Extract
Note: Please read the complete
full text with Figures and Tables at
Introduction
Patients with type 2 diabetes frequently present with
metabolic abnormalities such as postprandial hyperglycemia,
postprandial lipemia, atherogenic dyslipidemia, and elevated
levels of free fatty acids (FFA) and asymmetric
dimethyl-arginine (ADMA). All of these alterations are known risk
factors for atherosclerosis. Dyslipidemia in type 2 diabetic
patients is characterized by increased levels of triglycerides
and reduced levels of high-density lipoprotein cholesterol
(HDL-C), while total cholesterol and low-density lipoprotein
cholesterol (LDL-C) may be either normal or
elevated[1]. Nateglinide, a D-phenylalanine
derivative, has been shown to be effective in restoring early-phase insulin secretion and
therefore reducing postprandial hyperglycemia and glucose
excursion[2]. It has been demonstrated that the level of
postprandial triglycerides is reduced by nateglinide in a diabetic
animal model, but not in diabetic
patients[3,4]. In a type 2 diabetic model, nateglinide reduced FFA levels in the portal
blood after administration of a single
dose[3]. However, the effects of nateglinide on FFA levels in diabetic patients have
yet to be clarified.
Vascular endothelial dysfunction has been demonstrated
as a risk factor for atherosclerosis in type 2 diabetic
patients[5]. Although endothelial dysfunction was improved by using a
single dose of nateglinide[6], the contribution of lowering
postprandial hyperglycemia or improving early-phase
insulin secretion to this effect remains unknown. ADMA, a
specific endogenous inhibitor of nitric oxide (NO) synthase, has
been found to be elevated with the condition of chronic
hyperglycemia[7,8]. Moreover, high ADMA level is
associated with either chronic or acute endothelial dysfunction in
patients at risk for
atherosclerosis[9,10]. We hypothesize,
therefore, that the effect of nateglinide on insulin secretion
may be beneficial to the postprandial lipid profiles and other
metabolic parameters as is seen with postprandial
hyper-glycemia. This crossover clinical trial aims to investigate the
acute and chronic effects of nateglinide versus acarbose on
plasma ADMA levels and lipid profiles in patients with
newly-diagnosed type 2 diabetes.
Materials and methodsS
SStudy design Sixteen drug-naïve patients (5 males and
11 females, 49.4±1.6 years) with newly-diagnosed type 2
diabetes were screened and included in this study. Inclusion
criteria were fasting plasma glucose £11 mmol/L, 6.5%_10.0% HbA1c, a body mass index of 22_30
kg/m2, and treatment with diet alone for at least 2 weeks and without any
medication prior to enrolment for the study. In addition, 6
age-matched, healthy subjects with normal glucose
tolerance were used as normal controls. The study protocol was
approved by the Ethical Committee of Peking University
Health Science Center. All subjects gave informed consent.
This study was conducted in a crossover open-labeled
prospective design in order to diminish the statistical bias as
much as possible. After 2 weeks of diet treatment, all
subjects were randomized into group A and group B (8 subjects
for each group). At the initiation of the study, the patients in
group A were assigned to take nateglinide (120 mg, tid) just
before meals; the patients in group B were assigned to take
acarbose (50 mg, tid) together with meals. At the end of the
fourth week, medication was discontinued. After a 1 week
period of washout, the patients in group A were switched to
acarbose (50 mg, tid); the patients in group B to nateglinide
(120 mg, tid) for an additional 4 weeks (Figure 1).
Sample collection Before and after treatment with
nateglinide or acarbose, fasting blood samples were collected
after 14 h of overnight fasting for all subjects. In order to
assess the acute effects of nateglinide or acarbose on
postprandial lipid profiles and the ADMA levels, a 75 g instant
noodle ration served as a standard meal test and was
consumed with the first dose of medication on the first day of
treatment. Blood samples were collected from both arms at
30, 60, 120, and 240 min after the standard meal test.
Laboratory evaluation Plasma glucose levels were
measured using the glucose-oxidase method, while HbA1c
values were assessed by HPLC. Insulin was measured by
IMMULITE 1000 (DPC, Los Angeles, CA, USA).
Trigly-cerides, total cholesterol, LDL-C, and HDL-C were analyzed
using a standard technique. FFA levels were assayed using
commercially available kits (Randox Laboratories Ltd,
Co Antrim UK).
Plasma ADMA was determined with a modification of the
HPLC method previously described[11] using pre-column
derivatization with o-phthalaldehyde (OPA). Prior to analysis,
plasma samples and standards (Sigma, St Louis, MO USA)
were extracted on OASIS solid phase extraction cartridges
(Waters, Milford, MA, USA). The eluents were dried over
nitrogen and dissolved in 50% methanol for HPLC analysis.
The analysis was carried out on a HPLC system (TSP, San
Jose, CA, USA) consisting of Spectra-Physis SP8810 pumps,
a spectral fluorescence detector FL3000, and ThermoQuest
work station. Samples and standards were incubated for
exactly 3 min with the OPA (Sigma, USA) reagent (5.4 mg/mL
OPA in 1 mmol/L borate buffer, pH 9.5, containing 0.4%
β-mercaptoethanol) before injection into the HPLC. The
OPA-derivatives of ADMA were separated on a 5 µm Kromasil
C18 column (Dalian Elite Analytical Instruments,Dalian,
China) with the fluorescence monitor set at lex 338 nm and
lem 425 nm. Samples were eluted from the column with
0.96% citric acid/methanol 2:1, pH 6.8, at a flow rate of 1 mL/min.
The detection limit of the assay was 0.1 mmol/L.
Statistical analyses Values are expressed as the mean±
SEM. The statistical significance of variation between means
was tested using two-tailed paired Student's
t-test. Values of P<0.05 were considered statistically significant.
Results
Five male and 11 female patients with newly-diagnosed
type 2 diabetes were selected to enter this study. The mean
fasting plasma glucose and HbA1c levels of 16 diabetic
patients were 8.2±0.4 mmol/L and 7.6%±0.3%, respectively. Six
healthy subjects with normal glucose tolerance were enrolled
as normal controls, and their mean fasting plasma glucose
and HbA1c levels were 4.9±0.1 mmol/L and 5.0%±0.1%,
respectively. The mean age of the 16 diabetic patients and 6
normal controls was 49.4±1.6 and 48.8±5.3 years, respectively.
Acute effects of nateglinide versus acarbose on plasma
glucose and serum insulin levels The efficiencies of a single
dose of nateglinide or acarbose for lowering postprandial
hyperglycemia were similar in the patients with type 2
diabetes (Figure 2A). Nateglinide seemed to be slightly more
efficacious at lowering postprandial 120 min hyperglycemia,
while acarbose seemed to be better at lowering postprandial
60 min hyperglycemia after a standard meal test, but there
was no significant difference between these 2 medications.
Compared to acarbose, a single dose of nateglinide
significantly increased postprandial insulin release in patients with
type 2 diabetes (Figure 2B).
Acute effects of nateglinide versus acarbose on
postprandial lipid profiles Triglyceride levels were slowly
elevated with time after a standard meal test in normal
subjects and in the type 2 diabetic patients taking a single dose
of nateglinide or acarbose. There was no significant
difference between nateglinide and acarbose treatment (Figure
3A). The levels of serum FFA were remarkably decreased
and reached a nadir at 120 min after the standard meal test in
normal subjects. Fasting FFA levels in type 2 diabetics were
significantly higher than in normal subjects. Nateglinide
decreased postprandial 120 min FFA concentration more
profoundly than acarbose (181.7 mmol/L vs 257.8 mmol/L,
P=
0.019; Figure 3B). LDL-C and HDL-C levels after the
standard meal test were unchanged by these 2 medications (data
not shown).
Chronic effects of nateglinide versus acarbose on
fasting lipid profiles after 4 weeks of treatment
Fasting HDL-C levels increased and LDL-C levels decreased significantly
after 4 weeks of treatment with nateglinide in a sum of 2 arms
of the study (P<0.05). The triglyceride and total cholesterol
levels were unchanged by nateglinide treatment. Nateglinide
led to the decrease of fasting FFA by about 10% after 4
weeks of treatment, but there was no statistically significant
difference between pre- and post-treatment. Acarbose did
not affect fasting lipid profiles after 4 weeks of treatment. In
addition, there were no significant differences of fasting
lipid profiles between nateglinide and acarbose treatments (Table 1).
Acute and chronic effects of nateglinide versus acarbose
on plasma ADMA levels The fasting plasma ADMA level
was significantly higher in the patients with type 2 diabetes
than in normal subjects. Compared with a single dose of
acarbose, a single dose of nateglinide decreased
postprandial plasma ADMA concentration 240 min after the standard
meal test in type 2 diabetic patients (P<0.05). However, the
level of fasting ADMA was not significantly altered after 4
weeks of treatment with either nateglinide or acarbose (Figure 4).
Discussion
This crossover study showed that, as expected, insulin
secretion after a standard meal test is delayed in type 2
diabetic patients, and that a single dose of nateglinide could
partially restore early-phase insulin secretion. The data also
indicated that nateglinide and acarbose had similar
hypoglycemic effects on postprandial hyperglycemia.
It is well known that insulin inhibits the production of
FFA from lipolysis. An increased hepatic FFA flux is
postulated to be a major contributor of
diabetic dyslipidemia
because it leads to the overproduction of triglyceride-rich
lipoproteins by the liver[12]. On the other hand, there are
convincing data that show that plasma FFA levels modulate
the severity of insulin resistance in type 2
diabetes[13,14]. Most humans are non-fasting throughout most of the day, and
prolonged postprandial hyperlipidemia is
an important characteristic of diabetic
dyslipidemia[15]. Therefore, therapeutic
effect of antihyperglycemic agents on postprandial lipemia,
if any, may play a role in the prevention of atherosclerosis.
In this study, a single dose of nateglinide significantly
decreased the postprandial 120 min FFA level after a standard
meal in comparison with acarbose. The level of fasting FFA
after 4 weeks of treatment with nateglinide had a trend to
decline from the baseline, although no significant difference
was presented. In parallel with these results, nateglinide
was found to acutely increase portal insulin levels and
decrease portal FFA levels 15, 30, and 60 min after sucrose
loading in diabetic rats[3]. Since nateglinide had similar
effects on postprandial hyperglycemia as acarbose (which
did not alter postprandial FFA levels), the reduction of
postprandial 120 min FFA levels by nateglinide was likely to be
the result of the partial restoration of early-phase insulin
secretion rather than from the improvement of postprandial
hyperglycemia. In fact, in a previous study,
peripherally-administered insulin resulted in early insulin augmentation
that reduced the glycemic and FFA responses to a meal in
diabetic patients[16]. Furthermore, the improvement of
postprandial FFA concentrations by nateglinide in diabetics was
demonstrated to be associated with suppression of
hormone-sensitive lipase, which induces FFA release from adipose
tissue[17]. All these results suggest that the defect of
early-phase insulin secretion might be one of the reasons why
hyperlipidemia develops in type 2 diabetics.
In agreement with previous
reports[4], this study showed that nateglinide had neither acute nor chronic effects on
serum triglyceride levels in type 2 diabetics. The analogous
results were also found in subjects at risk for type 2
diabetes[18]. Insulin has potential effects on postprandial lipid metabolism.
It can affect either production or removal of triglycerides
and triglyceride-rich
lipoproteins[19,20]. So far, the reason why
nateglinide has no effect on triglyceride metabolism, despite
significantly increasing insulin secretion and improving
postprandial hyperglycemia, remains unclear. It has been
speculated that the acute increase of insulin by nateglinide might
be not enough to overcome insulin resistance and then to
substantially lower triglyceride concentrations in diabetic
patients[18].
The present study also showed that nateglinide increased
HDL-C levels and decreased LDL-C levels after 4 weeks of
treatment. Similarly, repaglinide, another rapidly-acting
prandial glucose regulator, was reported to decrease serum total
cholesterol concentration in type 2 diabetic
patients[21]. Whether nateglinide affects serum HDL-C and LDL-C levels
in this manner deserves further investigation as it
may indicate a cardiovascular advantage if it can be confirmed.
In the present study, nateglinide was found to decrease
postprandial ADMA concentrations. To our knowledge, this
is the first description of the effect of nateglinide on ADMA
in type 2 diabetics. ADMA, an endogenous competitive
inhibitor of NO synthase, is synthesized in many tissues,
including vascular endothelial cells. Inconsistent changes
in ADMA levels in diabetes have been described in the
literature, most of which showed that ADMA levels were
elevated[7,22], as seen in our study. Moreover, the
improvement of hyperglycemia was associated with lowered plasma
ADMA concentrations[23,24]. However, there was 1 study
reporting that ADMA levels in diabetic patients were lower
than in healthy subjects and were inversely correlated with
HbA1c levels[25]. In that study, the selected diabetic
patients had increased glomerular filtration rates compared to
healthy subjects, and the lowered ADMA in type 2 diabetic
patients was accordingly attributable to increased GFR. This
speculation, however, needed to be clarified, as the
difference of GFR between 90 mL/min/1.73
m2 and 98 mL/min/1.73 m2 might have no clinical significance. Therefore, the reason
of inconsistent results of ADMA levels in diabetes was
unclear. There is an intriguing relationship between insulin
resistance and ADMA levels reported in some of the literature.
Plasma concentrations of ADMA have been demonstrated
to be elevated in several clinical syndromes associated with
insulin resistance and in apparently-healthy insulin
resistant individuals[26,27]. Rosiglitazone treatment or weight loss
by non-pharmacological means in these subjects resulted in
both an enhancement in insulin sensitivity and a fall in plasma
ADMA concentrations[27,28]. On the basis of the
above-mentioned data, it is reasonable to think that the plasma levels of
ADMA were elevated rather than decreased in the patients
with type 2 diabetes characterized by hyperglycemia and
insulin resistance.
Reduced production of NO in endothelial cells by elevated
ADMA leads to the abnormality of endothelial cell-mediated
vasodilation[29], which has been demonstrated in patients
with established atherosclerosis[30]. Interestingly, it has been
shown that post-challenge hyperglycemia caused the
alteration of vascular function, but did not lead to elevation in
ADMA concentration in patients with impaired glucose
tolerance[31]. Indeed, our data also suggested that acarbose
did not affect postprandial ADMA concentration. In this
regard, we speculate that diminished postprandial ADMA
levels produced from nateglinide might result from the
partial restoration of early-phase insulin secretion or from
nateglinide per se. The latter is yet to be elucidated.
More-over, several clinical studies have demonstrated that
nate-glinide is capable of improving insulin sensitivity in patients
with established diabetes[32,33]. Therefore, it is conceivable
that the effect of nateglinide on postprandial plasma ADMA
concentration is possibly associated with its ability to
improve insulin sensitivity as well. However, the results of
our study also indicated that nateglinide did not alter the
fasting ADMA concentration after 4 weeks of treatment. The
reason for this phenomenon remains unknown. The results
of this study need to be confirmed in a larger clinical trial.
Nevertheless, the fact that nateglinide not only lowers
postprandial ADMA level, but also improves endothelial
dysfunction[34] suggests its therapeutic advantage in the
prevention of atherosclerosis.
Taken together, the results of our study demonstrate that
nateglinide partially restores early-phase insulin secretion,
decreases postprandial serum FFA and ADMA
concentra-tions, and possibly modulates serum HDL-C and LDL-C
abnormalities indicating that nateglinide has a cardiovascular
advantage over acarbose.
Acknowledgments
We are grateful to Qiu-ming GENG, Lu ZHANG, Zheng
MA, and Guo-quan LI for their excellent technical assistance.
Nateglinide and acarbose were kindly provided by Beijing
Novartis Pharma, Ltd (Beijing, China) and Bayer Healthcare
Company, Ltd (Beijing, China) respectively.
References
1 Buse JB, Tan MH, Prince MJ, Erickson PP. The effects of oral
anti-hyperglycemic medications on serum lipid profiles in
patients with type 2 diabetes. Diabetes Obes Metab 2004; 6:
133_56.
2 Ikenoue T, Akiyoshi M, Fujitani S, Okazaki K, Kondo N, Maki
T. Hypoglycemic and insulinotropic effects of a novel oral
anti-diabetic agent, (-)-N-(trans-4-isopropylcyclohexane-carbonyl)-
D-phenylalanine (A-4166). Br J Pharmacol 1997; 120: 137_45.
3 Mori Y, Kitahara Y, Miura K, Tajima N. Comparison of voglibose
and nateglinide for their acute effects on insulin secretion and
free fatty acid levels in OLETF rat portal blood after sucrose
loading. Endocrine 2004; 23: 39_43.
4 Vakkilainen J, Mero N, Schweizer A, Foley JE, Taskinen MR.
Effects of nateglinide and glibenclamide on postprandial lipid
and glucose metabolism in type 2 diabetes. Diabetes Metab Res
Rev 2002; 18: 484_90.
5 de Koning EJ, Rabelink TJ. Endothelial function in the
postprandial state. Atheroscler Suppl 2002; 3: 11_6.
6 Shimabukuro M, Higa N, Takasu N, Tagawa T, Ueda S. A single
dose of nateglinide improves post-challenge glucose metabolism
and endothelial dysfunction in type 2 diabetic patients. Diabet
Med 2004; 21: 983_6.
7 Abbasi F, Asagmi T, Cooke JP, Lamendola C, McLaughlin T,
Reaven GM, et al. Plasma concentrations of asymmetric
dim-ethylarginine are increased in patients with type 2 diabetes mellitus.
Am J Cardiol 2001; 88: 1201_3.
8 Xiong Y, Lei M, Fu S, Fu Y. Effect of diabetic duration on serum
concentrations of endogenous inhibitor of nitric oxide synthase
in patients and rats with diabetes. Life Sci 2005; 77: 149_59.
9 Annuk M, Zilmer M, Fellstrom B. Endothelium-dependent
vasodilation and oxidative stress in chronic renal failure: impact
on cardiovascular disease. Kidney Int Suppl 2003; 84: S50_3.
10 Stuhlinger MC, Oka RK, Graf EE, Schmolzer I, Upson BM, Kapoor
O, et al. Endothelial dysfunction induced by
hyperhomo-cysteinemia: role of asymmetric dimethylarginine. Circulation
2003; 108: 933_8.
11 Bode-Boger SM, Boger RH, Kienke S, Junker W, Frolich JC.
Elevated L-arginine/dimethylarginine ratio contributes to
enhanced systemic NO production by dietary
L-arginine in hypercholesterolemic rabbits. Biochem Biophys Res Commun 1996;
219: 598_603.
12 Taskinen MR. Diabetic dyslipidemia: from basic research to
clinical practice. Diabetologia 2003; 46: 733_49.
13 Kelley D, Williams K, Price J, McKolanis T, Goodpaster B, Thaete
F. Plasma fatty acids, adiposity and variance of skeletal muscle
insulin resistance in type 2 diabetes mellitus. J Clin Endocrinol
Metab 2001; 86: 5412_9.
14 Santomauro AT, Boden G, Silva ME, Rocha DM, Santos RF,
Ursich MJ, et al. Overnight lowering of free fatty acids with
acipimox improves insulin resistance and glucose tolerance in obese
diabetic and nondiabetic subjects. Diabetes 1999; 48: 1836_41.
15 Karpe F. Postprandial lipemia - effect of lipid-lowering drugs.
Atheroscler Suppl 2002; 3: 41_6.
16 Bruce D, Chisholm D, Storlien L, Kraegen E. Physiological
importance of deficiency in early prandial insulin secretion in
non-insulin dependent diabetes. Diabetes 1988; 37: 736_44.
17 Dimitriadis G, Boutati E, Lambadiari V, Mitrou P, Maratou E,
Brunel P, et al. Restoration of early insulin secretion after a
meal in type 2 diabetes: effects on lipid and glucose metabolism.
Eur J Clin Invest 2004; 34: 490_7.
18 Johanson EH, Jansson PA, Gustafson B, Sandqvist M, Taskinen
MR, Smith U, et al. No acute effect of nateglinide on
postprandial lipid and lipoprotein responses in subjects at risk for type 2
diabetes. Diabetes Metab Res Rev 2005; 21: 376_81.
19 Malmstrom R, Packard CJ, Caslake M, Bedford D, Stewart P,
Yki-Jarvinen H, et al. Defective regulation of triglyceride
metabolism by insulin in the liver in NIDDM. Diabetologia 1997;
40: 454_62.
20 Panarotto D, Remillard P, Bouffard L, Maheux P. Insulin
resistance affects the regulation of lipoprotein lipase in the
postprandial period and in an adipose tissue-specific manner. Eur J
Clin Invest 2002; 32: 84_92.
21 Damsbo P, Marbury TC, Hatorp V, Clauson P, Muller PG.
Flexible prandial glucose regulation with repaglinide in patients with
type 2 diabetes. Diabetes Res Clin Pract 1999; 45: 31_9.
22 Masuda H, Goto M, Tamaoki S, Azuma H. Accelerated intimal
hyperplasia and increased endogenous inhibitors for NO
synthesis in rabbits with alloxan-induced hyperglycaemia. Br J Pharmacol
1999; 126: 211_8.
23 Asagami T, Abbasi F, Stuelinger M, Lamendola C, McLaughlin T,
Cooke JP, et al. Metformin treatment lowers asymmetric
dimethylarginine concentrations in patients with type 2 diabetes.
Metabolism 2002; 51: 843_6.
24 Yasuda S, Miyazaki S, Kanda M, Goto Y, Suzuki M, Harano Y,
et al. Intensive treatment of risk factors in patients with type-2
diabetes mellitus is associated with improvement of endothelial
function coupled with a reduction in the levels of plasma
asymmetric dimethylarginine and endogenous inhibitor of nitric oxide
synthase. Eur Heart J 2006; 27: 1159_65.
25 Paiva H, Lehtimaki T, Laakso J, Ruokonen I, Rantalaiho V,
Wirta O, et al. Plasma concentrations of
asymmetric-dimethyl-arginine in type 2 diabetes associate with glycemic control and
glomerular filtration rate but not with risk factors of vasculopathy.
Metabolism 2003; 52: 303_7.
26 Vallance P. Importance of asymmetrical dimethylarginine in
cardiovascular risk. Lancet 2001; 358: 2096_7.
27 Stuhlinger MC, Abbasi F, Chu JW, Lamendola C, McLaughlin TL,
Cooke JP, et al. Relationship between insulin resistance and an
endogenous nitric oxide synthase inhibitor. JAMA 2002; 287:
1420_6.
28 McLaughlin T, Stuhlinger M, Lamendola C, Abbasi F, Bialek J,
Reaven GM, et al. Plasma asymmetric dimethylarginine
concentrations are elevated in obese insulin-resistant women and fall
with weight loss. J Clin Endocrin Metab 2006; 91: 1896_900.
29 Harrison DG. Cellular and molecular mechanisms of endothelial
cell dysfunction. J Clin Invest 1997; 100: 2153_7.
30 Ross R. Atherosclerosis _ an inflammatory disease. N Engl J
Med 1999; 340: 115_26.
31 Wascher TC, Schmoelzer I, Wiegratz A, Stuehlinger M,
Mueller-Wieland D, Kotzka J, et al. Reduction of postchallenge
hyperglycaemia prevents acute endothelial dysfunction in
subjects with impaired glucose tolerance. Eur J Clin Invest 2005;
35: 551_7.
32 Hazama Y, Matsuhisa M, Ohtoshi K, Gorogawa S, Kato K,
Kawamori D, et al. Beneficial effects of nateglinide on insulin
resistance in type 2 diabetes. Diabetes Res Clin Pract 2006; 71:
251_5.
33 Mari A, Gastaldelli A, Foley JE, Pratley RE, Ferrannini E.
Beta-cell function in mild type 2 diabetic patients: effects of 6-month
glucose lowering with nateglinide. Diabetes Care 2005; 28: 1132_8.
34 Shimabukuro M, Higa N, Takasu N, Tagawa T, Ueda S. A single
dose of nateglinide improves post-challenge glucose metabolism
and endothelial dysfunction in type 2 diabetic patients. Diabet
Med 2004; 21: 983_6.
|
