucose threshold values for cortisol were 2.7 2 0.1 mmol/L in the healthy volunteers and 2.3 ? 0.1 and 2.9 rt 0.2 mmol/L in the well and poorly controlled IDDM groups, respectively (P < 0.05, well controlled IDDM VS.other two groups; Table 2). At the nadir glucose, there was a significant correlation between cortisol and ACTH (r = 0.43; P < 0.05) in all 27 diabetic patients combined. 3.9 Glucose (minutes) B 3.3 2.0 2.2 (mmol/l) 800 600 s E c = .; 400 s 200 n Basal 5 4.4 3.9 Glucose 3.3 2.8 2.2 (mmol/l) FIG. 2. A, ACTH levels during hypoglycemic clamp studies in 20 healthy subjects (shaded bars), 13 subjects with well controlled IDDM (open bars), and 14 subjects with poorly controlled IDDM (solid bars). *, P < 0.05, healthy subjects us. well controlled IDDM. B, Cortisol levels during hypoglycemia in 20 healthy subjects (shaded bars), 13 subjects with well controlled IDDM (open bars), and 14 subjects with poorly controlled IDDM (solid bars). *, P < 0.05, healthy subjects us. well controlled IDDM. Catecholamines Basal epinephrine values before the hypoglycemic clamp study were not different among groups (238 2 33 pmol/L in the healthy volunteers and 289 t 44 and 235 2 44 pmol/L in the well and poorly controlled IDDM groups, respectively). However, epinephrine levels at the nadir of hypoglycemia were 5638 2 551 pmol/L in the healthy volunteers and 2320 t 497 and 4311 t 933 pmol/L in the well and poorly controlled IDDM groups, respectively (P < 0.001, healthy volunteers US. well controlled IDDM; Fig. 3A). Glucose thresholds for activation of epinephrine secretion were 3.1 + 0.1 mmol/L in the healthy volunteers and 2.7 + 0.2 and 3.4 rfr 0.3 mmol/L well controlled and poorly controlled IDDM groups, respectively (P < 0.05, well controlled IDDM VS. other two groups; Table 2). Basal norepinephrine levels were not different among study groups (0.97 + 0.06 nmol/L in healthy controls and 0.98 + 0.20 and 0.82 k 0.09 nmol/L in the well and poorly controlled IDDM groups, respectively). Norepinephrine values at the glucose nadir were lower in the well controlled IDDM group (1.67 + 0.17 nmol/L) than in the healthy volunteers (2.45 t 0.19 nmol/L; P < 0.011,and the norepinephrine level was 2.17 ? 0.28 nmol/L in the poorly controlled IDDM group (P = NS US.other two groups; Fig. 38). HPA RESPONSE A TO HYPOGLYCEMIA IN IDDM 687 6000 5000 g g s 4000 .f 2 3000 .K w” 2000 1000 Glucose (mmol/l) ” Basal 5 4.4 B 3~ 3.9 Glucose 3.3 2.6 2.2 (mmol/l) FIG. 4. GH levels during hypoglycemic clamp studies in 20 healthy subjects (shaded bars), 13 subjects with well controlled IDDM (open bars), and 14 subjects with poorly controlled IDDM (solid bars). *, P < 0.05, healthy subjects vs. well controlled IDDM. well and poorly controlled IDDM groups (P = NS among groups). Paired studies with metyrapone At the start of the experiment the mean basal glucose level was 5.1 t 0.1 mmol/L in both the control and metyrapone studies, whereas basal insulin levels were 24 t 6 and 26 t: 7 pmol/L, respectively. After the start of the clamp, glucose levels did not differ significantly between the groups, and the glucose nadir was 2.4 t 0.1 mmol/L for both studies. The mean steady state total insulin concentrations did not differ during the paired studies (840 + 120 ‘us.888 + 66 pmol/L; P = NS between groups). Plasma glucose and insulin. Glucose (mmol/l) FIG. 3. A, Epinephrine levels during hypoglycemic clamp studies in 20 healthy control subjects (shaded bars), 13 subjects with well controlled IDDM (open bars), and 14 subjects with poorly controlled IDDM (solid bars). *, P < 0.01, healthy subjects us. well controlled IDDM; #, P < 0.001, healthy subjects vs. well controlled IDDM. B, Norepinephrine levels during hypoglycemic clamp studies in 20 healthy controls (shaded bars), 13 subjects with well controlled IDDM (open bars), and 14 subjects with poorly controlled IDDM (solid bars). *, P < 0.01 healthy subjects us. well controlled IDDM. TABLE 2. Glucose thresholds for release of counter-regulatory hormones in healthy subjects and in patients with well controlled IDDM and poorly controlled IDDM IDDM patients Healthy subjects LTH Cortisol Epinephrine GH 2.9 20 t 0.1 2.7 i 0.1 3.1 t 0.1 2.9 t 0.1 Well controlled 2.7 2.3 2.7 3.1 t130.2 2 O.lb t 0.2b t 0.2 Poorly controlled 3.7 2.9 3.4 3.2 14 0.3” -c t 0.2 + 0.3 t 0.2 a P < 0.01 us. healthy subjects and well controlled IDDM. ‘P < 0.05 vs. healthy subjects and poorly controlled IDDM. GH Basal GH levels before the clamp were 4.3 -t 1.0 pg/L in the healthy volunteers and 10.5 + 3.0 and 8.4 t 2.0 &L in the well and poorly controlled IDDM groups, respectively (P < 0.05 between healthy volunteers and the well controlled IDDM group). GH values at the glucose nadir were 36 t 7 @g/L in the healthy volunteers and 59 t 9 /J&L in the well controlled IDDM (P < 0.05 between groups); this value was 37 t 8 pg/L in the poorly controlled IDDM group (Fig. 4). Glucose thresholds for GH were 2.9 2 0.1 mmol/L in the healthy volunteers and 3.1 + 0.2 and 3.2 t 0.2 mmol/L in the The basal ACTH level before hypoglycemia was 3.5 t 0.9 pmol/L, whereas the basal ACTH level during metyrapone treatment was 50 2 8 pmol/L (P < 0.001). ACTH levels remained significantly higher during metyrapone treatment at plasma glucose plateaus of 5.0, 4.4, and 3.9 mmol/L (Fig. 5A). The ACTH levels at nadir glucosevalues were 54 + 20 pmol/L during hypoglycemia alone and 68 t 23 pmol/L during hypoglycemia plus metyrapone (P = NS among groups). ACTH. Cortisol. Basalcortisol levels did not differ on the 2 study days (411 -+ 69 nmol/L for hypoglycemia alone vs. 372 -+ 72 nmol/L for hypoglycemia plus metyrapone). During hypoglycemia alone, cortisol levels rose to 720 5 80 nmol/L at the glucose nadir (P < 0.001 compared to basal cortisol). The cortisol level at the glucose nadir during metyrapone treatment was 312 +- 55 nmol/L (P = NS compared to basal cortisol during metyrapone). Cortisol levels were significantly lower during metyrapone treatment at plasma glucose levels of 2.8 and 2.2 mmol/L (P < 0.05 vs. hypoglycemia alone; Fig. 5B). Basal ll-deoxycortisol levels were significantly higher in the hypoglycemia plus metyrapone study (4011 + 346 nmol/L) compared with the basal level during hypoglycemia alone (69 2 6 nmol/L; P < O.OOl),consistent with effective adrenal cortical blockade. ll-Deoxycortisol levels were significantly higher at all levels of plasmaglucose IZ-Deoxycortisol. KINSLEY IB i 535 0S 600 400 200 800 I I1 E 0 Basal 5 4.4 3.9 3.3 Glucose (mmol/l) Basal - s z a .E 2 E ‘E w 4.4 3.9 3.3 Glucose (mmol/l) ** 1I JCE & M . 1996 Vol RI l No 2 AND SIMONSON 5 4.4 Glucose 3.9 3.3 (mmol/l) 2.8 6000 4000 2000 Basal 5 4.4 3.9 3.3 Glucose (mmol/l) 2.2 II I‘c 2.8 2.2 FIG. 5. A, ACTH values in 11 healthy control subjects during paired clamp studies of hypoglycemia (open bars) and hypoglycemia during adrenocortical blockade with metyrapone (solid bars). *, P < 0.001. B, Cortisol levels in 11 healthy subjects during paired clamp studies of hypoglycemia (open bars) and hypoglycemia during adrenocortical blockade with metyrapone (solid bars). *, P < 0.05. C, ll-Deoxycortisol levels in 11 healthy subjects during paired clamp studies of hypoglycemia (open bars) and hypoglycemia during adrenocortical blockade with metyrapone (solid bars). * P < 0.001. D, Epinephrine levels in 11 healthy subjects during paired clamp studies of hypoglycemia (open bars) and hypoglycemia during adrenocortical blockade with metyrapone (solid bars). from 2.2-5.0 mmol/L during hypoglycemia pone than during those during hypoglycemia 0.001; Fig. 5C). plus metyraalone (P < Cute&&mines. Basal epinephrine levels did not differ on the 2 study days (225 2 46 pmol/L during hypoglycemia alone and 207 ? 40 pmol/L during hypoglycemia plus metyrapone; P = NS between groups). Similarly, epinephrine responses did not differ throughout the clamp studies on the 2 study days, and epinephrine levels at the glucose nadir were 5266 ? 731 pmol/L during hypoglycemia alone and 4448 -C 699 pmol/L during hypoglycemia plus metyrapone (P = NS between groups; Fig. 5DJ. Norepinephrine levels similarly did not differ during the clamp studies on each study day (data not shown). GH. The basal GH value during hypoglycemia alone (4.7 -t 1.5 pg/L) was not significantly different from that during hypoglycemia plus metyrapone (5.7 2 1.6 Fg/LJ. There were no differences Jn GH JeveJs fJ~~z@ouf the cJ~~P &z&es on the 2 days, and the peak GH level at the nadir glucose achieved during the study was 49 2 10 pg/L during hypoglycemia alone US. 53 2 14 pg/L during hypoglycemia plus metyrapone (P = NS between groups). Discussion Strict glycemic control of IDDM is associated with a reduced counterregulatory hormone response to hypoglycemia and a 3-fold increase in the incidence of severe hypoglycemic events (1,3). Many recent studies have shown that exposure to recurrent hypoglycemia reduces the counterregulatory hormone responses to subsequent hypoglycemia in both normal subjects and patients with IDDM (20-26). Most of these studies show significant alterations in catecholamine, cortisol, and growth responses (5-11,20-26). We have recently examined the relationships between these hormonal responses and glycemic control in a large series of IDDM patients and healthy controls and have shown that catecholamine and cortisol secretion are consistently impaired whereas GH secretion is significantly enhanced in well controlled IDDM (50). The divergence in GH dynamics from the other counterregulatory hormones is most likely due to the fact that both hyperglycemia and hypoglycemia independently regulate GH secretion. Thus, the reduction in exposure to hyperglycemia that results from improved control may mitigate the inhibitory effect of glucose on GH secretion (50). However, no study to date has clarified the relative central HPA RESPONSE TO HYPOGLYCEMIA US. peripheral effect of glycemic control on hypothalamicpituitary-adrenal (HPA) responsiveness to hypoglycemia in IDDM. To address this issue, we performed stepped hypoglycemic insulin clamp studies to assess HPA axis responses to acute hypoglycemia. In addition, we used a model of acute adrenal cortical blockade with metyrapone to study the effect of reduced cortisol levels on the epinephrine response to hypoglycemia in a group of normal subjects. In the first study we have shown that strict glycemic control of IDDM reduces the ACTH and cortisol responses to hypoglycemia. The significant correlation between these two variables suggests that the ability of strict glycemic control to impair cortisol secretion during hypoglycemia is likely to reside at a central (hypothalamic-pituitary) site rather than the peripheral (adrenal) site. In the second study, adrenal cortical blockade with metyrapone abolished the cortisol response to hypoglycemia, while the epinephrine response remained intact. Thus, the epinephrine response to acute hypoglycemia is not dependent on intact ACTH-cortisol dynamics; this, again, suggests a central rather than peripheral origin for the adaptations in counterregulation. There are few previous data on ACTH responses to hypoglycemia in IDDM. Frier et al. (37) found no effect of the duration of IDDM on ACTH responses to hypoglycemia, but the researchers did not examine the effect of glycemic control in this study. Gallucci et al. (51) studied the effect of glycemic control of IDDM on ACTH responses to injection of ovine CRH and found that total integrated plasma ACTH, cortisol, and urinary free cortisol levels were higher in the IDDM subjects. They suggested that the degree of HPA axis disturbance appears to be associated with worse glycemic control. Lingenfelser et al. (25) showed a nonsignificant trend toward a decreased ACTH response in 11 IDDM subjects after recurrent hypoglycemia. Our data confirm this suggestion, as a previous study by our group using smaller numbers of patients showed a nonsignificant trend toward a reduction in ACTH responses with improved glycemic control (52). However, in this present study using larger numbers of subjects, we were able to observe a significant effect of glycemic control on ACTH responses to hypoglycemia. Thus, HPA axis activity appears to be abnormal in IDDM subjects in response to both hypoglycemic and nonhypoglycemic stimuli, and the degree of the abnormality may be determined by glycemic control. Data from animal studies have shown that ACTH release during hypoglycemia occurs mainly in response to hypothalamic stimuli (53-56). At levels of hypoglycemia comparable to those used in this study, corticotropin-releasing factor appears to be the major factor involved in ACTH release (56). Studies in humans have suggested that pituitary corticotrophs receive maximal stimulation from endogenous CRF during hypoglycemia (57). To date, endogenous CRF responses to hypoglycemia have not been studied in IDDM in man. Thus, strict glycemic control of IDDM may reduce the CRF response to hypoglycemia, although a direct effect on pituitary corticotrophs cannot be excluded. Possible roles for vasopressin (58-60), which can potentiate ACTH secretion, and P-endorphin (61, 62), which is cosecreted with ACTH, will need to be explored in future studies. The cellular mechanism underlying this alteration in HPA IN IDDM axis function in patients with IDDM remains uncertain. In animal studies, chronic hypoglycemia is associated with an increase in hexose transport across the blood-brain barrier, with a resultant increase in cerebral glucose utilization and less deterioration in cerebral function during hypoglycemia (33-35). Boyle et al. (36) detected a similar adaptation in brain glucose utilization in healthy humans after 4 days of hypoglycemia, with a decrease in the epinephrine response to hypoglycemia on day 4 compared with day 1 of the study. The reduction in glucose thresholds for ACTH and cortisol release in this study also suggests that strict glycemic control up-regulates glucose transport across the blood-brain barrier, thereby inducing an alteration in the glucose level at which hypoglycemia activates the hypothalamic-pituitary response and decreasing the degree of physiological stress associated with subsequent hypoglycemia. Prior exposure to recurrent hypoglycemia is an important determining factor in these adaptations (20-26). Could a central neural adaptation in the ACTH (and, consequently, cortisol) response, as seen in our study, contribute to the reduced epinephrine response in the well controlled IDDM group through a direct effect at the adrenal gland? There is evidence that the adrenal cortex and adrenal medulla are structurally and functionally interconnected. The adrenal medulla is exposed to levels of glucocorticoids many times higher than those in the systemic circulation (63, 64), and glucocorticoids were found to act as cofactors in the production of catecholamines in the adrenal medulla (65). Hypophysectomized animals have reduced tyrosine hydroxylase, dopamine P-hydroxylase, and phenylethanolamineN-methyl transferase levels that increase with either replacement of ACTH or treatment with large doses of glucocorticoids (65). In human studies, a subject with IDDM and selective ACTH deficiency had impaired epinephrine responses to hypoglycemia that improved with cortisol replacement (66), whereas in children with hypopituitarism, the epinephrine response to exercise was reduced in a group with ACTH deficiency (67). Based on this suggestive evidence, we studied the effect of acute adrenal cortical blockade on the adrenal medullary response to hypoglycemia in normal subjects. We chose metyrapone to induce acute adrenal cortical blockade. Although metyrapone effectively abolished the cortisol response to hypoglycemia, epinephrine responses did not differ on the 2 study days. Thus, the modifications in epinephrine responses induced by strict glycemic control do not appear to result from a decrease in cortisol secretion in the adrenal cortex. Models of chronic glucocorticoid deficiency, however, may have a greater effect. In conclusion, this study has shown that glycemic control of IDDM modulates the ACTH, cortisol, and catecholamine responses to acute hypoglycemia. The most likely mechanism for these adaptations is a central adaptation to recurrent hypoglycemia that maintains cerebral glucose utilization during hypoglycemia, resulting in decreased activation of the cerebral glucose sensors. Thus, there is less activation of the hypothalamic-pituitary-adrenal axis and sympatho-adrenal medullary system with resultant reduction in the counterregulatory hormone response to hypoglycemia. 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