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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.
Our data
do not support a direct adrenal contribution
to these adap-
690
KINSLEY AND SIMONSON
tations. These findings may have important implications for
our understanding of the stress response in patients with
IDDM.
24
25
Acknowledgments
We thank Susan Fritz, R.N., C.D.E., and Inga Liberman
for expert
assistance with the clinical protocols,
and Irene Reske and Marta Grinbergs for careful performance
of the laboratory
assays.
26
27
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