CURRENT THERAPEUTIC RESEARCH® VOL. 59, NO. 12, DECEMBER1998

EFFECT OF ENALAPRIL ON INSULIN SENSITIVITY IN PATIENTS WITH HYPERTENSION AND TYPE 2 DIABETES MELLITUS: A PILOT STUDY USING THE GLUCOSE CLAMP TECHNIQUE SATORU TSUJII1 AND HIDESHI K U Z U Y A 2 rDiabetes Center, Tenri Yorozu-Sodansho Hospital, Nara, and 2Institute of Endocrine and Metabolic Disease, Division of Clinical Research, Kyoto National Hospital, Kyoto, Japan

ABSTRACT Enalapril is the most widely used angioteusin-converting enzyme (ACE) inhibitor in Japan, but few studies have examined its effect on insulin sensitivity in Japanese patients. Therefore, we used the glucose clamp technique to study the effects o f enalapril on insulin sensitivity in nine patients with hypertension and type 2 diabetes mellitus (non-insulin-dependent diabetes mellitus [NIDDM]). Sixteen weeks after beginning t r e a t m e n t with enalapril, systolic and diastolic blood pressures had decreased significantly compared with baseline values; no significant changes were seen in glycemic control, body-mass index, or heart rate. During treatment, glucose infusion rates (M values) increased significantly, reflecting the degree of insulin sensitivity during the low insulinemic, hyperglycemic glucose clamp phase. In contrast, rates did not increase significantly during the high insnlinemic, euglycemic glucose clamp phase. N o significant changes in either insulin secretion or metabolic insulin clearance were observed. In addition, ACE activity significantly decreased throughout the glucose clamp procedure. Enalapril treatment did not have a significant effect on plasma angiotensin II conc e n t r a t i o n s across the glucose clamp; plasma bradykinin levels increased significantly, but only immediately before the clamp. The results o f this study suggest that enalapril may affect insulin sensitivity through its effect on bradykiniu levels rather than insulin secretion, thus improving glucose disposal. K e y w o r d s : t y p e 2 diabetes mellitus, hypertension, enalapril, insulin sensitivity, glucose clamp.

INTRODUCTION Two of the most common disorders in Japan are hypertension and type 2 diabetes mellitus (non-insulin-dependent diabetes mellitus [NIDDM]). Approximately 40% of patients with type 2 diabetes mellitus also have h y p e r t e n s i o n . 1 I n s u l i n s e n s i t i v i t y is believed to be r e d u c e d in p a t i e n t s w i t h h y p e r t e n s i o n a n d t y p e 2 diabetes mellitus; therefore, t r e a t m e n t of p a t i e n t s Address correspondence to: Satoru Tsujii, MD, Diabetes Center, Tenri Yorozu-SodanshoHospital, 200 Mishima, Tenri, Nara 632-0015,Japan. Received for publication on June 24, 1998. Printed in the USA. Reproduction in whole or part is not permitted. 863

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with hypertension and type 2 diabetes mellitus requires the use of an antihypertensive agent that consistently reduces blood pressure without affecting insulin sensitivity. Of the antihypertensive agents currently available, angiotensin-converting enzyme (ACE) inhibitors, alphaadrenergic antagonists, and several calcium channel blockers reportedly improve insulin sensitivity. 2'3 Insulin sensitivity is generally estimated based on blood glucose and insulin concentrations during glucose loading. The minimal model method,4 steady-state plasma glucose evaluation, 5 and the glucose clamp technique were designed for the quantitative assessment of insulin sensitivity. Although enalapril is the most commonly used ACE inhibitor both in Japan and worldwide, few studies in Japanese patients have examined the effect of this drug on insulin sensitivity using the glucose clamp technique. 6 Therefore, we investigated the effect of enalapril on insulin sensitivity in Japanese patients with hypertension and type 2 diabetes mellitus by means of the glucose clamp technique. PATIENTS AND METHODS Patients

Nine patients (6 men and 3 women; mean age, 55.9 + 9.4 years; mean body-mass index, 25.2 + 2.2 kg/m2) with both hypertension and type 2 diabetes mellitus were enrolled in the study. Glycated hemoglobin Ale (Hb AI~) measurements in these patients indicated that glycemic control had been stable without antidiabetic medication for 8 weeks. All patients gave written informed consent before participating in the study. Methods

Patients participated in a run-in period of at least 4 weeks before beginning treatment with enalapril 5 mg once daily. During this time, height, body weight, blood pressure, heart rate, I-Ib Ale level (by highperformance liquid chromatography; Kyoto Daiichi Kagaku Co., Kyoto, Japan), and fasting plasma glucose levels (by the glucose oxidase method) were measured. A 90-minute hyperglycemic clamp phase followed by a 90-minute euglycemic glucose clamp phase were integrated to study insulin sensitivity according to a previously described method. 7 Serum ACE activity (by the method of Kasahara and Ashihara s) and plasma concentrations of angiotensin II (AID and bradykinin (BK) (by radioimmunoassay; SRL Inc., Tokyo, Japan) were measured immediately before the clamp procedure and at the end of the hyperglycemic clamp and the euglycemic clamp phases. After 16 weeks of enalapril treatment, all measurements and the glucose clamp technique were repeated. Patients were instructed to refrain 864

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from taking enalapril on the morning of the glucose clamp procedure and blood sampling. Neither the use of antihypertensive agents other than enalapril nor the use of concomitant drugs for the treatment of complications of diabetes was permitted during the study period.

Glucose Clamp Technique After a 12-hour overnight fast, the glucose clamp procedure was started using an artificial pancreas (Model STG-22; Nikkiso Co., Ltd., Tokyo). The hyperglycemic glucose clamp procedure (target blood glucose level: fasting blood glucose level + 125 mg/dL) was performed for 90 minutes, followed by the euglycemic clamp procedure (target blood glucose level: 100 mg/dL) for 90 minutes with the aim of producing steady insulin concentrations of 100 ~U/mL following the algorithm described by DeFronzo et al. 7 Serum immunoreactive insulin (IRI) concentrations were determined by radioimmunoassay every 5 minutes for the first 15 minutes and every 15 minutes thereafter until the end of the clamp procedure. The M value, defined as the mean glucose infusion rate during the last 30 minutes of both clamp phases, was used as an indicator of insulin sensitivity. The area under the serum IRI concentration-time curve (AUCIRI) during the 90-minute hyperglycemic glucose clamp phase was calculated according to the trapezoidal rule and used as an indicator of insulin secretion. The insulin clearance rate during the 90-minute euglycemic clamp phase was calculated using the method described by Elahi et al. 9

Statistical Analysis All data are expressed as mean + SD. Measurements taken before and after enalapril treatment were compared using the paired t test if the measured values were normally distributed and the Wilcoxon signed-rank test if they were not. The relationships between variables were investigated using the Pearson product moment correlation. A probability level of <5% was considered significant. RESULTS

Table I shows blood pressure changes during enalapril treatment. The mean values for systolic blood pressure, diastolic blood pressure, and mean blood pressure during treatment were significantly lower than those before treatment, and decreases in all values at 8 weeks were maintained throughout the remainder of the 16-week treatment period. Body-mass index and heart rate did not change significantly during the study period, and no significant changes in mean Ht) Ale or mean fasting plasma glucose levels were noted during the 16-week treatment period (Table I). 865

ENALAPRIL A N D INSULIN SENSITIVITY IN H Y P E R T E N S I V E PATIENTS W I T H N I D D M

Table I. Changes in body-massindex (BMI), glycemiccontrol, heart rate, and blood pressure during enalapril treatment. Data are expressed as mean + SD. Treatment Duration

BMI {kg/m2} Hb A~ c- ,o~-(Vo) Fasting plasma glucose (mg/dL) Heart rate (beats/rain) Blood pressure Systolic (ram Hg) Diastolic (mm Hg) Mean (mm Hg)

Week 0

Week 4

Week 8

Week 12

Week 16

25.2 + 2.2 5.8 ± 0.7 106 + 17 76 ± 12

26.3 + 2.1

26.1 ± 2.1

26.4 ± 22

76 ± 9

78 ± 16

82 ± 22

25.0 + 2.4 5.7 ± 0.8 118 ± 25 80 ± 12

160±10 99 ± 4 119±5

140±15" 90 ± 75 106±55

133 + 111. 89 ± 8* 104±41.

140+115 86 ± 8t 104±45

132±121" 82 ± 125 9g~: 111.

Hb Alc = glycated hemoglobin Alc. *P< 0.05, tP< 0.001, -tP< 0.01 versus week O, by the paired t test.

Figure 1 shows changes in M values betv~een baseline and week 16, and Figure 2 shows changes in AUC-IRI and insulin clearance rates calculated from the data obtained during the glucose clamp procedure. Serum insulin concentrations during the final 30 minutes of the hyperglycemic and the euglycemic glucose clamp phases did not change significantly during enalapril treatment (hyperglycemic, 26.0 ± 15.1 ~U/mL vs 25.1 ± 17.0 }~U/mL; euglycemic, 87.6 ± 33.4 ~U/mL vs 78.5 ± 28.2 ~U/mL). However, the M values during the hyperglycemic glucose clamp phase increased significantly from 4.75 ± 2.56 mg/kg per minute at week 0 to 5.66 + 3.44 mg/kg per minute at week 16 (P < 0.05), whereas no significant change was seen in M values during the euglycemic glucose clamp phase (6.49 ± 3.18 mg/kg per minute vs 6.23 ± 2.45 mg/kg per minute). In addition, differences were not significant between pretreatment and posttreatment AUCIRI values measured during the hyperglycemic glucose clamp phase (1856 + 1179 ~U/mL per minute and 1859 ± 1326 ~U/mL per minute, respectively), nor was there any consistent pattern of change in the insulin clearance rate during the euglycemic glucose clamp phase (0.62 ± 0.16 mL/m 2 per minute before treatment vs 0.76 ± 0.39 mL/m 2 per minute during treatment). Changes in serum ACE activity and plasma AII and BK levels during the glucose clamp procedure are shown in Table II. Posttreatment ACE activity levels were significantly lower than pretreatment levels before the clamp procedure and at the end of the hyperglycemic clamp and euglycemic clamp phases (P < 0.05). AII levels did not change significantly during treatment or during the clamp procedure, although the values immediately before the clamp procedure were lower at week 16 than at week 0. BK values immediately before the clamp procedure increased significantly (P < 0.05) with enalapril treatment but did not change from baseline at the end of either the hyperglycemic or euglycemic clamp phase. Analysis of the correlation coefficient between the variables measured 866

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Figure 2. Changes in (A) area under the serum immunoreactive insulin concentration-time curve (AUC-IRI) during the hyperglycemic glucose clamp phase and (B) insulin clearance rate during the euglycemic glucose clamp phase before and during enalapril treatment. a c t i v i t y or A I I levels w a s noted. T h e c o r r e l a t i o n b e t w e e n p l a s m a B K levels a n d A U C - I R I v a l u e s w a s positive a n d significant (Figure 4). I n c o n t r a s t , no significant c o r r e l a t i o n b e t w e e n p o s t t r e a t m e n t c h a n g e s in p l a s m a B K levels a n d p o s t t r e a t m e n t c h a n g e s in blood p r e s s u r e levels w a s noted. 868

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Table II. Changes in angiotensin-converting enzyme (ACE) activity a n d plasma angiotensin II (AID a n d bradykinin (BK) levels during enalapril treatment. Data are expressed as m e a n ± SD.

Treatment Week

Clamp Phase Before

ACE Activity (IU/L per 37°C)

0 16 0 16 O 16

Hyperglycemic Euglycemic

10.8 6.9 11.0 7.5 9.9 6.8

All (pg/mL)

+ 4.3 ± 4.0* ± 4.2 + 3.6* ± 4.7 ± 2.7*

25.0 16.0 13.8 17.9 22.9 18.9

+ ± + + ± ±

18.8 10.6 7.1 13.9 22.7 15.5

BK (pg/mL) 22.2 39.8 19.1 23.4 19.2 19.7

± ± ± ± ± ±

8.1 24.1 * 5.5 8.5 6.8 9.6

*P < 0.05 versus week O, by the paired t test.

DISCUSSION

Two studies :°'1: using the glucose clamp technique have suggested that ACE inhibitors improve insulin sensitivity by reducing sympathetic adrenergic activity, in association with a decrease in AII production, and exhibiting insulinlike activity by causing an increase in local BK levels or 80

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C h a n g e in Plasma BK Level ( % ) Figure 3. Correlation between p o s t t r e a t m e n t changes in plasma bradykinin (BK) level immediately before t h e glucose clamp procedure a n d p o s t t r e a t m e n t changes in glucose infusion rate (M value) during the hyperglycemic clamp phase. The distribution of percent changes in the two variables a n d the regression line (Y = 0.189X + 2.89) are shown. (A positive correlation was evident between these variables [r = .590; P = 0.095].) 869

ENALAPRILAND INSULIN SENSITIVITY IN HYPERTENSIVE PATIENTS WITH NIDDM

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Plasma BK Level (pg/mL) F i g u r e 4. C o r r e l a t i o n b e t w e e n a r e a u n d e r t h e s e r u m i m m u n o r e a c t i v e i n s u l i n c o n c e n t r a t i o n t i m e c u r v e (AUC-IRI) a n d p l a s m a b r a d y k i n i n (BK) level i m m e d i a t e l y before t h e glucose c l a m p procedure. T h e d i s t r i b u t i o n of t h e two v a r i a b l e s a n d t h e r e g r e s s i o n line (Y = 39X + 639) a r e s h o w n . (A s i g n i f i c a n t positive correlation w a s e v i d e n t b e t w e e n t h e s e v a r i a b l e s [r = .640; P < 0.05].)

increasing glucose uptake by skeletal muscle through peripheral vasodilatory activity. However, the precise mechanism by which ACE inhibitors alter insulin sensitivity remains to be determined. In the present study, systolic and diastolic blood pressures decreased significantly in patients with type 2 diabetes mellitus during 16 weeks of enalapril treatment, but changes in glycemic control, body-mass index and heart rate were not significant. However, M values increased significantly, reflecting the degree to which insulin sensitivity improved during the low insulinemic, hyperglycemic clamp phase. The absence of any significant change in Hb Ale and fasting plasma glucose levels during treatment could be explained by the presence of many factors in addition to medication that can influence glycemic control in patients with diabetes, such as diet, physical activity, and stress, although these conditions were supposed to be similar before and during treatment. The Appropriate Blood Pressure Control in Diabetes trial 12 showed a similar trend in Hb Ale and total cholesterol concentrations during a 5-year period in patients assigned to receive enalapril and nisoldipine, the two study medications; there appeared to be no significant change in Hb A1¢ levels for at least 3 years during enalapril treatment. This study also indicated that enalapril reduces the incidence of cardiovascular events fivefold compared with nisoldipine. 870

S. TSUJII AND H. KUZUYA

In the present study, no consistent pattern of change in AUC-IRI levels or insulin clearance rates was evident, suggesting that enalapril may act on glucose metabolism by improving insulin sensitivity rather than by promoting insulin secretion. During the treatment period, ACE activity decreased significantly at each time point during the glucose clamp procedure, and AII levels immediately before the clamp procedure fell nonsignificantly. The lack of correlation between changes in these variables and changes in M values suggests that the mechanism responsible for the enalapril-induced improvement in insulin sensitivity does not involve the renin-angiotensin system. The findings that plasma BK levels immediately before the clamp procedure increased significantly during enalapril treatment and that a positive correlation was evident between changes in M values during the hyperglycemic clamp phase and changes in plasma BK levels immediately before the clamp procedure suggest that the improvement in insulin sensitivity produced by enalapril may be induced by an increase in plasma BK levels caused by inhibition of kinin degradation. In addition, we found a significant positive correlation between AUCIRI values and plasma BK levels, suggesting a possible relationship between plasma BK levels and the promotion of insulin secretion. We hypothesize that the improvement in insulin sensitivity produced by enalapril might have an effect on insulin secretion that is related to changes in BK levels, although insulin secretion did not increase after enalapril treatment in this study. Haenni et a113 used the euglycemic clamp technique to study the effect of four ACE inhibitors on insulin sensitivity in 22 obese patients with essential hypertension. During treatment with enalapril 10 to 20 mg/d for 26 weeks, M values increased by 6%; this increase was not significant. Vuorinen-Markkola and Yki-J/irvinen la reported a significant reduction in ACE activity, a 30% improvement in insulin sensitivity, and an improvement in Hb A1¢ levels during 4 weeks of treatment with enalapril 20 to 40 mg/d. During enalapril treatment in the present study, ACE activity was reduced significantly and BK levels increased; M values during the euglycemic and hyperglycemic clamp phase were increased by 3% and 57%, respectively. The absence of a significant improvement in M values during the euglycemic clamp phase might be attributed to the small dose (5 mg) of enalapril given, the wide variation in the degree of insulin resistance of the study patients, and perhaps the small sample size. Large, long-term, randomized, controlled studies are needed in Japan to provide further insight into the effect of enalapril treatment on insulin sensitivity and other metabolic processes. CONCLUSIONS

Insulin sensitivity improved during enalapril treatment, as evidenced by higher glucose infusion rates during the hyperglycemic glucose clamp 871

ENALAPRILANDINSULINSENSITIVITYIN HYPERTENSIVEPATIENTSWITHNIDDM

phase, which were associated w i t h i n c r e a s e d p l a s m a B K levels. T h e findings of this s t u d y s u g g e s t t h a t e n a l a p r i l m a y a l t e r i n s u l i n s e n s i t i v i t y t h r o u g h a n effect o n B K l e v e l s r a t h e r t h a n a n effect o n i n s u l i n s e c r e t i o n , thus improving glucose disposal.

Acknowledgment This study was supported by a research grant from Banyu Pharmaceutical Co., L t d . , T o k y o , J a p a n . References:

1. Kuzuya T, Akonuma Y, Akazawa Y, et al. Prevalence of chronic complications in Japanese diabetic patients. Diabetes Res Clin Pract. 1994:24(Suppl):S159-S164. 2. Rett K, Jauch KW, Wicklmayr M, et al. Angiotensin converting enzyme inhibitors in diabetes: Experimental and human experience. Postgrad Med J. 1986;62(Suppl 1):59-64. 3. Pollare T, Lithell H, Selinus I, et al. Application ofprazosin is associated with an increase of insulin sensitivity in obese patients with hypertension. Diabetologia. 1988;31:415-420. 4. Bergman RN, Prager R, Volund A, Olefsky JM. Equivalence of the insulin sensitivity index in man derived by the minimal model method and the euglycemic glucose clamp. J Clin Invest. 1987;79:790-800. 5. Harano Y, Ohgaku S, Hidaka H, et al. Glucose, insulin and somatostatin infusion for the determination of insulin sensitivity. J Clin Endocrinol Metab. 1977;45:1124-1127. 6. Uehara M, Kishikawa H, Isami S, et al. Effect of insulin sensitivity of angiotensinconverting enzyme inhibitors with or without a sulphydryl group: Bradykinin may improve insulin resistance in dogs and humans. Diabetologia. 1994;37:300-307. 7. DeFronzo RA, Tobin JD, Andres R. Glucose clamp technique: A new method for quantifying insulin secretion and resistance. A m J Physiol. 1979;237:E214-E223. 8. Kasahara Y, Ashihara Y. Colorimetry of angiotensin-I converting enzyme activity in serum. Clin Chem. 1981;27:1922-1925. 9. Elahi D, Nagulesparan M, Hershcopf RJ, et al. Feedback inhibitionof insulin secretion by insulin: Relation to the hyperinsulinemia of obesity. NEJM. 1982;306:1196-1202. 10. Rett K, Lotz N, Wicklmayr M, et al. Improved insulin action by ACE inhibition in type II diabetes. Dtsch Med Wochenschr. 1988;113:243-249. 11. Berne C, Pollare T, Lithel H, et al. Effects of antihypertensive treatment on insulin sensitivity with special reference to ACE inhibition. Diabetes Care. 1991;14(Suppl 4):3947. 12. Estacio RO, Jeffers BW, Hiate WR, et al. The effect of nisoldipine as compared with enalapril on cardiovascular outcomes in patients with non-insulin dependent diabetes and hypertension. NEJM. 1998;338:645-652. 13. Haenni H, Berlund L, Reneland R, et al. The alterations in insulin sensitivity during angiotensin converting enzyme inhibitor treatment are related to changes in the calcium/ magnesium balance. A m JHypertens. 1997;10:145-151. 14. Vuorinen-Markkola H, Yki-J~irvinen H. Antihypertensive therapy with enalapril improves glucose storage and insulin sensitivity in hypertensive patients with non-insulindependent diabetes mellitus. Metabolism. 1995;44:85-89. 872