(Hypertension. 2000;36:208.)
© 2000 American Heart Association, Inc.
Scientific Contributions |
Augmented Sympathetic Response to Bradykinin in the Diabetic Heart Before Autonomic Denervation
From the Department of Anesthesiology and Pain Management (Z.P., S.F.R.), Cook County Hospital, and the Department of Pharmacology (S.V.), University of Illinois College of Medicine, Chicago, and the Hypertension and Diabetes Research Unit (G.J.D.), Max Grundig Clinic, Buehl, Germany.
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Abstract |
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Abstract—We studied whether diabetes mellitus affects the bradykinin (BK)-induced release of norepinephrine (NE) from rat cardiac sympathetic endings in situ. Three groups were studied. Group A (n=12) was rendered diabetic with streptozotocin (STZ), group B (n=13) received STZ and insulin, and group C (n=14) received citrate buffer only. NPH insulin was given to group B from day 7 after STZ. Atria were paced (3Hz) with rectangular voltage pulses at mechanical threshold intensity (0.15 V/cm). The release of NE was assessed through its effects on contractile force in the presence of atropine (1 µmol/L). Intensifying the field stimulation above the neural threshold (
0.4
V/cm) produced a graded positive inotropic effect that was due to the
release of NE from sympathetic nerve endings. The additional effect of
0.1 µmol/L BK on the force of contraction was determined at
half-maximal neural stimulation (ie, at 0.65 V/cm). Then, after
washing out BK and lowering the stimulation intensity to mechanical
threshold, a cumulative dose-response curve for added NE was generated,
allowing the positive inotropic effects of neural stimulation (with or
without BK) to be expressed in terms of an equivalent inotropic
concentration of added NE ([NEeq]). Neural stimulation,
in the absence of BK, gave an [NEeq] of 32±3 nmol/L in
group A, 44±6 nmol/L in group B, and 37±6 nmol/L in group C. BK
increased [NEeq] by a factor of 6.2±0.9 in group A,
4.5±0.5 in group B, and 3.7±0.3 in group C. This factor was greater
in group A than in group C but indistinguishable in groups B and C.
Atria from normal and diabetic rats were incubated in
3[H]NE for 60 minutes. Excess tracer was removed, and
atria were stimulated during a series of 1-minute episodes at
half-maximal neural stimulation to cause exocytotic
3[H]NE release. BK augmented 3[H]NE release
in normal (n=4) and in diabetic (n=4) atria. This BK-induced increase
of 3[H]NE overflow (expressed as a fraction of tissue
3[H]NE radioactivity) was 4 times greater in diabetic
than in normal preparations. The response to BK in releasing
sympathetic neurotransmitter is augmented in diabetic rats, recovering
in a manner dependent on insulin.
Key Words: bradykinin • diabetes mellitus • norepinephrine • insulin
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Introduction |
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In chronic diabetes, there is a cardiomyopathy involving cardiac contractile performance that is related to diabetic autonomic neuropathy (DAN).1 DAN is a common complication of diabetes and has been proposed as a cause of sudden cardiac death in diabetic patients.2 3 Cardiac denervation results in a fixed heart rate that is unresponsive to exercise or stress. This absence of heart rate variability secondary to DAN is predictive of left ventricular failure4 5 and increased mortality.6 Diabetic rats have defects in the intrinsic electrical and contractile properties of the heart7 and in adrenergic and cholinergic receptor populations,7 and they also exhibit reduced norepinephrine (NE) turnover.8 These alterations may lead to impaired cardiac function. Moreover, the streptozotocin (STZ)-induced diabetic rat model is known to develop DAN after 16 weeks of induction.9 We have shown that in isolated rat atria, bradykinin (BK) affects cardiac contractile force by its modulation of evoked NE release from cardiac sympathetic nerve endings.10 BK was without effect on atrial contractile force when stimulation intensity was set to low levels that excite heart muscle but not neurons.10
The present investigation was undertaken to determine whether (1) the development of diabetes mellitus affects the BK-induced NE release, as determined by the measurements of twitch contractile force and exocytotic 3[H]NE release, and (2) insulin therapy reverses the contractile alterations in response to BK.
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Methods |
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After our Animal Care and Use Committee approved the protocol, 49 adult Sprague-Dawley rats (250 to 300 g) were randomly divided into 3 groups: (1) diabetic (n=16), (2) insulin-treated diabetic (n=13), and (3) control (n=20). Diabetes was induced with a single injection of STZ (55 mg/kg) in 0.05 mol/L citrate buffer, pH 4.5. Control rats received citrate buffer (1 mL/kg). Injections were given intravenously through a tail vein while the animals were under light general anesthesia with 5% sevoflurane. Diabetic and control rats were caged separately but housed under similar conditions. Both groups of animals were fed the same diet and water ad libitum until the day before the experiment. Rats were considered diabetic if fasting blood glucose exceeded 400 mg/dL 2 days after STZ. NPH human insulin was given to the second group at 3.3 U/d SC at
4:00 PM, from day 7 after
STZ until the day before the experiment. Blood glucose, body weight,
and water and food intake were measured before the injection of STZ and
at weekly intervals thereafter. To measure blood glucose, rats were
placed under light general anesthesia with 5% sevoflurane,
and a drop of blood was collected from a tail vein with a 25-gauge
needle. Blood glucose concentration was measured with an Accu-Chek III
blood glucose monitor (model 766, Boehringer-Mannheim Corp).
Measurement of Contractile Force in Isolated Heart Muscle
Twelve to 16 weeks after the administration of STZ or citrate
buffer, the rats were anesthetized with halothane. The hearts
were removed and immediately perfused to remove the blood. The left
atria were excised and attached between a force-displacement transducer
(Grass FT 0.3) and a fixed point with use of a pair of stainless-steel
hooks. The atria were immersed in a water-jacketed glass chamber (70-mL
capacity) containing heated (33°C) and oxygenated (100%
O2) Krebs-Henseleit solution. After a 30-minute
equilibration period, the resting tension of the tissue was adjusted to
give half-maximal twitch developed force. The muscle was then
stimulated at a frequency of 3 Hz with rectangular current pulses
delivered via a pair of platinum electrodes on either side of the
preparation. The field strength (mV/cm) of current pulses was measured
from a pair of silver wires on either side of the preparation at a
distance of 1 cm; these recording electrodes were insulated
except for their tips. Twitch contractions were recorded on a
flatbed recorder (model 45, IITC) and simultaneously
displayed on the video monitor of a computer (DELL 325P) after being
digitized (Labmaster Board, Tecmar, Inc). Online automated measurements
of the peak amplitude of twitch contractions were made and stored in a
file for later analysis.
Determination of Indirect Positive Inotropic Effect of
BK
Field stimulation was intensified for excitation of the
sympathetic nerve endings embedded in atrial muscle. Atropine (1
µmol/L) was used to block possible cholinergic effects of intensified
stimulation. To standardize excitation of sympathetic terminals, the
electrical stimulus was set to the voltage giving half-maximal inotropy
(V1/2) of the evoked sympathetic
catecholamine. V1/2 was determined by
increasing the stimulation intensity above the mechanical threshold in
steps of 5 or 10 V and recording the resultant positive
inotropic effects. V1/2 was estimated from a plot of
effect versus voltage. Next, with stimulation intensity set at
V1/2, the contractile force was measured until stable.
BK (0.1 µmol/L) was added to the bath, and its positive
inotropic effect was monitored until it developed fully. To prevent BK
degradation by angiotensin I–converting enzyme
(ACE)/kininase II, 2.6 µmol/L enalaprilat was added to the bath
1 minute before BK administration.
Inotropic Action of NE and Tyramine
The bath liquid was replaced several times with drug-free
solution to remove BK. After the recording of baseline
contractile force, a cumulative concentration-effect curve for added NE
(3 nmol/L to 6.7 µmol/L) was constructed (stimulation lowered to
mechanical threshold). NE concentration was raised only after the
twitch contractile force reached a steady state after the previous NE
concentration. In some atria, once the concentration-effect curve for
NE was completed, a baseline for contractility was
reestablished, and the positive inotropic effect of tyramine (100
µmol/L) was determined.
Calculation of Equivalent Inotropic Concentration of
NE
The effects of intensified field stimulation on contractile
force, with or without BK, were compared with the effect of added NE to
determine the equivalent inotropic concentration of NE
([NEeq]). [NEeq] was
calculated from
Kd,NExE/(Emax,NE-E),
where Kd,NE is the NE concentration
producing the half-maximal effect, Emax,NE is the
experimentally determined maximal inotropic effect of NE, and E is the
observed stimulation effect (at V1/2), with or without
the addition of BK (Figure 1).
Kd,NE was calculated by fitting a sigmoidal
function to the log NE concentration versus effect data. This
calculation relies on the fact that the entire inotropic effect of
field stimulation and BK are due to NE released from sympathetic nerve
endings.10
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Measurement of Exocytotic 3[H]NE Release
Isolated atria were prepared for simultaneous
measurements of twitch contractile force and
3[H]NE release. Preparations were attached to a
force displacement transducer and paced at mechanical threshold (0.4
Hz, 0.2 V/cm). A smaller experimental vessel (10-mL capacity) was used.
Atria were labeled for 60 minutes with 27 µCi of DL-NE
hydrochloride [7-3H(N)]. To remove excess tracer, the
bath liquid was replaced 12 times at 5-minute intervals. At the end of
each period, a 100-µL sample was withdrawn from the bath to be used
for later determination of tracer washout kinetics.
Protocol for Stimulation-Evoked Release of
3[H]NE
Preparations were bathed in Krebs’ solution supplemented with
3 µmol/L cocaine for the remainder of the experiment. This
portion of the experiment was divided into seven 10-minute periods (S0
to S6). For S0, no stimulation was applied; for S1 to S6, the
preparations received (during the first minute only) field stimulation
above neural threshold (0.7 V/cm, 2 Hz; see Figure 2A legend) to evoke exocytotic NE
release. At the end of each 10-minute period, a 100-µL sample was
withdrawn from the bath for later determination of tritium, and the
bath liquid was replaced with fresh cocaine-containing Krebs’
solution. BK (0.1 µmol/L) was administered immediately before
S5. At the conclusion of the experiment, the atria were blotted,
weighed, and placed overnight in a glass vial containing 1 mL of 2%
(vol/vol) perchloric acid. Radioactivity in tissue extracts and
collected fractions was determined by mixing them with a biodegradable
counting cocktail (Econo-Safe) by liquid scintillation counting
(Packard 2000 Tri-Carb, Packard Instrument Co).
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,
n=14), diabetic (DM, 
; n=12), and insulin-treated diabetic (DM+Ins,
; n=13) rats. Preparations were isolated 12 to 16 weeks after the
administration of STZ or diluent. V1/2 was 45.8±2.3 V
in control rats, 43.9±2.3 V in DM rats, and 42.5±2.2 V in DM+Ins
rats. B, Relationship between field strength (V/cm) and stimulation
intensity (V). The regression line through the data points has a slope
of 0.0145. In general, the slope obtained depends on bath volume and
geometry of stimulating electrodes. Inotropic effect is expressed as
percentage of basal contractile force that was 334±53 mg in the
control group, 298±27 mg in the DM+Ins group, and 427±75 mg in the DM
group. Values are mean±SEM.
Tissue Uptake of 3[H]NE
By use of the tracer washout samples (see above), the number of
3[H]NE counts remaining in unstimulated tissue
was plotted against time. A double-exponential function was fitted to
the data. The fast component (time constant 5 to 10 minutes)
represented interstitial tracer washout. The
slow component (time constant 90 to 110 minutes) accounted for 90%
of the tissue uptake; extrapolation of this exponential component to 0
minutes gave an estimate of initial tissue uptake of tracer (see Figure 5 legend).
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Exocytotic Release of 3[H]NE
From samples S1 to S6, stimulated 3[H]NE
release (overflow) was expressed as a fraction of tissue radioactivity
with correction for fractional background (unstimulated) release of
tritium. The latter was obtained as the 3[H]NE
release during S0 (expressed as a fraction of tissue radioactivity).
The BK effect for each preparation was taken as the difference between
3[H]NE overflow in S6 and S4.
Drugs and Solutions
Drugs
Atropine, NE, BK, STZ, ascorbic acid, and all salts used in the
Krebs-Henseleit solution were obtained from Sigma Chemical Co.
Enalaprilat was a gift from Merck Research Laboratories (Rahway, NJ).
Sevoflurane was obtained from Abbott Laboratories, and halothane was
from Ayerst Laboratories Inc. NPH human insulin (recombinant DNA
origin) was acquired from Novo Nordisk Pharmaceuticals Inc.
DL-NE hydrochloride [7-3H(N)] was purchased
from American Radiolabeled Chemicals, Inc, and Econo-Safe was from
Research Products International Corp. Cocaine was obtained from the
University of Illinois Hospital Pharmacy.
Solutions
The composition of the Krebs-Henseleit solution was as follows
(mmol/L): NaCl 118, KCl 4.7, CaCl2 0.2,
H2O 1, HEPES (acid) 5.55,
Na+-HEPES 4.45, MgCl2 0.25,
glucose 10, and Na4-EDTA 0.025. A stock solution
of BK (0.1 mmol/L) was prepared in advance in 154 mmol/L NaCl
and kept at -20°C until used. A stock solution of NE (1.0
mmol/L) was prepared before each experiment in Krebs-Henseleit solution
containing equimolar ascorbic acid.
Statistical Analysis
Results are expressed as mean±SEM. Statistical comparisons
among group means were made by 1-way ANOVA with the Newman-Keuls
multiple comparison post hoc test. Differences were considered
statistically significant at P<0.05.
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Results |
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General Observations
After STZ administration, the animals exhibited the common characteristics of diabetes: polyuria, polydipsia, and polyphagia. As shown in the Table, at the time of the experiment, STZ-treated rats had significantly lower body weight and elevated blood glucose levels compared with control rats, which received citrate buffer only, indicating that the STZ-treated rats became diabetic. Insulin treatment partially reversed the changes observed in diabetic rats. Although diabetic and insulin-treated diabetic rats had smaller hearts, the heart weight/body weight ratio was significantly increased in diabetic rats, indicative of myocardial hypertrophy, but the ratio was not different from control in diabetic rats receiving insulin.
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Indirect Inotropic Action of BK in Isolated Atria From Normal and
Diabetic Rats
Effect of Increasing Stimulation Intensity on Atrial
Contractility
Increasing the intensity of field stimulation produced graded
positive inotropic effects in all groups. Atropine prevented muscarinic
cholinergic effects on the atrial preparation from released
parasympathetic agonists (see Methods). The relationship between
stimulation intensity and force of contraction for each group was
sigmoidal (Figure 2). Positive inotropy due to the release of NE
from sympathetic nerve endings was evident at intensities >30 V
(Figure 2A), corresponding to a neural threshold value of 0.4
V/cm (Figure 2B). The maximal inotropic effect of stimulation,
which occurred at a field strength of >0.8 V/cm, was markedly
depressed in diabetic animals (Figure 2A). Insulin treatment did
not reverse this change. The V1/2 values were
statistically identical among the 3 treatment groups (see Figure 2 legend).
Effect of NE on Atrial Contractility
To determine whether the diabetic state causes a reduction in NE
sensitivity, the inotropic responses to cumulative doses of added NE
were obtained. As shown in Figure 3, atria from control animals gave the largest increase in contractile
force in response to increasing concentrations of NE. The most
depressed response was observed in atria from diabetic animals. Insulin
treatment produced only a minimal improvement in NE responsiveness. The
average EC50 values were 94±14 nmol/L in the
control group, 108±15 nmol/L in the diabetic group, and 119±15 nmol/L
in the insulin-treated diabetic group. Because these values were not
significantly different from one another, the reduced sensitivity to NE
in left atrial preparations from diabetic rats is attributed to a
smaller NE maximal effect.
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Effect of STZ and Insulin on Sympathetically Mediated Contractile
Response to BK
Figure 4 summarizes the data for the
effect of diabetes on the response to BK. Diabetes did not
significantly affect the baseline inotropic response
([NEeq]) to stimulation of atria at
V1/2 intensity (32±3 nmol/L in diabetic rats, 44±6
nmol/L in insulin-treated diabetic rats, and 37±6 nmol/L in control
rats). The addition of enalaprilat before BK produced no measurable
inotropic action. However, the BK-induced increase in
[NEeq] above baseline values differed
significantly in the 3 groups. [NEeq] was
increased by a factor of 6.2±0.9 in the diabetic group, 4.5±0.5 in
the diabetic group receiving insulin, and 3.7±0.3 in the control
group. The effect of BK was significantly greater (P<0.05)
in atria from diabetic rats than in those from control rats. The BK
effect in atria from insulin-treated diabetic rats was not
significantly different from that in atria from control rats,
indicating that insulin treatment reversed the higher responsiveness to
BK in diabetic rats.
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Effect of Tyramine on Atrial Contractility
We measured the indirect positive inotropic effect of tyramine as
a functional test for possible differences in the extent of sympathetic
innervation in the atrial preparations. Tyramine causes the release of
NE from sympathetic nerve endings. At a concentration of 100
µmol/L, the relative inotropic response to tyramine was virtually
identical in atria from diabetic and control rats. The results indicate
that sympathetic denervation had not occurred in the diabetic
preparations used.
BK-Induced 3[H]NE Overflow in Atria Isolated From
Normal and Diabetic Rats
Tissue Uptake of 3[H]NE
In unstimulated atria, the tissue uptake of
3[H]NE was estimated by extrapolation of the
tracer washout curve to time 0 (Figure 5). In all preparations, the washout was
adequately described as a double-exponential decaying function, which
was essentially the same in normal and diabetic groups. The fast and
slow time constants were 9±0.8 and 102±8 minutes, respectively, for
the normal group and 8±0.9 and 93±2 minutes, respectively, for the
diabetic group. The fast and slow time constants were statistically
indistinguishable in the 2 groups. The tissue uptake of
3[H]NE averaged 40 359±4350 and 27 647±5717
counts per milligram wet atrial weight for normal and diabetic rats,
respectively, and the difference between the mean values was
statistically significant (P<0.05). This difference,
however, was attributable to the higher wet weight of atria of diabetic
origin (49.3±9.4 mg) versus control atria (29.7±2 mg). The difference
in tissue uptake was scaled by the same factor as for the difference in
weight. This indicated that the magnitude of tissue loading of tracer
was equivalent in both groups.
3[H]NE Overflow
Figure 6 illustrates our method for
determining the effects of BK on 3[H]NE
overflow. BK increased both 3[H]NE overflow and
the twitch contractile force more in the diabetic rats. Figure 7 summarizes the data for BK-induced
3[H]NE overflow in atria from normal and
diabetic rats. In the diabetic group, the BK effect was 4 times
greater than that observed in the control group (0.025±0.005 versus
0.006±0.002).
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Discussion |
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The present study indicates that in diabetic rats the response to BK in releasing sympathetic neurotransmitters is augmented and that this high BK sensitivity is an early functional marker of autonomic neuropathy before the occurrence of sympathetic neuronal degeneration.
Because the diabetic heart muscle has an altered baseline contractility and a reduced sensitivity to catecholamines,11 we assessed evoked NE release in our myocardial preparation as [NEeq], thus facilitating comparison of BK responses among the different study groups. [NEeq] refers to the exact concentration of added NE that would produce the observed effects of intensified electric stimulation in the absence and presence of BK. This analysis makes the assumption that the positive inotropic effect of intensified stimulation and the further contractile effect of BK are due to NE release from sympathetic nerve endings, provided that atropine is present to block cholinergic effects. We previously demonstrated this to be the case in isolated rat left atrial preparations.10 In the present study, only minor variations in basal NE liberation at V1/2 stimulation occurred in the 3 treatment groups, whereas the action of BK to augment the evoked release of NE was decidedly more pronounced in diabetic preparations compared with normal preparations. Moreover, insulin treatment produced partial recovery of the enhanced BK sensitivity. Because we performed an NE concentration-effect curve in each preparation in which we tested BK, our analysis was able to take into account possible differences in NE sensitivity among preparations and study groups. We indeed found, as reported,11 marked differences in sensitivity to added NE among the 3 treatment groups. Dose-response analysis of NE demonstrated that atria from diabetic animals have a drastically reduced maximal NE effect compared with atria from control animals. Insulin treatment gave a partial recovery of the NE sensitivity toward the control response. Interestingly, the values obtained for NE EC50 (Kd) were not significantly changed in any group, indicating that changes in receptor affinity to NE do not account for the differences in overall response to NE in rat atria.
On the basis of the results of the inotropic response to BK, we predicted that the atria from diabetic animals would show a greater induced release of NE from sympathetic nerve endings in response to BK. The fact that (unstimulated) tracer uptake and efflux were equivalent in both groups indicates that the difference in the BK response observed cannot be attributed to differences in tracer loading or rate of background release of tracer. In addition, because cocaine was present during all stimulation periods, differences in NE uptake cannot explain the observed differences in BK action. Although tritiated metabolites of NE could have been a contaminant in the measured tracer release, the fact that 3[H]NE overflow was correlated with simultaneously recorded twitch contractile force suggests that the tracer released actually reflected a physiologically relevant NE pool. These findings independently confirm the conclusions drawn from the [NEeq] data.
The significance of the enhanced sympathetic response to BK in atria from diabetic rats is difficult to assess at the present time. It could be suggested that an increased formation of kinins, as observed in the diabetic state,12 is a compensatory mechanism to improve autonomic function. In fact, ACE inhibitors, which are known to potentiate BK actions,13 exert a beneficial effect on peripheral neuropathy in STZ-induced diabetic rats.14 15 Although this beneficial effect could be due to vasodilatation,16 a direct influence on sympathetic nerve endings, via BK potentiation, should now be considered.
Rats treated with STZ develop cardiac autonomic dysfunction.17 18 19 The duration of diabetes appears to play a significant role determining the type of functional and morphological changes that take place in the autonomic nervous system after STZ administration. In a recent study in which biotelemetry techniques were used to determine the effect of STZ on autonomic nervous control of cardiac function in conscious unrestrained rats, it was found that STZ-induced diabetes is associated with time-dependent alterations in resting heart rate and its circadian variation, pulse pressure, and cardiac autonomic nervous system control.20 Analysis of NE content and innervation pattern of the heart of STZ-induced diabetic rats indicated that the duration of diabetes is particularly important. During the early stages, cardiac NE levels are increased, whereas in long-term diabetes, cardiac NE levels either return to normal or are decreased compared with control levels. No changes in the pattern of noradrenergic innervation were noted in the heart of diabetic rats at any age of onset or duration of STZ-induced diabetes, although the hearts of rats with short-term (1-month) diabetes appeared to be more densely innervated and to have more branching fibers than did the hearts of control rats.9 The fact that in the present study both stimulation (V1/2)-induced and tyramine-induced NE release were statistically indistinguishable in the 3 groups examined indicates that sympathetic neurons in diabetic tissue had a normal capacity to release NE. This is in accord with the assessment that overt DAN morphological changes after STZ do not take place before 6 months.18 However, because of the abnormal response to BK, it is now clear that at an early stage in the development of DAN, there are functional alterations of sympathetic nerve endings in the heart. Thus, our method could be a valuable tool to investigate incipient cardiac functional changes of diabetes.
We also found that the enhanced sympathetic response to BK reverses in
a manner dependent on insulin. Although many studies have confirmed the
beneficial effects of improved glycemic control on the progression of
peripheral somatic nerve deficit, including neuropathic
symptoms,21 definitive evidence that good glycemic control
prevents or delays the progression of DAN is still lacking. So far,
tests in humans have given variable results. Improved
metabolic control was reported to slow the progression of
DAN, reflected as deficits in heart rate variability in
insulin-dependent diabetic patients in some studies22 23 24
but not in others.25 26 Because very little, if any, is
known about the reversibility of the altered atrial response to BK
observed in diabetic animals, a secondary purpose of the present
study was to examine the effect of insulin therapy on atrial
contractile responses in diabetes. We found that at the dose used, 3.3
U/kg per day administered at 4:00 PM, insulin produced
only a partial recovery of the metabolic and mechanical
changes induced by STZ. As reflected by the blood glucose level
measured at 10:00 AM, this dose of insulin was probably
inadequate to produce optimal metabolic control.
Nevertheless, the results of the present study (Figure 4)
give a direct indication that improved metabolic control
with insulin indeed reverses some of the functional changes induced by
diabetes. This finding indicates that insulin plays a role in the
regulation of the BK B2 receptor on sympathetic
nerve endings. HOE 140 completely blocks the effect of BK on atria from
normal10 and diabetic rats (S. Vogel and S.F. Rabito,
unpublished data, 1999). In addition, neither Lys-BK nor
Des-Arg9 -BK, agonists on the BK B1
receptor, modify the twitch contractile force in atria from diabetic
rats (S. Vogel and S.F. Rabito, unpublished data, 1999). These
findings are in agreement with a recent study that demonstrated that
the expression of the BK B1 receptor was not
detectable in the myocardium of normal or diabetic
rats.27
We cannot exclude the possibility that differences in localized degradation of BK could account for the enhanced NE-releasing effect of BK in the diabetic group. Though we used a high concentration of BK in the conducted experiments and treated the preparations with enalaprilat to block the degradation of BK by ACE, the myocardium contains at least a minimal amount of neutral endopeptidase28 and kininase I activity29 that were not blocked with the ACE inhibitor.
In summary, our results indicate that the response to BK in releasing sympathetic neurotransmitters is augmented in diabetic rats and that BK responsiveness recovers toward control values with insulin treatment. Because the adult rat myocardium lacks inotropically functional postsynaptic BK receptors, these results can be interpreted as resulting from an increased number of BK receptors per nerve ending or an improved efficiency of the BK B2 receptor, as a consequence(s) of diabetes. In addition, because these changes were partially prevented in animals treated with insulin, it could be inferred that insulin plays a role in the regulation of the BK B2 receptor on sympathetic nerve endings.
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Footnotes |
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Reprint requests to Sara F. Rabito, MD, Associate Professor of Anesthesiology, Rush Medical College of Rush University, Department of Anesthesiology and Pain Management, Cook County Hospital, 637 South Wood St, Room 427, Chicago, IL 60612.
Received December 21, 1999; first decision January 12, 2000; accepted April 3, 2000.
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