Exercise-induced changes in insulin action are associated with ACE ...

Physiol Genomics 11: 73–80, 2002. First published September 11, 2002; 10.1152/physiolgenomics.00048.2002.

Exercise-induced changes in insulin action are associated with ACE gene polymorphisms in older adults DONALD R. DENGEL,1,2 MICHAEL D. BROWN,3 ROBERT E. FERRELL,4 THOMAS H. REYNOLDS IV,3 AND MARK A. SUPIANO3 1 School of Kinesiology, University of Minnesota, Minneapolis 55455; the 2Minneapolis Veterans Affairs Medical Center, Minneapolis, Minnesota 55417; 2Department of Internal Medicine, Division of Geriatric Medicine, and Geriatric Research, Education and Clinical Center (GRECC), Ann Arbor Veterans Affairs Medical Center, Ann Arbor, Michigan 48105; and 4 Department of Human Genetics, University of Pittsburgh Graduate School of Public Health, Pittsburgh, Pennsylvania 15261 Received 23 April 2002; accepted in final form 4 September 2002

Dengel, Donald R., Michael D. Brown, Robert E. Ferrell, Thomas H. Reynolds IV, and Mark A. Supiano. Exercise-induced changes in insulin action are associated with ACE gene polymorphisms in older adults. Physiol Genomics 11: 73–80, 2002. First published September 11, 2002; 10.1152/physiolgenomics.00048.2002.—We evaluated the association between insulin resistance and the angiotensin-converting enzyme (ACE) insertion (I)/deletion (D) gene polymorphism in a group of older hypertensive subjects (63 ⫾ 1 yr, n ⫽ 35) before and after a 6-mo aerobic exercise program (AEX). Insulin sensitivity index (SI), assessed by the frequently sampled intravenous glucose tolerance test, was significantly (P ⫽ 0.0001) increased following AEX. In addition, there was a significant (P ⫽ 0.001) interaction between AEX and ACE genotype. SI increased significantly (P ⬍ 0.05) more in those with the II (2.5 ⫾ 0.8 ␮U ⫻ 10⫺4 䡠 min⫺1 䡠 ml⫺1) ACE genotype compared with both the DD and ID (0.7 ⫾ 0.1 and 0.7 ⫾ 0.2 ␮U ⫻ 10⫺4 䡠 min⫺1 䡠 ml⫺1, respectively) ACE genotypes. Similarly, there was a significant (P ⫽ 0.036) decrease in the acute insulin response to glucose (AIRG) and a significant (P ⫽ 0.05) interaction between AEX and ACE genotype. AIRG decreased significantly (P ⬍ 0.05) more in those with the II (⫺17.6 ⫾ 5.6 mU/ml) ACE genotype compared with both the DD and ID (⫺1.4 ⫾ 6.2 and ⫺3.6 ⫾ 2.5 mU/ml) ACE genotypes. In conclusion, we demonstrated that those older hypertensives with the ACE II genotype have the greatest improvement in insulin action following AEX. angiotensin-converting enzyme; blood pressure; genetics; glucose metabolism

THE ANGIOTENSIN-CONVERTING ENZYME

(ACE) plays a key role in regulating blood pressure (BP) as well the regulation of vascular tone through the activation of the vasoconstrictor angiotensin II (35, 45) and inactivation of the vasodilatory peptide bradykinin (10). Interindividual differences in serum ACE levels are due in part to the presence of an insertion/deletion (I/D) polymorphism in intron 16 of the ACE gene. Individu-

als homozygous for the deletion allele (DD) have serum ACE levels almost twice that of individuals homozygous for the insertion allele (II) (40, 43). In addition, a number of studies have identified the presence of the DD genotype as a significant risk factor for myocardial infarction and cardiovascular disease (3, 6, 44). Insulin resistance is frequently associated with hyperinsulinemia, increased BP, and elevated cholesterol levels. This constellation of metabolic as well as cardiovascular diseases is commonly referred to as the insulin resistance syndrome. The exact mechanism(s) responsible for the development of the insulin resistance syndrome is unknown; however, genetic factors may exert some influence. Pharmacological inhibitors of ACE have been shown to improve glucose metabolism (22, 38). Recently, the D allele of the ACE gene has been associated with insulin resistance in nevertreated hypertensives (37). This would imply that the ACE gene is related to insulin resistance; however, it should be noted that other studies have reported the I allele of the ACE gene is associated with insulin resistance (34, 41). In addition, Jeng et al. (23) reported no difference in insulin sensitivity between those individuals with the D or I allele of the ACE gene. A sedentary lifestyle has been shown to result in a decrease in insulin sensitivity (17). We have previously shown in middle-aged to older normotensive subjects as well as in hypertensive subjects that aerobic exercise (AEX) training significantly increases insulin sensitivity (8, 9, 39). However, the increase in insulin sensitivity in response with exercise training is highly variable. Thus, based upon our previous studies, we sought to evaluate the hypothesis that ACE DD genotype is associated with insulin resistance and may play a role in explaining the variability in the exercise training-induced increase in insulin sensitivity. METHODS

Article published online before print. See web site for date of publication (http://physiolgenomics.physiology.org). Address for reprint requests and other correspondence: D. R. Dengel, Univ. of Minnesota, 1900 Univ. Ave. SE, 110 Cooke Hall, Minneapolis, MN 55455 (E-mail: [email protected]).

Study Population Thirty-five subjects (14 males and 21 females) with mild hypertension were recruited for study. Subjects were recruited through newspaper advertisement, from the Univer73

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ACE GENE POLYMORPHISM, INSULIN ACTION, AND EXERCISE

sity of Michigan Turner Geriatric Clinic, and from the Human Subject Research Participation Core of the University of Michigan Geriatrics Center. All subjects were community dwelling and in good health apart from their hypertension. Subjects were screened prior to participation with a medical history and physical examination, a complete blood count, and routine chemistries, and a urinalysis. Individuals were excluded from the study if they had clinically significant concomitant medical illness such as cardiac, renal (serum creatinine greater than 135 mmol/l), hepatic, or gastrointestinal disease, or required medications that might affect glucose metabolism, BP, or renal function. Also excluded were individuals with a recent history of smoking or drug or alcohol abuse, or clinically relevant mental health disorders. Absence of diabetes mellitus according to American Diabetes Association criteria (2) was confirmed in all subjects by a 2-h, 75-g oral glucose tolerance test. The presence of hypertension was defined in subjects who were receiving antihypertensive treatment or had a seated systolic BP ⱖ140 mmHg and/or a seated diastolic BP ⱖ90 mmHg (25). General Study Protocol Following a screening visit to determine their eligibility for participation as described above, subjects signed an informed consent form approved by the University of Michigan Institutional Review Board. Hypertensive subjects who were being treated with antihypertensive medications were tapered off their medications and were studied following a 4-wk period during which no antihypertensive medications were taken. Measurement of body composition. The waist-to-hip circumference ratio (WHR) was calculated as the ratio of the minimal circumference of the abdomen to the circumference of the buttocks at the maximal gluteal protuberance. Body fat, lean body mass (LBM), and percent body fat were determined by dual-energy X-ray absorptiometry (DXA, model DPX-IQ; Lunar Radiation, Madison, WI). ˙ O2 max). A Measurement of maximal oxygen consumption (V maximal exercise test was performed at baseline, after 3 mo of exercise training, and again after 6 mo of exercise training. The initial treadmill speed was set to elicit 75% of each ˙ O2 max measured during their screening treadmill subject’s V test. The treadmill elevation was increased every 2 min until ˙ O2 and the subject was exhausted and could not continue. V ˙ CO2) were measured continucarbon dioxide production (V ously, and BP and a 12-lead electrocardiogram were recorded ˙ O2 max was considered to every 3 min during the test. A true V be attained if two of the following three criteria were achieved: 1) respiratory exchange ratio greater than 1.10, 2) maximal heart rate greater than 90% of age-predicted max˙ O2 (change in V ˙ O2 imum (220 ⫺ age), and 3) a plateau in V ⬍0.2 l/min). AEX training protocol. Exercise training consisted of three sessions per week of supervised treadmill walking. Target heart rate was calculated for each individual with the equation of Karvonen et al. (27). The intensity and duration of exercise was progressively increased so that subjects completed 40 min per session at 75–85% of their heart rate reserve for the last 3 mo of training. Compliance with the training program was ⬃91%. Measurement of BP. Three BP measurements were made 1 wk apart in the morning (0700–0900 h) by auscultation using the appropriate cuff size. Subjects had been seated comfortably for ⬎15 min with the cuffed arm supported at heart level before measurements were taken. The mean of these three BP measurements is reported. Physiol Genomics • VOL

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Frequently sampled intravenous glucose tolerance test. Prior to the frequently sampled intravenous glucose tolerance test (FSIVGTT), subjects were placed on a controlled diet for 7 days. The diets at baseline and after exercise training were identical in carbohydrate (50–55%), fat (30– 35%), protein (15–20%), and sodium (200 mmol/day). All meals during the 7-day diet period were prepared by the University of Michigan General Clinical Research Center (GCRC) Metabolic Kitchen. The FSIVGTT was performed as previously described by Bergman (4). In all subjects the FSIVGTT included an injection of insulin (Humulin-R; Eli Lilly, Indianapolis, IN) to enhance precision of the estimates of insulin action (50). Subjects were studied in the supine position. Briefly, an intravenous catheter was inserted into an antecubital vein in one arm for the injection of insulin and glucose. Another catheter was inserted in a retrograde manner into a dorsal hand vein of the contralateral arm, which was placed in a thermostatically controlled (60°C) warming box to arterialize venous blood samples for the measurement of glucose and insulin (15). Catheters were kept patent by a slow infusion of 0.45% saline (⬍50 ml/h). Beginning 20 min after the insertion of intravenous lines, three baseline blood samples for glucose and insulin were obtained, and BP and heart rate were measured at 5-min intervals. Baseline values were calculated as the mean of these three measurements for each variable. Fifty percent glucose (300 mg/kg) was given as an intravenous push over 30 s. Blood samples (3 ml) were collected at 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 19, 22, 23, 24, 25, 27, 30, 40, 50, 70, 80, 90, 100, 120, 140, 160, and 180 min after the glucose bolus. Insulin (0.02 U/kg) was given intravenously over 30 s, 20 min after the glucose injection to further stimulate insulin secretion. Blood samples for plasma glucose and insulin were collected into chilled glass tubes containing heparin sodium, stored on ice, and separated immediately following each study. Plasma was stored at ⫺70°C until assay. Plasma glucose was measured by the autoanalyzer glucose oxidase method and plasma insulin by RIA in the Core Laboratory of the Michigan Diabetes Research and Training Center. Samples from each of the subject’s two studies were analyzed together in the same assay. The insulin sensitivity index (SI) and glucose effectiveness (SG) were calculated from a least-squares fitting of the temporal pattern of glucose and insulin throughout the FSIVGTT using the MINMOD program (4). SI is a measure of the effect of an increment in plasma insulin to enhance the fractional disappearance of glucose. SG is a measure of the fractional glucose turnover rate at the basal insulin level. The acute insulin response to intravenous glucose (AIRG) was calculated as the mean rise in plasma insulin above basal from 2 to 10 min after intravenous glucose administration. Disposition index (DI) was calculated as the product of AIRG and SI. KG, a measure of glucose tolerance, is the rate of plasma glucose disappearance calculated as the least square slope of the natural logarithm of absolute glucose concentration between 10 and 20 min after the glucose bolus (a normal nondiabetic value for KG is greater than 1%/min). The reproducibility for the minimal model approach for determining insulin sensitivity has been reported to be ⬃16% (1, 12). ACE genotyping. High-molecular-weight genomic DNA was isolated from EDTA-anticoagulated whole blood by standard procedures (32). Subjects were genotyped for the ACE intron 16 Alu insertion by the method of Tiret et al. (46). The I (insertion) allele yields a fragment of 490 bp, and the D (deletion) allele yields a product of 190 bp. Heterozygotes were characterized by the presence of both bands plus a www.physiolgenomics.org

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slower migrating heteroduplex. Alleles were scored by direct comparison to sequence-verified controls run on the same gel, and subjects were classified as II, ID, or DD. DD genotypes were confirmed using deletion-specific primers as described by Lee and Tsai (31). Statistical analysis. Data were analyzed using Statview (Abacus Concepts, Berkeley, CA). An alpha level of 0.05 was accepted for statistical significance. Comparison between the characteristics of the ACE genotype groups was made using analysis of variance. A repeated measures two-way ANOVA with group (ACE genotype) as one variable and training status (pre- and post-exercise) as the other was utilized to examine within and between group differences. All data are reported as the means ⫾ SE. RESULTS

Baseline Physical characteristics of subjects. The distribution of the ACE genotypes in this sample population of older hypertensives was 23% II, 57% ID, and 20% DD (Table 1). This distribution is similar to what has been reported in the general population (II, 23%; ID, 49%; DD, 38%) (42). Baseline subject characteristics for the three ACE genotypes are presented in Table 1. There were no statistically significant differences in age, weight, percent body fat, body mass index (BMI), or WHR at baseline between the three ACE genotype groups. Glucose metabolism. At baseline, SI was significantly (P ⬍ 0.05) lower in those individuals with DD compared with the ID ACE genotype (Fig. 1). There was no significant difference in SI between the ID and II ACE genotypes. No significant differences were found in the AIRG (P ⫽ 0.933), DI (P ⫽ 0.346), KG (P ⫽ 0.942), or SG (P ⫽ 0.115) between the three ACE genotype groups at baseline (Table 2). Blood pressure. At baseline there was no significant difference in mean arterial (P ⫽ 0.753), systolic (P ⫽ 0.355), or diastolic (P ⫽ 0.936) BP between the three ACE genotypes (Table 1).

Table 1. Characteristics of subjects at baseline

Number/distribution (%) Sex (M/F) Ethnicity (African American/White) Age, yr Height, cm Weight, kg BMI, kg/m2 Body fat, % WHR Blood pressure, mmHg Systolic Diastolic Mean arterial ˙ O2max, ml 䡠 kg⫺1 䡠 min⫺1 V

II

ID

DD

8 (23%) 3/5

20 (57%) 8/12

7 (20%) 3/4

0/8 63 ⫾ 2 165 ⫾ 5 81 ⫾ 7 28.5 ⫾ 1.8 37.8 ⫾ 1.5 0.88 ⫾ 0.03

4/16 62 ⫾ 2 168 ⫾ 2 82 ⫾ 3 28.1 ⫾ 0.9 38.4 ⫾ 2.3 0.87 ⫾ 0.03

1/6 65 ⫾ 2 166 ⫾ 4 89 ⫾ 7 32.4 ⫾ 1.8 41.7 ⫾ 3.2 0.85 ⫾ 0.02

155 ⫾ 4 89 ⫾ 4 112 ⫾ 4 19.1 ⫾ 1.2

149 ⫾ 2 88 ⫾ 2 109 ⫾ 2 18.1 ⫾ 0.9

155 ⫾ 5 88 ⫾ 2 111 ⫾ 4 16.3 ⫾ 1.0

Effects of AEX Training Intervention Changes in physical characteristics (Table 3). Thirtyone of the thirty-five subjects completed the AEX training program. Three subjects dropped out due to time constraints of the AEX training program and one subject dropped out due to unrelated medical reasons. There was a small (1%) but significant (P ⫽ 0.038) decrease in body weight following the 6-mo program of AEX in all genotype groups. There was no interaction between the ACE genotype and the effects of 6 mo of AEX on body weight. The decrease in body weight resulted in a significant (P ⫽ 0.031) decrease in the BMI. However, similar to body weight, there was no interaction between the change in BMI with AEX and the ACE genotype. There was a 4% decrease (P ⫽ 0.001) in percent fat following 6 mo of AEX in all three ACE genotype groups. There was no interaction between the change in percent fat and ACE genotype. Although LBM did not significantly change (P ⫽ 0.378) following the AEX program, there was a 5% decrease (P ⫽ 0.001) in fat mass with the AEX program. There was no interaction between ACE genotype and the effect of 6 mo of AEX on fat mass. The 6-mo program of AEX did not significantly (P ⫽ 0.825) alter WHR. Table 2. Baseline parameters of glucose metabolism from the frequently sampled intravenous glucose tolerance test II

Values are means ⫾ SE as indicated. There were no statistically significant differences in these characteristics with respect to genotype group. M, male; F, female; II, homozygous for insertion allele; ˙ O2 max, maxID, heterozygote; DD, homozygous for deletion allele; V imal oxygen consumption; BMI, body mass; WHR, waist-to-hip ratio. Physiol Genomics • VOL

Fig. 1. Insulin sensitivity index at baseline in subjects with the insertion (I)/deletion (D) genotype of the angiotensin-converting enzyme (ACE) gene. * Significantly lower (P ⬍ 0.05) insulin sensitivity index than in ID ACE genotype group.

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Number SG, min⫺1 AIRG, ␮U/ml DI, 10⫺4/min KG, %/min

ID

DD

8 20 7 0.019 ⫾ 0.002 0.016 ⫾ 0.001 0.021 ⫾ 0.002 44.9 ⫾ 10.3 41.2 ⫾ 5.3 42.9 ⫾ 7.6 92.0 ⫾ 19.3 127.2 ⫾ 19.5 81.0 ⫾ 33.9 1.905 ⫾ 0.167 1.955 ⫾ 0.122 1.886 ⫾ 0.165

P Value

0.115 0.933 0.346 0.942

Values are means ⫾ SE. SG, glucose effectiveness; AIRG, acute insulin response; DI, disposition index; KG, intravenous glucose tolerance. www.physiolgenomics.org

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Table 3. Changes in physical characteristics

Body wt, kg Body fat, % ˙ O2max, ml 䡠 kg⫺1 䡠 min⫺1 V MAP, mmHg Insulin, pmol/l Glucose, mmol/l

Baseline

Post Training

P Value

80.8 ⫾ 4.0 44.7 ⫾ 1.4 16.8 ⫾ 0.9 109 ⫾ 3 88.6 ⫾ 14.4 5.3 ⫾ 0.1

78.3 ⫾ 4.2 42.9 ⫾ 1.5 19.3 ⫾ 0.9 104 ⫾ 3 74.4 ⫾ 11.5 5.2 ⫾ 0.2

0.042 0.006 0.001 0.001 0.074 0.741

Values are means ⫾ SE. MAP, mean arterial pressure.

Following the 6-mo program of AEX training, there ˙ O2 max in all three was a 12% increase (P ⬍ 0.0001) in V groups. However, there was no significant (P ⫽ 0.484) ˙ O2 max impact of ACE genotype on the change in V with AEX. Changes in glucose metabolism. There was a 48% increase (P ⫽ 0.0001) in SI following the 6 mo of AEX in all genotype groups. At the end of the AEX program there were no significant differences in SI between the ACE genotypes (DD, 2.17 ⫾ 0.57; II, 4.34 ⫾ 1.07; ID, 4.04 ⫾ 0.46 ␮U ⫻ 10⫺4 䡠 min⫺1 䡠 ml⫺1; P ⫽ 0.136). There was a significant (P ⫽ 0.011) interaction between ACE genotype and the change in SI, indicating that those individuals homozygous for the insertion allele (II) had the greatest improvement in SI compared with those individuals who were homozygous for the D allele (DD) or heterozygous (Fig. 2). Following AEX there was a 10% decrease in the AIRG (P ⫽ 0.036) (Fig. 3). Following the 6-mo AEX program, there were no significant differences in AIRG between the ACE genotypes (DD, 40.7 ⫾ 8.9; II, 34.1 ⫾ 7.8; ID, 39.1 ⫾ 6.8 ␮U/ml; P ⫽ 0.877). Similar to the exercise-induced change in SI, there was a significant (P ⫽ 0.05) interaction between ACE genotype and the change in AIRG, indicating that those individuals who were homozy-

Fig. 3. Exercise-induced changes in acute insulin response (AIRG) in subjects with the I/D polymorphism of the ACE gene. * Significant difference in AIRG following exercise training (P ⬍ 0.05) than II ACE genotype group.

gous for insertion allele (II) had the greatest decline in AIRG compared with those individuals who were homozygous for the D allele (DD) or heterozygous (Fig. 3). There was a 29% increase (P ⫽ 0.04) in DI with 6 mo of AEX training. However, there was no interaction between ACE genotype and the change in DI (P ⫽ 0.456). There was no significant change in SG (P ⫽ 0.207) or KG (P ⫽ 0.967) following the 6-mo AEX program. In addition, there was no interaction between the change in SG or KG with AEX and ACE genotype. Changes in blood pressure. There was a significant 4% decrease in both diastolic (P ⫽ 0.0003) and mean arterial (P ⬍ 0.0001) BP following AEX. Similarly, there was a 7% exercise-induced decrease (P ⬍ 0.0001) in systolic BP. Unlike SI there were no significant interactions between ACE genotype and the exercise training-induced change in systolic, diastolic, and mean arterial BP. DISCUSSION

Fig. 2. Exercise-induced changes in insulin sensitivity index in subjects with the I/D polymorphism of the ACE gene. * Significantly lower increase in insulin sensitivity index following exercise training (P ⬍ 0.05) than II ACE genotype group. Physiol Genomics • VOL

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The present study demonstrates an association between the insertion/deletion polymorphism in the ACE gene and insulin sensitivity in older hypertensives. Subjects homozygous for the insertion (II) and deletion (DD) allele at the ACE locus demonstrated significantly lower sensitivity to insulin compared with heterozygous subjects (Fig. 1). In addition, we also observed that the exercise-induced improvement in insulin sensitivity was significantly greater in II compared with either the ID or DD ACE genotype groups (Fig. 2). The existence of an association between the ACE gene and glucose metabolism is controversial (21, 28, 30, 34, 37, 41). Paolisso et al. (34) reported in a group of healthy older Italian subjects that the degree of insulin resistance estimated by the homeostatic www.physiolgenomics.org


method assessment (HOMA) was higher in those individuals with the II ACE genotype than those individuals with either the DD or ID ACE genotype. Similarly, Ryan et al. (41) reported that overweight women who were homozygous for the D allele of the ACE gene were more insulin sensitive than those women who were homozygous for the I allele. The results of this study may have been confounded by the effect of medications (16 of the 66 women studied were taking calcium channel blockers or ACE inhibitors, which have been shown to affect glucose metabolism) and the presences of type 2 diabetes in a number of the women studied. In fact, the authors report that 78% of the women who were homozygous for the I allele at the ACE locus had either impaired glucose tolerance or type 2 diabetes compared with only 24% of the individuals with the ACE DD genotype having impaired glucose tolerance or type 2 diabetes. Huang et al. (21) reported higher blood glucose levels and a greater degree of glucose intolerance in type 2 diabetes patients with the DD genotype. Recently, Perticone et al. (37) reported that in a group of never-treated hypertensives, individuals with the ACE DD genotype were more insulin resistant as determined by the HOMA method than those individuals with either the ACE ID or II genotype groups. The results of the present study demonstrating a greater degree of insulin resistance in older hypertensives with the DD ACE genotype support the previous results of Perticone et al. (37) in never-treated hypertensives. The ability of the ACE genotype to influence glucose metabolism is not understood; however, one possible mechanism explaining the link may be the elevated ACE levels that are associated with the ACE DD genotype (40). ACE catalyzes the conversion of angiotensin I to angiotensin II and the degradation of bradykinin to inactive products (5). Bradykinin has been demonstrated to produce endothelial-dependent increases in blood flow and glucose uptake (47). Brown et al. (5) recently demonstrated that the half-life of bradykinin was significantly lower in individuals with the DD ACE genotype. Therefore, the increased ACE activity and the decreased half-life of bradykinin in those individuals with the DD ACE genotype may decrease insulin sensitivity in these individuals by decreasing glucose delivery to skeletal muscle. Although the mechanism responsible for the association between insulin resistance and hypertension is not known, the ability of insulin to cause vasodilation has been postulated (19). A number of studies have reported that endothelium-dependent vasodilation is impaired in hypertensive individuals. In support of this, Perticone et al. (36) reported that the ACE DD genotype was associated with an impairment of endothelium-dependent vasodilation in a group of nevertreated hypertensive patients compared with those hypertensive individuals with either the ID or II ACE genotype. Therefore, the ACE DD genotype may influence glucose metabolism by attenuating insulin’s action to increase muscle blood flow. Recently, it has been demonstrated that angiotensin II impairs insulin’s ability to promote insulin receptor substrate-1 (IRS-1) Physiol Genomics • VOL

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phosphorylation and activates the phosphatidylinositol 3-kinase pathway (13). This disruption in insulin signaling would provide another link between the vascular system and glucose metabolism. In support of this hypothesis, the use of ACE inhibitors (22, 38) or angiotensin II receptor blockers not only lowers BP in hypertensive individuals, but also improves glucose metabolism (20). To the best of our knowledge this is the first study to describe a link between the ACE gene polymorphism and AEX training-induced changes in insulin sensitivity. In the present study, we observed that there was a significant increase in insulin sensitivity with AEX training in all subjects; however, those individuals with the II genotype had the greatest improvement in insulin sensitivity with AEX training. AEX training results in a significant increase in insulin sensitivity; however, there is a great deal of variability in this response (8, 9, 39). In the present study, variations in the ACE gene accounted for 14% (P ⫽ 0.04) of the variability in changes in insulin sensitivity with AEX training. The increase in insulin sensitivity in those individuals with the ACE II allele may be related to the effects of bradykinin. Wicklmayer et al. (48) reported that bradykinin is liberated by working skeletal muscles in healthy individuals, but not in individuals with type 2 diabetes (49). In addition, Kishi and associates (29) demonstrated in cultured cells that bradykinin triggers GLUT4 translocation and stimulates glucose uptake. Therefore, it is possible that the attenuated increases in insulin sensitivity due to AEX training in individuals with the ACE DD allele may be due to a shorter half-life of bradykinin or possibly to lower levels of bradykinin liberated during exercise. In addition to the exercise-induced improvement in insulin sensitivity observed in the present study, we also observed a significant decrease in the AIRG during the FSIVGTT. Our data are similar to that of Kahn et al. (26), who also reported a decrease in AIRG with AEX training in older adults. These two studies demonstrate that in addition to improving insulin sensitivity in older individuals, AEX training also results in significant decrease in the response of the ␤-cell to plasma glucose. Taken together these two studies demonstrate that in older adults AEX training may decrease the response of the ␤-cell to glucose. Of greater importance is the fact that variations in the ACE gene accounted for 22% (P ⫽ 0.008) of the variability in changes in AIRG with AEX training. In the present study, those individuals homozygous for the insertion allele of the ACE gene had a significantly greater decline in AIRG than those individuals in the other two ACE genotypes. It is possible that the greater increase in insulin sensitivity in the II ACE genotype is coupled with a greater decline in AIRG. Since defects in both insulin sensitivity and secretion are necessary for the development of type 2 diabetes (11, 16), the fact those individuals with the ACE II genotype have significantly greater changes in both with AEX training is of great interest. Future studies will be required to examine the role of ACE gene polymorphisms and AEX trainingwww.physiolgenomics.org

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induced enhancements in insulin sensitivity and secretion in individuals with impaired glucose tolerance and the progression to type 2 diabetes is warranted. To the best of our knowledge this the first study to examine the effects of AEX training on DI in either young or older adults. The DI is considered to be a measure of the ability of the ␤-cell to compensate for changes in insulin sensitivity by increasing insulin secretion (7). An increase in the DI would indicate that more insulin is secreted for a given change in plasma glucose concentration. Given the fact that those individuals with the II ACE genotype had significantly greater changes in both SI and AIRG, one might expect this group to have a greater increase in DI with AEX training than the other two ACE genotypes. However, since the disposition index is the product of SI and AIRG, the significantly greater in SI coupled with the significantly greater decrease in AIRG produced a similar increase in the DI as the two other ACE genotypes. The association between the ACE gene polymorphism and essential hypertension is controversial, with a few studies demonstrating an association (14, 52) but others failing to find such an association (24, 46, 51). In the present study, we did not observe any significant differences in BP due to ACE genotypes. Given the lack of conclusive evidence between the ACE gene and BP and the heterogeneity in backgrounds of the individuals examined in this study, this finding is not too surprising. As we have previously reported in older hypertensives, the AEX training program resulted in significant decreases in systolic, diastolic and mean arterial BPs (5, 18). However, similar to our results at baseline, we found no association between the AEX training-induced changes in BP and ACE genotype. Previously, we reported a greater reduction in resting diastolic BP, but not systolic or mean arterial BPs, in individuals with ACE I allele compared with those individuals homozygous for the ACE D allele after a 9-mo exercise training program (18). The difference in results between the previous study and the current one may be due to the larger sample size in the present study that allowed us to examine each of the ACE gene polymorphisms separately. In addition, subjects in our previous study also underwent a weight loss program in conjunction with the exercise training program. The results in the present study are similar to Montgomery et al. (33), who examined the effect of 9 mo of physical training in 460 normotensive males. Montgomery et al. (33) found no association between the change in BP with AEX training and the ACE genotype. One limitation of this study is the small number of subjects examined. Most studies assessing the effect of genes on particular disease or outcome have substantially larger sample sizes. However, it should be noted that the longitudinal design of this study allows one to assess the actual changes resulting from AEX training within an individual and account for possible baseline differences between genotype groups and individuals that cannot be accounted for in cross-sectional studies. Second, the intervention was standardized across subPhysiol Genomics • VOL

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jects, ensuring that subjects had been subjected to an AEX training program that was sufficient to elicit substantial change in glucose metabolism. However, we cannot rule out the possibility of a type II statistical error. In conclusion, an increase in insulin sensitivity and a decrease in ␤-cell secretion due to AEX training are greatest in those with the II genotype of the ACE gene. Future studies will be needed to examine the mechanism that contributes to the observed interaction between exercise-induced changes in insulin sensitivity, ␤-cell function, and the ACE genotype. We thank all the subjects who volunteered, and we thank the nursing and dietary staffs at the University of Michigan General Clinical Research Center for assistance with the research studies. This work was supported by National Institutes of Health Research Scientist Development Award in Aging KO1-AG-0072301 (to D. R. Dengel), National Institutes of Health Grant U10-HL-54526 (R. E. Ferrell), National Institutes of Health Institutional National Research Service Award T32-AG-00114 (to M. D. Brown and T. H. Reynolds IV), the Department of Veterans Affairs Medical Research Service (to M. A. Supiano), and the Geriatric Research, Education and Clinical Center (to D. R. Dengel and M. A. Supiano) at Ann Arbor, University of Michigan, Claude D. Pepper Older Americans Independence Center (Grant AG-08808), University of Michigan Clinical Research Center (Grant RR-00042). REFERENCES 1. Abbate SL, Fujimoto WY, Brunzell JD, and Kahn SE. Effect of heparin on insulin-glucose interactions measured by the minimal model technique: implications for reproducibility using this method. Metabolism 42: 353–357, 1993. 2. American Diabetes Association. Report of the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus (Committee Report). Diabetes Care 21, Suppl 1: S5–S19, 1998. 3. Bauters C and Amouyel P. Association between the ACE genotype and coronary artery disease: insights from studies on restenosis, vasomotion, and thrombosis. Eur Heart J 19, Suppl J: J24–J29, 1998. 4. Bergman RN. Toward physiological understanding of glucose tolerance: Minimal model approach. Diabetes 38: 1512–1527, 1989. 5. Brown NJ, Blais C, Gandhi SK, and Adam A. ACE insertion/ deletion genotype affects bradykinin metabolism. J Cardiovasc Pharmacol 32: 373–377, 1998. 6. Cambien F, Poirier O, Lecerf L, Evans A, Cambou JP, Arveiler D, Luc G, Bard JM, Bara L, Ricard S, Tiret L, Amouyel P, Alhenc-Gelas F, and Soubrier F. Deletion polymorphism in the gene for angiotensin-converting enzyme is a potent risk factor for myocardial infarction. Nature 359: 641– 644, 1992. 7. Chen M, Bergman RN, and Porte D Jr. Insulin resistance and B-cell dysfunction in aging: the importance of dietary carbohydrate. J Clin Endocrinol Metab 67: 951–957, 1988. 8. Dengel DR, Galecki AT, Hagberg JM, and Pratley RE. The independent and combined effects of weight loss and aerobic exercise on blood pressure and oral glucose tolerance in older men. Am J Hypertens 11: 1405–1412, 1998. 9. Dengel DR, Pratley RE, Hagberg JM, Rogus EM, and Goldberg AP. Distinct effects of aerobic exercise training and weight loss on glucose homeostasis in obese sedentary men. J Appl Physiol 81: 318–325, 1996. 10. Erdos EG. Angiotensin I-converting enzyme and the changes in our concepts through the years. Hypertension 16: 363–370, 1990. 11. Ferrannini E. Insulin resistance versus insulin deficiency in non-insulin dependent diabetes mellitus: problems and prospects. Endocr Rev 19: 477–490, 1988. 12. Ferrari P, Alleman Y, Shaw S, Riesen W, and Weidmann P. Reproducibility of insulin sensitivity measured by the minimal model method. Diabetologia 34: 527–530, 1991. www.physiolgenomics.org

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