© 1999 American Heart Association, Inc.
Atherosclerosis and Lipoproteins |
Impaired Endothelium-Dependent Vascular Responses of Retinal and Intrarenal Arteries in Patients With Type 2 Diabetes
From the Second Department of Internal Medicine, Osaka City University Medical School, Osaka, Japan.
Correspondence to Takahiko Kawagishi, MD, Second Department of Internal Medicine, Osaka City University Medical School, 1-4-3, Asahi-machi, Abeno-ku, Osaka, 545-8585, Japan. E-mail takahiko{at}med.osaka-cu.ac.jp
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Abstract |
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Abstract—Endothelial dysfunction has been implicated in the pathogenesis of diabetic microangiopathies such as retinopathy and nephropathy as well as macrovascular diseases. The aim of the current study was to determine whether endothelial function in the retinal and renal arteries is impaired in type 2 diabetes mellitus. We examined the effects of an intravenous infusion of L-arginine and a sublingual administration of nitroglycerin on the brachial, retinal, and interlobar arterial hemodynamics in 20 type 2 diabetic patients (10 with normoalbuminuria and 10 with microalbuminuria) and 10 aged-matched control subjects. Despite no difference in the nitroglycerin-induced vascular response of the brachial or retinal artery among the 3 groups, the L-arginine–induced vascular response of each artery was significantly lower in both the normoalbuminuric and microalbuminuric patients than in the control subjects and the microalbuminuric patients showed the lowest value among the 3 groups (P<0.01, each artery, respectively). The L-arginine–induced vascular response of each artery was significantly correlated with HbA1c levels (brachial artery, r=0.617, P=0.0003; retinal artery, r=0.599, P=0.0005; interlobar artery, r=0.636, P=0.0002). In addition, stepwise multiple regression analysis of all subjects showed that HbA1c level was an independent determinant for the L-arginine–induced vascular response of each artery. The results showed that the endothelium-dependent vascular responses of the retinal and intrarenal arteries as well as the brachial artery were impaired in diabetic patients before the clinical manifestation of diabetic nephropathy, and suggest that endothelial dysfunction in these arteries is associated with hyperglycemia in these patients.
Key Words: L-arginine • diabetes mellitus • endothelium-dependent vasodilation • kidney • retina
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Introduction |
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Recently, the role of the L-arginine/nitric oxide (NO) pathway in the regulation of vascular smooth muscle tone has attracted increasing interest. NO is synthesized from the physiological precursor L-arginine by the stereospecific enzyme NO synthase.1 NO induces relaxation of the smooth muscle by activating soluble guanylate cyclase to increase cyclic 3', 5'-guanosine monophosphate (cGMP) levels. There is evidence from in vitro and in vivo studies that NO-mediated vasodilation may be impaired in type 1 and type 2 diabetes.2 3 4
Endothelium-derived NO has also been shown to regulate renal and ocular blood flow.5 6 7 In isolated ophthalmic arteries, NO is an important modulator of vascular tone,8 and systemic NO-synthase inhibition decreases choroidal blood flow in animals9 and humans.10 By using L-arginine analogues as probes for the renal NO pathway, several studies have demonstrated that NO acts as a potent vasodilator in the kidney.11 12 In addition, endothelial dysfunction, as estimated by the plasma von Willebrand factor concentration, precedes and may predict the development of microalbuminuria in type 1 diabetic patients.13 Although endothelial dysfunction is assumed to contribute to altered ophthalmic and intrarenal circulation and the development of diabetic retinopathy and nephropathy,7 14 there has been little research regarding endothelial function in both the retinal and renal arteries of type 2 diabetic patients.
On the basis of these observations, we hypothesized that either local NO synthesis/release or local sensitivity to exogenous NO might be impaired in patients with diabetes mellitus and that this may contribute to the development of diabetic retinopathy and nephropathy. Vascular responses after the intravenous administration of L-arginine were recently demonstrated to be a likely consequence of an increase in the endothelial production of NO.15 Accordingly, we compared the effects of L-arginine and NO donation with nitroglycerin on systemic, retinal, and renal hemodynamics in patients with and those without microalbuminuria.
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Methods |
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Subjects
We evaluated 20 Japanese type 2 diabetic patients. Ten patients were normoalbuminuric and 10 patients were microalbuminuric. The diagnosis of diabetes was based on a history of diabetes or criteria according to the Report of the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus.16 All patients were admitted to Osaka City University Hospital for an educational course on diabetes. As a control group, 10 age- and sex-matched healthy subjects were examined. No subjects were taking medication known to affect the retinal, renal, or systemic circulation. Control subjects were receiving no regular medication and had no evidence of macrovascular, renal, or retinal vascular disease as well as diabetes or hypertension. Patients with hypertension or a history of ischemic heart disease, those with advanced diabetic complications (macroalbuminuria or proliferative retinopathy), and those treated with insulin were excluded from the study. Subjects with nondiabetic or obstructive kidney diseases and smokers were also excluded from this study. Eleven of the diabetic patients had mild nonproliferative diabetic retinopathy. Each subject gave informed consent to participate in the study. The study methodology was approved by the Ethics Committee on Clinical Investigation of the Osaka City University Medical School.
Slit-lamp biomicroscopy showed no retinal vascular disease except diabetic retinopathy in each subject. Ten patients were treated with diet alone (25 to 30 kcal per ideal body weight) and 10 patients with sulfonylureas. The patients were seen at least at 14-day intervals during the study, and the dosage of sulfonylureas was not changed. Sulfonylureas were discontinued 24 hours before the study. After an overnight fast, the effects of L-arginine and nitroglycerin administrations on the hemodynamics were studied, and blood sampling was performed before and after the infusion of L-arginine in each subject.
L-Arginine Infusion
Intravenous lines were inserted into a large
antecubital vein of the left arm for L-arginine infusion
and into a dorsal vein of the right arm for blood sampling. Patency was
preserved by a slow saline infusion (0.9% NaCl). The subjects were
then instrumented for automatic measurements of blood pressure and
heart rate. The study was performed after the subjects had rested for
at least 30 minutes and after 3 consecutive measurements of blood
pressure and heart rate. L-Arginine (10%
L-arginine monochloride solution; Morishita
Pharmaceuticals Co) was infused at a constant rate of 10 mL/min (1
g/min) over 30 minutes with an infusion pump. Blood pressure and heart
rate were recorded at 5-minute intervals. Mean blood pressure was
calculated as the diastolic pressure plus one third of the
pulse pressure. Ultrasonographic parameters and blood
samples for the measurements of plasma glucose and insulin were
obtained at 0, 15, 30, and 50 minutes after the start of the
L-arginine infusion. Baseline serum concentrations of total
cholesterol, HDL cholesterol,
triglycerides, creatinine, and HbA1c were
obtained at 0 minutes. Control studies in which 300 mL of saline
replaced the L-arginine infusion in 5 healthy subjects were
performed; the testing procedure was otherwise identical to that in the
L-arginine infusion.
Vascular Responses of Brachial Artery to L-Arginine
and Nitroglycerin
L-Arginine–induced
(endothelium-dependent) and
nitroglycerin-induced
(endothelium-independent) vasodilations of the right
brachial artery were measured by an examiner on 2 different days within
7-day intervals. The measurements were performed according to the
method described by Celermajer et al17 in a
temperature-controlled (22°C) room. The arterial diameter
was measured with the use of an ultrasonic phase-locked echo-tracking
system, which was equipped with a high-resolution, real-time, 7.5-MHz
linear scanner in the B-mode (SSD 610, Aloka). The first ultrasound
examination was performed with subjects in the supine position after
they had rested for at least 15 minutes. A longitudinal section of the
right brachial artery 2 to 12 cm above the elbow was scanned. The ECG
was monitored continuously. Vessel diameter was measured by the same
observer, who was unaware of the clinical details or the stage of the
experiment. The arterial diameter was measured from the
anterior to the posterior interface between the media and adventitia
("m" line)18 at a fixed distance from an anatomic
marker.17 The mean diameter was calculated from 4 cardiac
cycles synchronized with the R-wave peaks on the ECG. All measurements
were made at end-diastole. The diameter changes at 15, 30,
and 50 minutes caused by the L-arginine infusion were
expressed as the percent change relative to that at the initial resting
scan. Also, intima-media thickness of the right brachial
arterial posterior wall was measured 2 cm proximal to the
elbow joint as previously reported.18 On a subsequent day,
after the resting scan and administration of a sublingual spray of
nitroglycerin (300 µg/spray), the diameter changes at
3 and 5 minutes caused by nitroglycerin administration
were measured in the same way and are presented as the percent
change relative to that at the resting scan.
Vascular Responses of Retinal and Intrarenal Arteries to
L-Arginine and Nitroglycerin
Ultrasound examinations of the central retinal and interlobar
arteries were performed simultaneously during the study of
the brachial artery by 2 examiners (one examined the retinal artery and
the other examined the interlobar artery). Images were obtained with a
duplex Doppler apparatus (Aloka SSD 2000, Aloka Co Ltd)
with a 5-MHz convex array probe.
Duplex Doppler Sonography of the Central Retinal
Artery
In the control subjects and the diabetic patients without
retinopathy, the right eye was studied. In the patients
with retinopathy, the eye with more advanced
retinopathy was studied. The color Doppler mode,
which shows arterial and venous flows in different colors,
was used to identify the appropriate position of the central retinal
artery for the pulsed Doppler recordings. The pulsed
Doppler mode was used to obtain quantitative measurements of
velocity. The sample volume was adjusted to a pulse length of 1.0
mm and was positioned so that its center was 
3.0 mm behind the
disk surface. The sample volume was estimated by use of the
angle-correction menu of the apparatus and by placing a
cursor along the course of the central retinal artery as previously
reported.19
Duplex Doppler Sonography of the Intrarenal Artery
(Interlobar Artery)
The ultrasound probe was positioned gently on the right flank in
an oblique projection, and the right kidney was visualized as a
longitudinal image. Sample volumes were obtained to position the cursor
of the pulsed Doppler mode at the mid-portion of the interlobar
arteries, which flow along the renal pyramid. The pulsed Doppler
mode was used to obtain quantitative measurements of velocity by
placement of a cursor along the course of the interlobar arteries. The
sample volume was adjusted to a pulse length of 1.0 mm and was
estimated by use of the angle correction menu of the
apparatus. The pulsed Doppler recordings for
the waveform analysis were obtained in the same position as the
interlobar artery during the ultrasonographic examination in each
subject, as previously reported.20
Flow Wave Analysis of the Pulsed Doppler Image
The peak systolic flow velocity (PSV), the
end-diastolic flow velocity (EDV), and the time-averaged
flow velocity (TAV) were automatically calculated by the ultrasound
apparatus. Flow velocities were determined from signals
that were stable for at least 5 seconds. Measurements represent
the average of 3 complete waveforms of the arteries. The resistance
index (RI) was determined as19 20 RI=(PSV-EDV)/PSV.
All measurements were performed by the same examiners, who were unaware of subject characteristics. The ultrasound studies of both the retinal and interlobar arteries were performed simultaneously when the brachial artery was assessed.
For both the retinal and interlobar arteries, the changes in the RI caused by the L-arginine and nitroglycerin administrations were expressed as the percent change relative to that at the initial resting scan. The responses of each artery to the L-arginine and nitroglycerin administrations were expressed as the area under the cumulative response curve (AUC) during each study period.
Reproducibility of the Ultrasound Study
Eight diabetic patients and 8 control subjects were examined on
2 different occasions separated by 7 days to estimate the intraobserver
variability of the values of nitroglycerin-induced
vasodilation of the brachial artery diameter by the same examiner, who
was unaware of the values from the first examination. The coefficient
of variance for the values was 3.8%. The RI values in the central
retinal artery and the interlobar artery are highly reproducible, as in
our previous reports.19 20 The coefficient of variation
for the RI values was 3.6% for the retinal artery and 3.3% for the
interlobar artery.
Biochemical Analysis
In each diabetic patient, the level of 24-hour urinary
albumin excretion (UAE) was the mean value from 3 consecutive
days. Normoalbuminuria was defined as UAE <20 µg/min and
microalbuminuria was defined as UAE 
20 µg/min and <200
µg/min. The plasma glucose and HbA1c levels were measured as
previously described.19 The plasma insulin levels were
measured with the use of a double-antibody radioimmunometric assay
(Insulin RIABEAD II, Dinabot Co, Ltd). Serum total
cholesterol, triglycerides, HDL
cholesterol, and creatinine levels were
measured with the autoanalyzer. Urinary albumin was
measured by use of immunoturbidimetry (TIA MicroAlb Kit, Nittobo). As
an insulin sensitivity index, we used the homeostasis model assessment
insulin resistance (HOMA-IR)21 : HOMA-IR=Fasting plasma
insulin (µU/mL) · Fasting plasma glucose (mmol/L)/22.5
Statistical Analysis
The AUC for the vascular response of each artery was calculated
by use of the trapezoidal rule. For L-arginine study, the
AUC was calculated as the incremental or decremental values from the
baseline obtained during the 50 minutes (from time 0 to 50 minutes
after L-arginine administration). For
nitroglycerin study, the AUC was calculated as the
incremental or decremental values from the baseline obtained during the
5 minutes (from time 0 to 5 minutes after nitroglycerin
administration). The AUC was expressed in arbitrary units. Data are
expressed as mean±SE unless otherwise indicated. Differences in
variables among the groups were analyzed by one-way ANOVA
with a Scheffé-type or Mann-Whitney U test. The relations
between the vascular parameters and biochemical
variables were examined by linear regression analysis.
Subsequently, variables whose correlation with the vascular
responses achieved near statistical significance (P<0.1)
were entered into a stepwise regression model to assess the magnitude
of their individual effects on the vascular responses. These procedures
were performed on a Macintosh computer with the StatView IV Statistical
System. A value of P<0.05 was accepted as statistically
significant.
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Results |
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All subjects tolerated the study well, and there were no adverse events. The clinical and biochemical characteristics of the control and diabetic subjects within each group are summarized in Table 1
. Fasting
plasma glucose and HbA1c levels were significantly higher in both the
normoalbuminuric and microalbuminuric patients than in
the control subjects (P<0.05 and P<0.01,
respectively). The serum triglyceride level was
significantly higher in the microalbuminuric patients than in
the control subjects (P<0.01). There was no significant
difference in the age, sex, body mass index (BMI), blood pressures,
serum total cholesterol, HDL cholesterol, and
baseline hemodynamic parameters of each
artery among the 3 groups.
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Effects of L-Arginine and Nitroglycerin
Administrations on Blood Pressures and Heart Rate
The systolic, diastolic, and mean blood
pressures decreased significantly after the infusion of
L-arginine compared with the baseline values in the
normoalbuminuric and microalbuminuric patients and
control subjects (P<0.01 in each group). The heart rate did
not show any significant change during the infusion in each group
(Table 2). The systolic,
diastolic, and mean blood pressures decreased and the heart
rate increased significantly after the administration of
nitroglycerin compared with the baseline values in each
group (P<0.01 in each group) (Table 3). There was no significant difference
in the changes in these parameters before and after the
administration of L-arginine or
nitroglycerin among the 3 groups.
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Vascular Responses to L-Arginine and Nitroglycerin
Administrations
Brachial Artery
The percent changes in the L-arginine–induced
vasodilation of the brachial artery at 15 and 30 minutes were
significantly lower in both the normoalbuminuric
(P<0.01) and microalbuminuric (P<0.01)
patients than in the control subjects (Figure 1a). The percent change in the
L-arginine–induced vasodilation of the brachial
artery at 50 minutes was significantly lower in the
microalbuminuric patients than in the control subjects
(P<0.05). The percent AUC for the
L-arginine–induced vasodilation was
significantly lower in both the normoalbuminuric (186±40) and
microalbuminuric (139±21) patients compared with the control
subjects (475±78) (P<0.05 and P<0.01,
respectively). Additionally, the percent AUC of the
L-arginine –induced vasodilation showed the
lowest value in the microalbuminuric patients among the 3
groups (P<0.01). On the other hand, there was no
significant difference in the percent change in the
nitroglycerin-induced vasodilation of the brachial
artery at 3 or 5 minutes or in the percent AUC among the
normoalbuminuric and microalbuminuric patients and the
control groups (Figure 2b).
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Central Retinal Artery
The percent change in the L-arginine–induced
reduction in the retinal arterial RI at 15 minutes was
significantly lower in the microalbuminuric patients than in
the control subjects (P<0.05) (Figure 2a
). The
percent changes in the L-arginine–induced
reduction in the retinal arterial RI at 30 and 50 minutes
were significantly lower in both the normoalbuminuric
(P<0.05) and microalbuminuric (P<0.01)
patients than in the control subjects. The percent AUC for the response
of the RI to L-arginine was significantly lower
in both the normoalbuminuric (103±57) and
microalbuminuric patients (41±17) than in the control subjects
(378±91) (P<0.05 and P<0.01, respectively).
Additionally, the percent AUC for the response of the RI to
L-arginine showed the lowest value in the
microalbuminuric patients among the 3 groups
(P<0.01). The percent AUC for the response of the RI to
L-arginine showed no significant difference
between the patients with and those without
retinopathy. On the other hand, there was no
significant difference in the percent change in the
nitroglycerin-induced reductions in the retinal
arterial RI at 3 minutes or 5 minutes or in the percent AUC
among the normoalbuminuric and microalbuminuric
patients and control groups (Figure 2b).
Intrarenal Artery (Interlobar Artery)
The percent change in the L-arginine–induced
reduction in the interlobar arterial RI at 15 minutes was
significantly lower in the microalbuminuric patients than in
the control subjects (P<0.01) (Figure 3a). The percent changes in the
L-arginine–induced reduction in the interlobar
arterial RI at 30 and 50 minutes were significantly lower
in both the normoalbuminuric and microalbuminuric
patients than in the control subjects (P<0.01,
respectively). The percent AUC for the response of the RI to
L-arginine was significantly lower in both the
normoalbuminuric (124±53) and microalbuminuric
patients (64±25) compared with the control subjects (369±29)
(P<0.01, respectively). Additionally, the percent AUC for
the response of the RI to L-arginine showed the
lowest value in the microalbuminuric patients among the 3
groups (P<0.01). On the other hand, the percent changes in
the nitroglycerin-induced reduction in the interlobar
arterial RI at 3 and 5 minutes were significantly lower in
the microalbuminuric patients than in the control subjects
(P<0.01, respectively). However, there was no significant
difference in the percent changes in the
nitroglycerin-induced reduction in this artery at 3 or
5 minutes or in the percent AUC between the normoalbuminuric
and microalbuminuric patients (Figure 3b).
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Effects of L-Arginine Infusion on Plasma Concentrations
of Glucose and Insulin
The plasma concentrations of glucose and insulin increased
significantly at 15 and 30 minutes compared with baseline in each group
(P<0.01, respectively). Although the plasma concentrations
of glucose at all 3 time points were significantly high in both the
normoalbuminuric and microalbuminuric patients compared
with the control subjects (P<0.01, respectively), there was
no significant difference in the plasma concentrations of insulin among
the 3 groups (Table 4).
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Relation Between Clinical Characteristics and Vascular
Responses
The percent AUC for the L-arginine–induced
vasodilation in the brachial artery of all subjects was significantly
correlated with fasting plasma glucose (r=0.531,
P=0.0025) and HbA1c (r=0.617,
P=0.0003) (Table 5). The
percent AUC for the response of the RI to
L-arginine in the retinal artery of all subjects
was significantly correlated with HbA1c (r=0.599,
P=0.0005). The percent AUC for the response of the RI to
L-arginine in the interlobar artery of all
subjects was significantly correlated with fasting plasma glucose
(r=0.571, P=0.001), HbA1c (r=0.636,
P=0.0002), and HOMA index (r=0.377,
P=0.0398). The nitroglycerin-induced
vascular response of the retinal artery was significantly correlated
with plasma fasting insulin (r=0.398, P=0.0294),
and that of the interlobar artery was significantly correlated with
HbA1c (r=0.452, P=0.0122). In multiple stepwise
regression analysis of factors affecting the vascular
responses, HbA1c level, BMI, and fasting plasma glucose were
independently associated with the vascular response of the brachial
artery (R2=0.558,
P<0.0001); HbA1c was independently associated with the
vascular response of the retinal artery
(R2=0.359, P=0.0005); HbA1c
and diastolic blood pressure were independently associated
with the vascular response of the intrarenal artery
(R2=0.522, P<0.0001)
(Table 6).
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Effects of 300-mL Saline Infusion on Hemodynamic
Parameters
The systolic, diastolic, and mean blood
pressures and vascular responses of each artery did not show any
significant change between before and after the saline infusion in 5
healthy subjects (data not shown).
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Discussion |
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We have demonstrated for the first time that not only the brachial artery but also retinal and intrarenal hemodynamic responses to an intravenous infusion of L-arginine are impaired in type 2 diabetic patients. Despite no significant difference in the vascular responses of the retinal and brachial arteries after the nitroglycerin administration between the patient and control groups, there were significant differences in the vascular responses of these arteries in both the normoalbuminuric and microalbuminuric patients compared with the control subjects after the L-arginine infusion, suggesting that the endothelium-dependent vascular responses in the retinal and brachial arteries are impaired but that the responsiveness of the smooth muscle to exogenous NO is not impaired in diabetic patients. On the other hand, the reduction in vascular resistance of the interlobar artery after the nitroglycerin administration was significantly lower in the microalbuminuric patients than in the control subjects, suggesting that the responsiveness of the smooth muscle to exogenous NO intrarenal artery is impaired in diabetic patients with microalbuminuria.
The blunted vascular response of the retinal and intrarenal arteries to L-arginine in the diabetic patients observed in our study is not in accordance with the findings of the study by Schmetterer et al7 showing that the effects of stimulation of NO synthesis by L-arginine on ocular hemodynamics were almost identical in diabetic patients and control subjects. However, the differences in our study methodology and patient selection from that of Schmetterer et al7 must be mentioned. They measured the fundus pulsation amplitude by using laser interferometry, which estimates the pulsatile blood flow in the choroid,10 whereas the RI measured with the use of Doppler sonography in our study estimates the vascular resistance of the retinal artery.19 Accordingly, we used an RI as a hemodynamic parameter indicating vascular compliance.22 Indeed, the RI of the retinal and renal arteries decreased significantly after L-arginine and nitroglycerin administration in the control subjects, whereas the change in the RI of each artery after L-arginine was significantly reduced in the diabetic patients compared with the control subjects. In addition, we studied the vascular responses in type 2 diabetic patients. A recent study showed that endothelial function was preserved in type 1 diabetic patients but not in type 2 diabetic patients compared with healthy subjects.23 These findings suggest that the endothelium-dependent vascular responses of these arteries may be impaired in type 2 diabetic patients.
The data concerning hemodynamic changes in the retinal arteries of diabetic patients are controversial. Retinal blood flow is increased in patients with early stages of diabetic retinopathy,24 and patients with proliferative retinopathy have reduced retinal and choroidal blood flow,25 26 whereas our previous report showed that the retinal artery blood flow velocities decreased and that vascular resistance increased in type 1 diabetic patients without clinical manifestations of diabetic retinopathy.19 Similarly, glomerular hyperfiltration has been shown to often exist in patients with early stages of diabetic nephropathy.27 Therefore, hemodynamic alterations may precede clinical manifestations of both retinopathy and nephropathy and may contribute to the development of these vascular complications. The results obtained during the L-arginine infusion indicate that either altered sensitivity of retinal and intrarenal arteries to NO or altered local NO synthesis/release may be involved in the pathology of diabetic retinopathy and nephropathy, which have already been hypothesized on the basis of human and animal studies.14 28 In diabetic patients, the systemic and ocular hemodynamic reactivity to NO synthase inhibition is reduced as compared with age-matched healthy subjects.7 In a diabetic rat model, increased endogenous NO activity may play a role in basal hyperfiltration and in the persistent renal vasodilation manifested at a reduced renal perfusion pressure.29 In the current study, no significant difference was observed in the nitroglycerin-induced vascular responses of the retinal and intrarenal arteries between the normoalbuminuric patients and the control subjects, suggesting that the impaired sensitivity of these arteries to NO may not be involved in diabetic patients without microalbuminuria. It must be determined whether the impaired vascular response to L-arginine is the cause or the effect of diabetic retinopathy and nephropathy. Since the vascular responses of these arteries to L-arginine were impaired even in the diabetic patients without retinopathy or nephropathy, it is unlikely that the impaired vascular responses to L-arginine are resultant phenomena. Therefore we suggest that endothelial dysfunction in these arteries precedes the clinical manifestations of diabetic microangiopathy in diabetic patients.
Recently, Giugliano et al30 demonstrated that the vascular effects of L-arginine are mediated partly by endogenously released insulin. In the current study, the insulin response to L-arginine was lower (but not significantly) in the diabetic patients compared with the control subjects, and therefore it is possible that the decrease in endogenous released insulin contributed in part to the impaired vascular responses to L-arginine in the diabetic patients. Additionally, it must be mentioned that a physiological link between insulin resistance and endothelial dysfunction has been shown in insulin resistance syndromes such as essential hypertension and obesity.31 32 Insulin sensitivity index correlated with the vascular response to L-arginine in the intrarenal artery in all subjects. This finding suggests that insulin resistance in this artery may in part contribute to the impaired vascular response to L-arginine in diabetic patients. However, because HOMA index is a rough method for estimating insulin resistance, further examination is required to determine the association between endothelial function and insulin resistance.
Sulfonylurea can impair dilation mediated by NO-mediated, ATP-sensitive K+ channel activation.33 However, no significant difference in the vascular responses observed between the diabetic patients treated with diet alone and those treated with sulfonylurea in the current study (data not shown) is consistent with the study by McVeigh et al.34
Another important factor contributing to endothelial dysfunction in diabetes mellitus is reported to be hyperglycemia in vivo.35 36 37 In the current study, both plasma glucose and HbA1c levels were significantly higher in the diabetic patients than in the control subjects. The L-arginine–induced vascular responses in these arteries were correlated with plasma glucose and/or HbA1c levels, whereas multiple regression analysis showed the independent association between the vascular response to L-arginine in each artery and HbA1c rather than plasma glucose, suggesting that long-term hyperglycemia may contribute to impairment of the vascular responses in these arteries. These data are consistent with the findings that chronic hyperglycemia is associated with impaired vascular endothelial function in diabetic patients.36
In conclusion, the results showed that vascular endothelial function was impaired in retinal and intrarenal arteries as well as the brachial artery in the type 2 diabetic patients with and those without microalbuminuria and therefore suggest that the endothelial dysfunction in these arteries may precede the clinical manifestations of diabetic microangiopathies. The endothelial dysfunction thus may be associated with poor glycemic control and may contribute to the development of microangiopathy and macrovascular diseases in type 2 diabetic patients.
Received January 11, 1999; accepted March 4, 1999.
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