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Original Studies |
Division of Endocrinology and Metabolic Diseases, University of Verona Medical School, and Medical Department, GlaxoWellcome (S.U.), 37126 Verona, Italy
Address all correspondence and requests for reprints to: Prof. Enzo Bonora, Endocrinologia e Malattie del Metabolismo, Ospedale Civile Maggiore, Piazzale Stefani 1, 37126 Verona, Italy. E-mail: malmetab{at}borgotrento.univr.it
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Abstract |
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 Top Abstract IntroductionSubjects and MethodsResultsDiscussionReferences |
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5 mmol/L)
hyperinsulinemic (300 pmol/L) clamp in combination with a dual
glucose tracer infusion ([3-3H]- and
[U-14C]D-glucose) and indirect calorimetry in
40 nonobese subjects with type 2 diabetes. Subjects were studied twice:
after a 4-week run-in period and after a 16-week period of double
blind, randomized treatment with 4–6 mg/day lacidipine, a calcium
channel blocker (n = 19), or 10–20 mg/day lisinopril, an
angiotensin-converting enzyme inhibitor (n = 21). Antihypertensive
treatment resulted in a significant increase in total glucose disposal
during insulin clamp as well as in basal and insulin-stimulated
nonoxidative glucose disposal rates. On the contrary, oxidative glucose
disposal was significantly decreased by antihypertensive treatment,
mainly in the basal state. The changes in glucose disposal rates were
not significantly different in subjects treated with lacidipine and in
those treated with lisinopril. The suppression of endogenous glucose
production during insulin clamp was significantly greater after
lacidipine than after lisinopril. These results suggest that treatment of subjects with type 2 diabetes with either lacidipine or lisinopril has no adverse effect on glucose metabolism. Conversely, both drugs seem to improve insulin sensitivity.
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Among available antihypertensive drugs, angiotensin-converting enzyme inhibitors (ACEI) seem to be devoid of unfavorable effects on glucose and lipid metabolism (4). Indeed, many reports documented the positive effects of these compounds on several metabolic parameters in type 2 diabetes, including insulin sensitivity (5, 6, 7, 8). As for calcium channel blockers (CCB), their effects on glucose/lipid metabolism in type 2 diabetes are poorly understood (9). To our knowledge, only one study compared a CCB with an ACEI in type 2 diabetes (10), but the number of subjects examined was rather small and the study was not controlled (randomization but not blindness). Thus, information about possible differences between CCB and ACEI are still insufficient. Furthermore, no study thoroughly examined the intracellular metabolic partition of plasma glucose during treatment with these compounds. Indeed, although few studies employed indirect calorimetry to distinguish between oxidative and nonoxidative glucose disposal rates (5, 6, 10), no study used more sophisticated techniques to precisely assess the metabolic fate of glucose taken up by the bloodstream.
In the present study we employed a dual glucose tracer technique associated with the euglycemic hyperinsulinemic clamp and indirect calorimetry to measure the fate of intracellular glucose in the basal state and during insulin infusion before and after chronic treatment with lacidipine, a CCB, and lisinopril, an ACEI, in patients with type 2 diabetes.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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We enrolled 40 nonobese [body mass index (BMI), <30 kg/m2] noninsulin-treated subjects with type 2 diabetes. Twenty of them were hypertensive according to conventional criteria (11), but virtually all the others had blood pressure values higher than those recommended in diabetes mellitus (3). Few subjects were treated for hypertension (3 with ACEI and 2 with CCB). Patients with concomitant chronic liver disease, renal failure, endocrine disease other than diabetes, or any other major disease were excluded. In subjects with high blood pressure, secondary hypertension was ruled out by standard clinical work-up.
The purpose and potential risks of the study were explained to all subjects, and their informed, written, voluntary consent was obtained before their participation. The protocol was reviewed and approved by the ethical committee of the University of Verona Medical School.
Study design
After the enrollment, all subjects were given placebo in a
single blind fashion for 4 weeks. During this period, ongoing
antihypertensive treatment, if any, was discontinued, and the current
hypoglycemic treatment was titrated with the goal of achieving a
fasting plasma glucose level not higher than 7.8 mmol/L. Nine patients
were treated with diet only, 12 were treated with sulfonylureas, and 19
were treated with a combination of sulfonylureas and metformin.
Thereafter, every effort was made to hold diet, physical activity, and
drug treatment of diabetes constant so that any change in glucose
levels and insulin sensitivity could be related exclusively to
antihypertensive treatment. After the run-in and wash-out period, the
baseline metabolic studies were performed (see below). Then, the
patients were randomized to a 16-week treatment with lacidipine (a CCB)
or lisinopril (an ACEI) with a double blind, double dummy, parallel
groups design. Nineteen subjects received lacidipine, and 21 subjects
received lisinopril. The 2 groups were well matched for sex, age, BMI,
waist/hip ratio (WHR), blood pressure, and degree of metabolic control
(Table 1
). Blood pressure measurements,
compliance, and clinical adverse effects were recorded every 4 weeks.
Lacidipine and lisinopril doses were titrated from 4–6 mg/day and from
10–20 mg/day, respectively, with the purpose of achieving a diastolic
blood pressure value less than 85 mm Hg.
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Blood pressure was measured to the nearest millimeter with a mercury sphygmomanometer in the right arm after at least 10 min of rest in the sitting position. Systolic and diastolic blood pressures were defined as Korotkoff phases I and V. The mean of three measurements taken at 2-min intervals was averaged and used for the analysis.
Anthropometry
Height (to the nearest 0.5 cm) and weight (to the nearest 0.5 kg) were recorded, whereas subjects were wearing only underwear garments. BMI was calculated as weight/height2. Waist circumference (widest between the lower rib margin and the iliac crest) and hip circumference (widest over the great trochanters) were measured in duplicate, and the values were averaged and used to compute WHR. Higher values of WHR indicated a predominantly central fat distribution.
Based upon the principle that resistance to a mild electrical current is related to total body water, and that the latter is highly correlated to fat-free mass, a tetrapolar impedance analyzer (BIA-103, Akern, Florence, Italy) was used to measure body electrical resistance and to derive an estimate of total body water, fat-free mass, and body fat (12). The measures achieved with this technique are highly correlated with those generated by more sophisticated methods, including isotope dilution in the body (13).
Metabolic studies
A 4-h oral glucose tolerance test (OGTT) and a 4-h euglycemic hyperinsulinemic clamp were performed on 2 separate days at least 3 days apart. During the OGTT, plasma glucose, insulin, and C peptide were measured. On the basal sample of the OGTT, glycohemoglobin [hemoglobin A1c HbA1c)] and plasma concentrations of total and high density lipoprotein (HDL) cholesterol and triglycerides were assessed. During the insulin clamp, several parameters of glucose metabolism as well as plasma free fatty acid (FFA) and lipid oxidation were measured (see below). Antihypertensive and hypoglycemic medications were withheld on the morning of the metabolic tests to avoid observing the acute effects of the drugs.
Glucose disposal
Glucose disposal in the basal state and during insulin infusion was measured by the combination of euglycemic hyperinsulinemic clamp, radioisotopic technique, and indirect calorimetry as previously described in detail (14) and as summarized below.
In the morning, after an overnight fast, a 20-gauge Teflon catheter was inserted into an antecubital vein for the infusion of all test substances. A second catheter was inserted retrograde into a wrist vein of the contralateral hand for blood sampling, and the hand was inserted into a hot (60 C) box to achieve the arterialization of venous blood. Both catheters were kept patent with the infusion of a normal saline solution.
Basal state. At 0800–0830 h (-150 min), a prime-constant
infusion of [3-3H]D-glucose (0.15 µCi/min)
and a prime-constant infusion of
[U-14C]D-glucose (0.075 µCi/min; DuPont-New
England Nuclear, Boston, MA) were started and continued for 150 min.
The prime of [3-3H]D-glucose and
[U-14C]D-glucose was associated with a bolus
injection of NaH14CO3 (8 µCi). The prime
for tracer glucose was calculated by dividing the glucose pool (plasma
glucose concentration times glucose distribution volume, assumed to be
25% of body weight) by the estimated basal glucose turnover (11
mmol/min·kg fat-free mass when fasting plasma glucose was <11.1
mmol/L, and 14 mmol/min·kg fat-free mass when it was 
11.1 mmol/L)
and then multiplying the result for the tracer infusion rate.
Insulin clamp. At the end of the basal state (time zero),
the dual glucose tracer infusion was discontinued, and a euglycemic
hyperinsulinemic (20 mU/m2 surface area·min) clamp was
initiated and continued for the subsequent 240 min. Plasma glucose was
left to drop until euglycemia (5 mmol/L) was reached, and then was
maintained at that level by a glucose infusion adjusted every 5–10 min
according to the variation in plasma glucose. Two hours after the
beginning of insulin clamp (120 min), the prime-constant infusions of
[3-3H]D-glucose and
[U-14C]D-glucose were resumed at rates of
0.30 and 0.15 µCi/min, respectively, and were continued until the end
of the study. A second bolus of NaH14CO3 was
administered (8 µCi) with the prime. The prime dose of labeled
glucose was calculated by dividing the glucose pool by the product of
1.1 by GIR100–120 and then multiplying the result by the
tracer infusion rate. GIR100–120 was the glucose infusion
rate during the interval from 100–120 min of the glucose clamp. It was
multiplied by 1.1 to take into account the expected 10% average
increase in glucose infusion from 100–120 to 180–240 min (15, 16).
The rationale for interrupting the tracer administration during the
first 2 h of the insulin clamp was that by resuming the tracer
infusion in a near-steady state, the time of equilibration for labeled
precursor (glucose) and products (water and CO2) would be
significantly shortened (14). Indeed, with such a methodological
approach, a good steady state specific activity was obtained during the
last 60 min of the clamp (coefficient of variation, <10%) (14, 16).
Expired air samples for the determination of CO2 specific activity were collected in the last hour of the basal period and the insulin clamp period. At the same time, blood was withdrawn for the determination of plasma [3-3H]D-glucose and [U-14C]D-glucose specific activities and plasma 3H2O and insulin and FFA concentrations. Blood was collected in heparinized tubes and promptly centrifuged, and the plasma was decanted and stored at -20 C until analyzed. Expired air was bubbled through a CO2 trapping solution (hyamine hydroxide-absolute ethanol-0.1% phenolphtalein, 3:5:1). The solution was titrated with 1 N HCl to trap 1 mmol/L CO2 in 3 mL solution. Part (0.5 mL) of the saturated solution was added to 5 mL scintillation liquid, and 14C radioactivity was measured using a ß-scintillation counter (Beckman Coulter, Inc., Fullerton, CA).
Computerized, open circuit, continuous, indirect calorimetry with a canopy system (Deltatrac, Sensormedics, Anaheim, CA) was employed for the determination of gaseous exchange in the last hour of the basal period and the insulin clamp period. Data were used to compute the rates of O2 consumption and CO2 production. Protein oxidation was calculated from the urinary nitrogen excretion measured before and during the insulin clamp. The rate of lipid oxidation in the last hour of the basal state and insulin clamp was measured according to standard equations (17).
Analytical determinations
Plasma glucose, HbA1c, serum total and HDL cholesterol, and triglycerides were measured by standard and quality-controlled techniques. Insulin and C peptide were measured by double antibody RIAs. Insulin assay was performed with an antibody without significant cross-reactivity with proinsulin (Linco Research, Inc., St. Louis, MO). The plasma FFA concentration was determined by a spectrophotometric method. The urinary nitrogen concentration in samples collected during the basal and insulin-stimulated periods was measured by the method of Kjeldhal (18).
[3-3H]D-glucose, [U-14C]D-glucose, and 3H2O specific activities in the plasma, and 14CO2 specific activity in the expired air were determined as described in detail previously (14).
Calculations
Total glucose disposal, endogenous glucose production,
glycolysis, glycogen synthesis, glucose oxidation, nonoxidative glucose
disposal, and nonoxidative glycolysis were computed as previously
reported (14). In particular, as
[3H]3-D-glucose specific activity and
[14C]U-D-glucose specific activity in the
plasma and 14CO2 specific activity in the
expired air were in steady state (Fig. 1), we could calculate glucose fluxes as
follows: 1) total glucose disposal by dividing the
[3-3H]D-glucose infusion rate by the steady
state [3-3H]D-glucose specific activity, 2)
glycolysis by dividing the 3H2O production rate
by the plasma [3-3H]D-glucose specific
activity, 3) glycogen synthesis as the difference between total glucose
disposal and glycolysis, 4) glucose oxidation from the
14CO2 production rate divided by the steady
state plasma [U-14C]D-glucose specific
activity, 5) nonoxidative glucose disposal by subtracting glucose
oxidation from total glucose disposal, and 6) nonoxidative glycolysis
by subtracting glucose oxidation from glycolysis. Endogenous glucose
production was equal to total glucose disposal in the basal state,
whereas during insulin clamp it was computed as the difference between
total glucose disposal and exogenous glucose infused to maintain
euglycemia. All fluxes were expressed as micromoles per min/kg fat-free
mass.
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All values are presented in figures and tables as the mean ± SE. The areas under the curve of plasma glucose, insulin, and C peptide were calculated according to the trapezoidal rule. Data were preliminarily log transformed for statistical analyses, when appropriate, and were back-transformed in natural units for presentation in text and tables. Student’s t test for unpaired data was used for baseline comparisons between groups undergoing treatment with lacidipine or lisinopril. To test changes in clinical and biochemical variables with treatment, ANOVA with repeated measures was performed with drug as a between-subjects factor and before-after treatment and basal insulin (when applicable) as within-subjects factors (19).
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Results |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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, body weight, body composition, and body
fat distribution did not change with treatment. As expected, blood
pressure significantly decreased during treatment (systolic, 15 mm
Hg; diastolic, 6 mm Hg), without any difference between the two
therapeutic regimens. Plasma lipid concentrations did not significantly
change after treatment, although a remarkable increase in HDL
cholesterol was noticed. Also, the areas under the plasma glucose,
insulin, and C peptide curves were not significantly changed by
treatment, although there was a trend toward lower postglucose plasma
insulin levels after treatment. HbA1c remained
unchanged.
Table 2 reports glucose, insulin, and FFA
plasma levels before and after treatment both in the basal state and
during the insulin clamp. In the latter condition (last hour of the
clamp), there was an approximately 4-fold increase in plasma insulin
levels concomitantly with a decrease in plasma glucose and FFA
concentrations. The decrease in plasma FFA was enhanced by
antihypertensive treatment. These changes were not different in
lacidipine- and lisinopril-treated subjects.
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reports glucose and lipid fluxes in the basal state and during
insulin clamp in subjects treated with lacidipine or lisinopril. When
glucose fluxes in the basal state before and after treatment were
compared, glucose oxidation was significantly decreased
(P = 0.007), and nonoxidative glucose disposal was
significantly increased (P = 0.006) by antihypertensive
treatment, with no difference between the two therapeutic regimens. As
expected, insulin infusion resulted in a suppression of endogenous
glucose production and lipid oxidation rate and a stimulation of total
glucose disposal (TGD) and all glucose fluxes: glucose oxidation,
nonoxidative glucose disposal, glycogen synthesis, and glycolysis.
Regardless of the drug employed, antihypertensive treatment yielded a
significant increase in TGD (P = 0.007) and
nonoxidative glucose disposal (P = 0.007). The increase
in glycogen synthesis after treatment was close to significance
(P = 0.1). No significant differences were found
between the two therapeutic regimens, with the exception of endogenous
glucose disposal, which was suppressed by insulin to a greater extent
during lacidipine than during lisinopril treatment (P =
0.02). The decline in FFA during the insulin clamp was inversely correlated to the increase over basal in TGD both before (r = -0.502; P = 0.002) and after treatment with antihypertensive agents (r = -0.539; P < 0.001). However, the increase in TGD during the clamp after treatment was not related to the greater suppression of FFA during insulin infusion observed after treatment (r = -0.16; P = NS).
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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When we compared the effects of treatment with lacidipine with those of treatment with lisinopril, we found that there were no major differences. Indeed, only the suppression of endogenous glucose production during insulin infusion was found to be significantly greater after lacidipine than after lisinopril, whereas the effects on peripheral glucose fluxes, if any, either in the basal state or during insulin clamp, were not different.
Data regarding the effects of ACEI on insulin resistance of type 2 diabetes are controversial. In fact, both positive effects (5, 6, 7, 8) and neutral effects (23, 24, 25) were found. As to CCB, the few studies carried out in diabetic subjects showed neutral effects (9, 10), and no study reported beneficial effects of treatment with CCB. Our results indicate that lacidipine and lisinopril are similarly effective in improving insulin sensitivity in type 2 diabetes. The increase in total glucose disposal that we observed with lisinopril was of the same extent of that reported by those investigators who used other ACEI, such as captopril or enalapril (5, 6, 7, 8). Also with these drugs, a preferential increase in nonoxidative glucose disposal was observed (5, 6). The amelioration of insulin sensitivity with lacidipine is of particular interest, as previous studies examining the metabolic effects of CCB failed to find positive effects of this class of drugs in subjects with type 2 diabetes (9, 10).
Besides the hemodynamic (vasodilatory) effect, lacidipine might result in increased glucose utilization through the effect of ameliorating deranged cation metabolism. Indeed, a drug-induced reduction of the intracellular Ca2+ concentration in skeletal muscle cells might be another mechanism underlying the favorable metabolic effect of lacidipine. Accordingly, Sowers and co-workers suggested that cation derangement contributes significantly to the genesis of insulin resistance in several clinical conditions, including type 2 diabetes and essential hypertension (26). As for ACEI, it has been suggested that, besides the vasodilatory effect, the reduced degradation of bradykinin after ACE inhibition might exert favorable metabolic effects due to an insulin-like activity of this molecule (27).
During insulin clamp, the suppressive effect of insulin on endogenous glucose production was greater during lacidipine than during lisinopril. The identification of the molecular mechanism of this effect is beyond the scope of our study. However, it might be related to the intracellular calcium concentration within the liver and, perhaps, the kidney. Accordingly, it has been reported that the intracellular calcium concentration can increase gluconeogenesis (28) and glycogenolysis (29), and that infusion of a calcium channel blocker (verapamil) into the rat liver diminishes hepatic glucose production (30). On the other hand, an increased cytosolic free calcium concentration has been related to insulin resistance in human adipocytes, a defect reversed by verapamil (31).
Whatever the mechanisms of action of these molecules, the results of the present study document that treatment with either lacidipine or lisinopril in subjects with type 2 diabetes exerts a beneficial effect on insulin-stimulated glucose metabolism by increasing the rate of nonoxidative glucose disposal, mainly glycogen synthesis. This aspect is particularly important, because a diminished glycogen synthesis within the skeletal muscle is one of the hallmarks of noninsulin-dependent diabetes mellitus (32) and seems to be the unique metabolic defect featuring essential hypertension (33). Lacidipine also seems to improve the suppression of endogenous glucose production by insulin, i.e. another defect featuring type 2 diabetes (32).
We were not able to observe a better degree of metabolic control after treatment with lacidipine or lisinopril. In fact, fasting glucose levels and glycated hemoglobin concentrations were not lowered by treatment. This finding is consistent with those obtained by other investigators who examined the metabolic effects of antihypertensive agents of the same class in diabetic subjects (8, 9, 10, 24). Such results might suggest that a mild to moderate increase in insulin sensitivity for a few months is not sufficient to yield a substantial amelioration of the daily glucose profile. Alternatively, it might be hypothesized that the beneficial effect on insulin sensitivity could have been counterbalanced by an adverse effect on insulin secretion. The slightly lower insulin and C peptide responses to oral glucose load might support this conclusion. However, the failure to observe a better metabolic control after 4 months of antihypertensive treatment should not deprive our results of clinical relevance, as an increased insulin sensitivity might exert positive effects on glucose control over a longer period and might result in other beneficial effects, for instance on atherosclerosis. As a matter of fact, long term improvement of insulin sensitivity secondary to treatment with lacidipine or lisinopril is expected to be associated with less hyperinsulinemia, a well known independent risk factor for cardiovascular disease (34, 35, 36, 37). Moreover, insulin resistance itself seems to be an independent risk factor for atherosclerosis (38, 39, 40, 41, 42), and its partial correction might be beneficial for the development of cardiovascular disease.
The results of two recent studies suggested that ACEI might be preferable over CCB in hypertensive diabetic patients because the former would result in a more favorable cardiovascular outcome on a long term basis (43, 44). Apart from the caution needed in the interpretation of these studies (45), we might conclude from our present data that the advantage of ACEI vs. CCB in terms of cardiovascular events, if any, is not mediated by differences in insulin resistance changes during the treatment.
In conclusion, our data indicate that blood pressure can be lowered successfully with either lacidipine or lisinopril in type 2 diabetes, with the confidence that no adverse metabolic effects will occur. On the contrary, either drug seems to have favorable effects on insulin resistance by increasing the stimulus exerted by insulin on nonoxidative glucose disposal and/or the inhibition exerted by the hormone on endogenous glucose production.
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Acknowledgments |
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Footnotes |
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Received June 10, 1998.
Revised January 8, 1999.
Accepted February 10, 1999.
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References |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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-adrenergic blockers on
glucose and lipid metabolism in NIDDM patients with hypertension. Diabetes. 44:665–671.[Abstract]
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