(Circulation. 1996;93:1905-1912.)
© 1996 American Heart Association, Inc.
Articles |
Aminoguanidine Prevents the Decreased Myocardial Compliance Produced by Streptozotocin-Induced Diabetes Mellitus in Rats
From the Departments of Physiology (G.R.N., A.J.W.) and Nuclear Medicine (G.C.), University of the Witwatersrand, Johannesburg, South Africa.
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Abstract |
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Background A decreased cardiac compliance is a major feature of the cardiomyopathy of diabetes mellitus. Either an increase in the resistance afterload to the LV or an increase in collagen cross-linking induced by the formation of advanced glycosylation end products (AGEs) of collagen may be responsible for the stiff myocardium. To evaluate these hypotheses, we examined the effect of captopril, an afterload-reducing agent, and aminoguanidine, a nucleophilic hydrazine that prevents the accumulation of collagen AGEs, on left ventricular end-diastolic (LVED) compliance after 4 months of streptozotocin (0.26 mmol/kg)–induced diabetes mellitus in rats.
Methods and Results Diabetes mellitus produced a decrease in LV chamber compliance as a result of an increased myocardial stiffness (slope of the linearized LVED stress–LVED strain relation [unitless]: diabetes mellitus, 47±4; control, 27±3; P<.001) and an increase in blood pressure as a result of an elevated vascular resistance. LV end-systolic elastance was unaltered by diabetes mellitus. The stiff myocardium was not associated with changes in the myocardial collagen volume fraction or total hydroxyproline concentration but was associated with an increased myocardial collagen fluorescence (fluorescence units/µg hydroxyproline) (diabetes mellitus, 11±1.1; control, 6.6±0.7; P<.01). Captopril therapy (0.22 mmol ·kg-1·d-1), despite producing a decrease in blood pressure through alterations in vascular resistance, failed to decrease myocardial stiffness in rats with diabetes mellitus. Alternatively, administration of aminoguanidine (7.35 mmol·kg-1·d-1) prevented both the enhanced myocardial collagen fluorescence (7.1±1.2) and the increased slope of the linearized LVED stress–LVED strain relation (29±2) but did not change markers of blood glucose control.
Conclusions These results demonstrate that diabetes mellitus can produce a stiff myocardium before the development of myocardial fibrosis. The stiff myocardium in the early stages of the development of the cardiomyopathy of diabetes mellitus is not a consequence of an increase in ventricular resistance afterload and in these circumstances is associated with the formation of collagen AGEs.
Key Words: aminoguanidine • diabetes mellitus • myocardium • diastole • cardiomyopathy
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Introduction |
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A decreased cardiac diastolic performance occurs in patients with diabetes mellitus before systolic cardiac disease occurs.1 The altered cardiac diastolic performance is thought to result from a reduced cardiac compliance as documented in animal models of diabetes mellitus.2 3 4 5 The mechanism(s) responsible for the stiff myocardium have not been elucidated.
A number of possibilities exist that may explain the stiff myocardium in diabetes mellitus. These include an enhanced myocardial collagen content,2 3 6 7 which in turn may result from either an increased resistance afterload to the LV8 or reduced collagen degradation.6 Nonenzymatic glycosylation of collagen results in an AGE that exhibits the characteristics of increased cross-linking.9 Reduced myocardial collagen acid solubility associated with increased myocardial hydroxyproline content, as demonstrated in the myocardium of a mouse model of diabetes mellitus,6 suggests that increased myocardial collagen cross-linking leads to reduced collagen degradation. The accumulation of collagen AGEs may therefore be responsible for the decrease in cardiac compliance produced by diabetes mellitus.
We decided to investigate the possibility that alterations in either resistance afterload or the accumulation of myocardial collagen AGEs may contribute to the decreased myocardial diastolic performance produced by diabetes mellitus. To examine the possible role of an increase in resistance afterload to the LV, we investigated the effect of the afterload-reducing agent captopril on myocardial stiffness in STZ-induced diabetes mellitus in rats. To test the hypothesis that an accumulation of myocardial collagen AGEs may contribute to the augmented myocardial stiffness, we examined the effect of aminoguanidine on cardiac compliance in the same model of diabetes mellitus in rats. Aminoguanidine, a nucleophilic hydrazine, decreases the formation of aortic collagen cross-linkages produced by the formation of AGEs.9
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Methods |
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The experimental procedures were approved by the Animal Ethics Committee of the University of the Witwatersrand (certificate numbers 93/18/4, 93/26/4, and 93/105/4). One hundred eight male Sprague-Dawley rats (OLAC, UK) weighing 100 to 150 g were assigned to eight different groups. Diabetes mellitus was induced in four of these groups with an intravenous injection (tail vein) of the pancreatic ß-cell toxin STZ (0.26 mmol/kg). This produces a nonketotic model of diabetes mellitus.10 The remaining four groups of rats received an intravenous injection of the saline vehicle; these groups acted as controls. Blood glucose measurements (Ames glucometer) on tail vein blood samples were performed before and 3 days after the STZ or saline injection to confirm the presence or absence of diabetes mellitus. These measurements were made at the same time of day to standardize for blood glucose fluctuations with feeding. Rats that failed to develop blood glucose concentrations >12 mmol/L were excluded from the study. Four rats were excluded for this reason (one rat in each of the STZ-injected groups).
Experimental Groups
From 2 weeks after the STZ or vehicle injection, aminoguanidine
(Sigma Chemical Co) at a dose known to inhibit the development of the
neuropathy produced by experimental diabetes mellitus
without altering polyol pathway metabolites (7.35
mmol·kg-1·d-1)11
was administered in the drinking water to one group of STZ-injected
rats (n=14) (group 1) and one group of control rats (n=11) (group 2).
In addition, a separate group of STZ-injected rats (n=17) (group 3) and
control rats (n=11) (group 4) received captopril (SQ 14,225, Squibb) in
the drinking water (0.22
mmol·kg-1·d-1).
The remaining STZ-injected (n=27) and control (n=24) rats were left
untreated for the experimental period. Seventeen rats in the
STZ-injected and 12 in the control group were used as hyperglycemic
(group 5) and euglycemic (group 6) controls, respectively,
for groups 1 through 4 above. In all of groups 1 through 6, cardiac
performance was determined from measurements of short-axis
external diameter of the LV. The remaining 10 untreated STZ-injected
rats (group 7) and 12 untreated euglycemic control rats
(group 8) were used to investigate cardiac performance as
determined from measurements of long-axis segmental length of the
LV.
Blood Chemistry
After completion of the cardiac function measurements, made 16
weeks after STZ or vehicle injection, blood samples were collected for
the determination of plasma glucose (via glucose oxidase, Glucoxact,
Clinical Sciences Diagnostics) and fructosamine
concentrations and the percentage of HbA1. Fructosamine
concentration was determined by a reduction method with nitroblue
tetrazolium (Boehringer Mannheim GmbH/Diagnostica).
HbA1 was determined electrophoretically at a wavelength of
415 nm with a REP Glyco kit (Helena Laboratories).
Noninvasive Blood Pressures
SBP was measured immediately before the commencement of either
captopril or aminoguanidine therapy in groups 1 through 4; from 2 weeks
after STZ or vehicle injection in groups 5 through 8; and thereafter at
regular intervals for 14 weeks in all groups as previously
described.12 SBP could not be measured in rats before
diabetes mellitus was induced because a tail pulse is undetectable in
small rats by this technique. STZ induces consistent diabetes
mellitus only in prepubertal rats. SBP measurements were performed in
all rats in groups 1 through 4. However, samples of rats from groups 5
through 8 were selected at random for SBP measurements (group 5, 11
rats; group 6, 10; group 7, 5; and group 8, 9). SBP was therefore
measured in a total of 16 untreated hyperglycemic and 19 control
rats.
Cardiac Function
The surgery, instrumentation, experimental techniques, and
calculations used in our present experiment have recently been
described and validated.13 14 At the end of the 4-month
period, LV performance was successfully measured in all of the
rats except for 1 rat in group 1 and 2 rats in group 3. In these rats,
measurements of short-axis diameter were inconsistent,
and the data collected were therefore excluded from the
analysis.
Cardiac dynamic responses were measured during IVC occlusion to obtain
a range of LVED short-axis diameters in groups 1 through 6 and a
range of LVES and LVED anterolateral wall long-axis segmental
length measurements in groups 7 and 8. Before IVC occlusion, a baseline
LVEDP of 
10 mm Hg was obtained in all rats by injection of a
modified Dextran-70 solution13 14 through the
arterial catheter. No significant alterations in heart rate
occurred during IVC occlusion. Data were recorded a minimum of
three times on each rat to ensure reproducibility. Typical examples of
the short-axis diameter or long-axis segmental length
measurements obtained during IVC occlusion have been published
previously.13 14
LVED chamber compliance was assessed from the slope of the relationship between LVEDP and LVED strain. LVED strain was calculated from short-axis external diameter and long-axis segmental length measurements at the end of diastole as previously described.13 14 LVED regional myocardial compliance was determined from the slope of the relationship between LVED stress and LVED strain as calculated from short-axis diameter measurements.13 14 A thick-walled spherical model of LV geometry was assumed in the calculation of wall stress.15 16 17 Cardiac contractility was determined in groups 7 and 8 from the slope of the relation of LVESP versus LVES strain (an index of LVES elastance, LV Ees)13 14 16 calculated from long-axis segmental length measurements. LV Ees could not be calculated from short-axis diameter measures, since we could not be sure that the piezoelectric transducers followed the heart during cardiac contraction.13 14
LV "Resistance Afterload"
To determine the effect of diabetes mellitus and captopril
therapy on ventricular resistance afterload, we measured
cardiac output and MAP at increasing LVEDP preloads in the
captopril-treated and untreated rats with diabetes mellitus and the
control rats at the same time as cardiac performance was
determined. We used the calculated TPR value obtained from these
measures as an index of the resistance afterload to the LV.
Ascending aortic blood flow, measured with a Narcomatic electromagnetic flowmeter (Narco Bio-Systems Inc), was used as a measure of cardiac output, since coronary blood flow represents only 3% to 4% of cardiac output in a rat.18 These measurements were made in open-chest, ventilated rats over a range of LVEDP values. Recordings were obtained at incremental LVEDP values with dextran infusion, only after phasic electromagnetic flow traces had stabilized. These recordings were therefore obtained before the measurements obtained during IVC occlusion. The experiment was successfully performed in a sample of rats from each group (control, 12 rats; diabetes mellitus, 17; control/captopril-treated, 11; and diabetes mellitus/captopril-treated, 16). The success of an experiment in each rat was determined by the quality of the phasic electromagnetic flow traces obtained.
Myocardial Collagen Content and Collagen
Fluorescence
At the end of the cardiac performance experiments, the
right ventricle was dissected free from the LV. A portion of the base
of the LV was oven-dried for 48 hours to determine dry heart
weight. A portion of the LV of a random sample of rats from groups 5
(n=8) and 6 (n=6) was formalin-fixed. The collagen-specific
stain Sirius red F3BA19 was used on 5-µm-thick,
paraffin-embedded coronal sections of the LV obtained from the
equator. From each cross section of the LV, 15 fields were randomly
selected and analyzed for collagen volume fraction, and 8
fields for perivascular collagen volume fraction by videodensitometry.
This analysis was performed on a computer-based flexible
image-processing system (FIPS; CSIR, SA). By gray-level
thresholding, only the stained areas (collagen) were selected, and the
area collagen volume fraction of stained versus nonstained tissue was
determined.
Samples of ventricular tissue from groups 1 (n=12), 2 (n=11), 5 (n=17), and 6 (n=12) were weighed and stored at -70°C for hydroxyproline and collagen fluorescence measurements. To determine myocardial hydroxyproline content, tissue was hydrolyzed with 6N HCl at 107°C for 16 hours in evacuated sealed tubes. HCl was evaporated off from hydrolyzed tissue under nitrogen, and the remaining tissue was redissolved in a buffer (pH 6.0) that has been described previously.20 After centrifugation to remove insoluble material, myocardial 4-trans-hydroxyproline concentration was determined by the method of Stegemann and Stalder.20
Myocardial collagen AGE formation was determined by collagen fluorescence by a similar method as previously described.21 Stored LV (10 mg) was minced and added to 1 mL of a 0.1 mol/L CaCl2/0.02 mol/L Tris-HCl buffer containing 0.05% toluene (buffer A) and 1 mg/mL of type IV collagenase (Sigma). The solution was sonicated to achieve a fine suspension and then incubated at 37°C for 24 hours before centrifugation at 15 000 rpm for 30 minutes. A 100-µL aliquot of the supernatant was used to determine the hydroxyproline concentration by the method of Stegemann and Stalder.20 A volume of buffer A was then added to the remaining supernatant of each sample to achieve a final hydroxyproline concentration of 5 µg/mL. The resultant supernatant was used to measure collagen-related fluorescence with excitation/emission at 370/440 nm with a Perkin-Elmer fluorimeter, model LS-3B.21 The measurements were made against a blank containing collagenase in buffer A. The degree of advanced glycosylation of myocardial collagen was determined by expression of collagen-specific fluorescence per microgram hydroxyproline.
Data Analysis
Regression analysis was used to determine the lines of
best fit for the cardiac function relations. A value of
r>.95 was considered to be a good fit. The relations of
LVEDP versus LVED strain and of LVED stress versus LVED strain were
found to best fit an exponential function: LVEDP (or LVED
stress)=b·e(-m·LVED
strain), which was linearized: ln LVEDP (or LVED
stress)=ln b-m(LVED strain) for statistical analysis. The
slope of the linearized LVED stress–strain and LVEDP-strain
relations are considered as the myocardial and the chamber stiffnesses,
respectively.15 19 The exponential function
y=axb was found to best fit the LVESP
versus LVES strain relation, which was linearized (log
y=mx+log b) for statistical analysis.
Multiple comparisons of the slopes of these functions were made between
the groups by one-way ANOVA followed by the Tukey test. The same
statistical procedure was used to establish differences between the
groups for body weight, heart weight, baseline MAP,
HbA1, plasma concentrations of circulating glucose
and fructosamine, myocardial hydroxyproline, and myocardial collagen
fluorescence. The unpaired Student's t test was
used to compare the collagen volume fraction of the untreated group of
rats with diabetes mellitus with that of the control group. To compare
the SBPs measured noninvasively as well as the TPR and cardiac output
values at increasing LVEDP preloads between groups, a repeated measures
ANOVA, followed by the Tukey multiple comparisons test, was used. To
compare SBPs after versus before therapeutic intervention, a repeated
measures ANOVA followed by Dunnett's test was used. All values in the
text are presented as mean±SEM. Values of n in the figures and
tables are sample numbers.
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Results |
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Table 1
summarizes the values obtained for blood
and plasma chemistry markers of glucose control in the different groups
of rats. STZ injection produced marked diabetes mellitus, with an
increase in blood glucose and fructosamine concentrations and an
increased percentage HbA1. Neither captopril nor
aminoguanidine treatment altered the plasma glucose concentrations or
the markers of chronic diabetes control.
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Fig 1 illustrates the effect of captopril and
aminoguanidine treatment on SBPs in rats with diabetes mellitus, as
measured by an indirect technique in awake rats. Diabetes mellitus
produced an increased SBP compared with the untreated control group of
rats. Administration of either captopril or aminoguanidine resulted in
a decrease in the SBP to values similar to control rat blood pressures
throughout the experimental period. Neither captopril nor
aminoguanidine therapy influenced SBP in control rats (not
illustrated).
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Table 2 summarizes the body and heart weights of the
treated hyperglycemic and euglycemic controls as well as
untreated rats. STZ-induced diabetes mellitus produced decreased body
and heart weights. The ratio of heart weight to body weight in the
hyperglycemic rats was increased compared with the
euglycemic controls. Aminoguanidine therapy failed to
influence body weight or heart weight. Alternatively, captopril therapy
produced a decrease in the heart weight and normalized the ratio of
heart weight to body weight in rats with diabetes mellitus but did not
influence the heart weight of euglycemic controls. No
differences in the ratio of the dry to wet LV weight were noted between
the control rats (0.205±0.010) and the untreated rats with diabetes
mellitus (0.213±0.005). In addition, neither captopril nor
aminoguanidine administration altered the ratio of dry to wet LV
weight.
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Fig 2 illustrates the effect of 4 months of STZ-induced
diabetes mellitus on the LVEDP versus LVED strain relation as
determined from short-axis diameter measurements (Fig 2A), the LVED
stress versus LVED strain relation as determined from short-axis
diameter measurements (2B), the LVEDP versus LVED strain relation as
determined from long-axis regional length measurements (2C), and
the LVESP versus LVES strain relation as determined from long-axis
regional length measurements (2D). Table 3 summarizes
the slopes and the intercepts of the linearized relations of the curves
depicted in Fig 2. The slopes of the linearized relations of the line
graphs depicted in Fig 2A and 2C represent LV chamber
compliance; 2B represents LV regional myocardial compliance
(the myocardial elastic stiffness); and 2D represents LV Ees.
Diabetes mellitus resulted in an increased LVED chamber stiffness as
determined from short-axis (2A) and long-axis (2C) diameter and
length measurements, respectively. The decreased chamber compliance is
explained by a reduced regional myocardial compliance (2B). Despite
these changes in diastolic performance, diabetes
mellitus failed to influence LV Ees (2D).
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Fig 3 illustrates the effect of 4 months of either
captopril or aminoguanidine therapy on the relation of LVED stress
versus LVED strain (regional myocardial compliance) in rats with
STZ-induced diabetes mellitus. Fig 4 summarizes the
effect of captopril or aminoguanidine therapy on the slope of the
linearized LVED stress–LVED strain relation in control or STZ-injected
rats. Regional myocardial compliance was unchanged with captopril
therapy. However, aminoguanidine therapy prevented the increase in LVED
stiffness in rats with diabetes mellitus. A similar effect of
aminoguanidine was also noted when we compared the slope of the
linearized LVEDP–LVED strain relation (not illustrated).
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Fig 5 illustrates the effect of captopril therapy on MAP
as measured from carotid artery pressures in closed-chest rats and
TPR as measured at increasing preload values in anesthetized,
open-chest, ventilated rats. Four months of STZ-induced diabetes
mellitus resulted in an increased MAP and TPR (as measured over a range
of LVEDP values), which were normalized with captopril therapy. Cardiac
output (per 100 g body weight) was not different at increasing LVEDP
values between the four groups (not illustrated).
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Four months of STZ-induced diabetes mellitus failed to influence the proportion of myocardium occupied by fibrillar collagen (collagen volume fraction) (diabetes mellitus, 4.95±0.29%; control, 5.04±0.34%) or the proportion of the perivascular area occupied by fibrillar collagen (perivascular collagen volume fraction) (diabetes mellitus, 12.3±1.6%; control, 12.6±1.1%).
Fig 6 summarizes the effect of 4 months of STZ-induced
diabetes mellitus and aminoguanidine treatment on the myocardial
hydroxyproline concentration and on myocardial collagen
fluorescence. In support of the results obtained with a
histological technique for assessing myocardial
collagen, STZ-induced diabetes mellitus failed to alter myocardial
hydroxyproline concentration (Fig 6, top). However, STZ-induced
diabetes mellitus resulted in an increase in myocardial collagen
fluorescence, and aminoguanidine treatment prevented this
effect (Fig 6, bottom).
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Discussion |
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Our results demonstrate that the increased myocardial stiffness that occurs as a consequence of STZ-induced diabetes mellitus is associated with increased vascular resistance and blood pressure as well as an accumulation of the AGEs of myocardial collagen. However, the increased myocardial stiffness was not associated with an increase in myocardial collagen content. Captopril administration failed to prevent the increased myocardial diastolic stiffness despite decreasing blood pressure and vascular resistance to control values. Alternatively, aminoguanidine treatment prevented the increase in myocardial stiffness without influencing blood glucose control. Moreover, the effect of aminoguanidine on myocardial stiffness was associated with a decrease in myocardial AGEs.
Alterations in the shape of the LV (spherical versus elliptical) in rats with diabetes mellitus may limit the interpretation of LVED compliance values as measured in only one axis of the heart. However, an increase in ventricular stiffness was demonstrated by use of either measures of short-axis diameter or long-axis segmental length in untreated rats with diabetes mellitus compared with the control groups. Since aminoguanidine failed to influence LV geometry or heart weight, it is unlikely to have altered ratios of LV long axis to short axis. Therefore, the beneficial effect of aminoguanidine on myocardial compliance cannot be ascribed to limitations of measurement techniques.
The increased chamber stiffness noted in the untreated rats with diabetes mellitus is consistent with previous findings in dog,3 4 monkey,2 and rat5 models of prolonged diabetes mellitus. However, these authors failed to examine diastolic stress-strain relations and therefore could not comment on the contribution of alterations in ventricular geometry versus myocardial stiffness to this shifted relationship. Concentric LV geometry is an independent determinant of chamber stiffness.22 Our results demonstrate that the altered chamber stiffness is due to an augmented regional myocardial stiffness. The increased myocardial stiffness may explain the altered indexes of diastolic function, as measured by echocardiography, in normotensive diabetic patients with a normal cardiac systolic function.1
It may be argued that STZ injection produces direct toxic effects on the myocardium, which are abolished by aminoguanidine therapy. This, however, is an improbable explanation, because myocardial compliance measured within the first 26 days after STZ administration is unaltered.23
A number of extramyocardial factors influence LVED pressure-volume relations, including pleural and pericardial forces as well as ventricular interaction.22 However, in our study we measured cardiac diastolic function in open-chest, ventilated rats after a parietal pericardectomy. Since pleural forces play a role only in the closed-chest situation22 and a pericardectomy will abolish pericardial forces as well as the effects of ventricular interaction, these factors are unlikely to have influenced our results.
Alterations in coronary perfusion pressure, possibly by influencing the degree of engorgement of coronary capillaries, also contribute to myocardial compliance.22 Diabetes mellitus resulted in an increased blood pressure (and hence a possible increase in coronary perfusion pressure), which was normalized by aminoguanidine therapy. Hence, an influence of aminoguanidine therapy on coronary vascular engorgement needs to be considered as a possible mechanism for the effect on myocardial compliance. However, captopril, which produced an equipotent blood pressure–lowering effect compared with aminoguanidine in rats with diabetes mellitus, failed to improve myocardial compliance. Hence, a possible effect of aminoguanidine on coronary vascular engorgement is unlikely to contribute to the prevention of the stiff myocardium in diabetes mellitus.
Diabetes mellitus resulted in an increase in blood pressure as a consequence of an elevated TPR. An enhanced vascular resistance leads to increased ventricular wall tension by augmenting LV systolic pressures. An elevated wall tension as a result of changes in resistance afterload is associated with the development of ventricular fibrosis19 and a decreased rate of ventricular relaxation, both of which may contribute to an increased diastolic stiffness.22 Aminoguanidine therapy decreased blood pressure, which may be explained by an effect on vascular collagen cross-linkages.9 A reduced blood pressure is likely to have resulted in a decreased ventricular wall tension, which in turn could explain the beneficial effect of aminoguanidine on myocardial compliance. However, the myocardial collagen content was unaltered in rats with chronic diabetes mellitus. In addition, captopril therapy, which produced an equipotent decrease in blood pressure (through alterations in TPR) compared with aminoguanidine, failed to influence regional myocardial compliance. The effect of aminoguanidine on LV stiffness is therefore not attributed to a decreased ventricular afterload and a subsequent influence on myocardial collagen content.
Our findings of a normal myocardial collagen content in diabetes mellitus are contrary to the findings of others using alloxan-induced,2 3 genetic,6 and STZ-induced7 models of diabetes mellitus. Myocardial fibrosis, however, is not a consistent finding in STZ-induced diabetes mellitus.24 25 A possible reason for the discrepancy between our findings and those of Kita et al7 is that different techniques were used to determine the degree of myocardial fibrosis. A likely reason for the discrepancy between our findings and those of Haider et al,2 Regan et al,3 and Khaidar et al,6 regarding the presence of myocardial fibrosis, is that we examined myocardial collagen content after a shorter duration of diabetes mellitus.
The genetic model of diabetes mellitus, namely the db/db mouse6 and the alloxan-induced model of diabetes mellitus in the monkey,2 led to a decrease in myocardial collagen acid solubility. This indicates that diabetes mellitus is associated with an increase in myocardial collagen cross-linkages. An increase in collagen cross-linkages may explain the stiff myocardium. Aminoguanidine decreases the formation of glucose-mediated AGEs of collagen and hence collagen cross-linkages.9 A similar effect of aminoguanidine on myocardial collagen in diabetes mellitus may explain the improved LVED compliance in our study. Indeed, we were able to demonstrate that the reduced myocardial compliance produced by aminoguanidine was associated with a concomitant reduction in the formation of myocardial collagen AGEs.
AGEs of proteins change cell function by interacting with specific membrane receptors as well as DNA.26 Alterations in myocardial cell membranes or DNA expression could lead to changes in those membrane channels, pumps, or enzymes responsible for controlling the myocyte intracellular Ca2+ concentration. An increased intracellular Ca2+ concentration during diastole will modulate the extent of ventricular relaxation and hence decrease myocardial compliance.22 Indeed, alterations in those processes responsible for controlling myocyte intracellular Ca2+ concentration have been documented in diabetes mellitus.27 Therefore, an effect of aminoguanidine on AGE formation may decrease intracellular Ca2+ concentration and hence myocardial stiffness. However, myocardial cell membrane AGEs are not altered by diabetes mellitus in rats.28 Hence, it is unlikely that aminoguanidine prevents the increased myocardial stiffness produced by diabetes mellitus by protecting cell membrane Ca2+ channels, pumps, or enzymes from the consequences of irreversible glycosylation.
In conclusion, we have demonstrated that aminoguanidine but not captopril treatment prevents the decreased myocardial compliance produced by diabetes mellitus in rats before the development of ventricular fibrosis. The lack of effect of captopril on myocardial diastolic function (despite preventing the increase in resistance afterload to the LV) and the absence of myocardial fibrosis suggest that alterations in myocardial stiffness produced by diabetes mellitus can occur independently of an increase in myocardial collagen or through mechanisms that are not determined by the effects of LV afterload on the myocardium. Taken together, our evidence demonstrating a parallel influence of aminoguanidine on diastolic myocardial stiffness and myocardial collagen AGEs and the evidence demonstrating a lack of effect of diabetes mellitus on cardiac myocyte AGEs28 supports the hypothesis that the mechanism of the decreased cardiac diastolic performance in diabetes mellitus is the result of alterations in myocardial collagen AGEs.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This research was supported by the University of the Witwatersrand Medical Faculty Research Endowment Fund, the H.E. Griffin Charitable Trust, the I.E. Hodges Cardiovascular Research Trust, and the Medical Research Council of South Africa. We also acknowledge the kind donation of captopril by Squibb Pharmaceuticals; the donation of aminoguanidine from Prof John Kalk of the Department of Medicine, University of the Witwatersrand; and the technical assistance of Matthew Laundy.
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Footnotes |
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Reprint requests to Dr Gavin Norton, MBBCh, PhD, or Angela Woodiwiss, MS, PhD, Department of Physiology, University of the Witwatersrand Medical School, 7 York Rd, Parktown, 2193, Johannesburg, South Africa.
Received September 18, 1995; revision received November 13, 1995; accepted November 21, 1995.
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