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(Circulation. 1995;92:1422-1429.)
© 1995 American Heart Association, Inc.

Articles

Insulin, Intact and Split Proinsulin, and Coronary Artery Disease in Young Men

Peter Båvenholm, MD; Anthony Proudler, MSc; Per Tornvall, MD; Ian Godsland, PhD; Christian Landou, MD; Ulf de Faire, MD; Anders Hamsten, MD

From the Departments of Medicine (P.B., P.T., U. de F., A.H.) and Thoracic Radiology (C.L.) and the Atherosclerosis Research Unit, King Gustaf V Research Institute (P.B., P.T., A.H.), Karolinska Hospital, Karolinska Institute, Stockholm, Sweden, and the Wynn Institute for Metabolic Research (A.P., I.G.), National Heart and Lung Institute, London, UK.

Correspondence to Dr Peter Båvenholm, Department of Internal Medicine, Karolinska Hospital, S-171 76 Stockholm, Sweden.

	Abstract

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Background Glucose intolerance and hyperinsulinemia are common disturbances in nondiabetic men with premature coronary artery disease (CAD). To investigate the relation between insulin-like molecules and severity of coronary atherosclerosis, 62 consecutive nondiabetic men presenting with a first myocardial infarction before the age of 45 were studied along with 41 healthy, age-matched, male, population-based control subjects.

Methods and Results Specific two-site immunoradiometric assays were used to distinguish intact proinsulin, (des 31,32)proinsulin, and "true" insulin in fasting plasma and during an oral glucose tolerance test (OGTT). Global coronary atherosclerosis and number and severity of distinct stenoses were determined in the patients in 15 proximal coronary arterial segments by use of separate semiquantitative classification systems. The patients had a two- to threefold increase in insulin and insulin propeptide concentrations in the fasting state as well as during the OGTT. Severity of coronary atherosclerosis correlated significantly (P<.05 to P<.01) with basal proinsulin (r=.40) and the proinsulin area under the curve (AUC) (r=.34), basal insulin (r=.31), basal C peptide (r=.30), and the glucose AUC (r=.30). In multiple stepwise regression analysis including insulin-like molecules, major plasma lipoproteins, and lipoprotein subfractions, basal proinsulin (increase in R²=.09) and dense LDL triglycerides (increase in R²=.10) predicted 19% of the variation of the global coronary atherosclerosis score after adjustment for age, body mass index, fasting insulin concentration, and VLDL triglycerides.

Conclusions This study shows that young, nondiabetic, male survivors of myocardial infarction are truly hyperinsulinemic during an OGTT and suggests a close association between proinsulin and coronary atherosclerosis.

Key Words: glucose • insulin • radioimmunoassay • atherosclerosis

	Introduction

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Large-scale prospective epidemiological studies have indicated that hyperinsulinemia^{1 2 3} and glucose intolerance⁴ are associated with the development of coronary artery disease (CAD). Fasting hyperinsulinemia and/or hyperinsulinemic responses to glucose challenge are also characteristic findings in subjects with manifest CAD.^{5 6} In addition, associations have been shown consistently between hyperinsulinemia, either fasting or after an oral glucose load, and established risk factors for cardiovascular disease, including abdominal obesity, hypertriglyceridemia, reduced HDL cholesterol concentration, and hypertension.^{7 8} However, the mechanisms underlying the relations of insulin to CAD remain unclear.

It has been argued recently that a standard radioimmunoassay for insulin may overestimate the true insulin levels in non–insulin-dependent diabetics and subjects with impaired glucose tolerance, due to cross-reaction with intact proinsulin and split proinsulin.^{9 10} The question now arises whether the hyperinsulinemia previously demonstrated in nondiabetic subjects with manifest CAD is to some extent accounted for by proinsulin-like molecules and whether proinsulin and split proinsulin are implicated in the development of coronary atherosclerosis and thrombosis. These issues were addressed in a cross-sectional angiographic study of men who had survived a first myocardial infarction before the age of 45, which also included comparisons with population-based, age-matched men serving as control subjects. Specific two-site monoclonal antibody–based assays were used for measuring intact proinsulin and (des 31,32)proinsulin in fasting samples and during an oral glucose tolerance test (OGTT).

	Methods

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Study Population and Protocol
Sixty-two men with a first myocardial infarction before the age of 45 were studied consecutively. The patients initially had been admitted between April 1989 and October 1990 to the 10 hospitals in Stockholm county equipped with a coronary or intensive care unit and were subsequently referred to the Karolinska Hospital for metabolic and cardiological studies. Acute myocardial infarction was diagnosed when at least two of the following three criteria were fulfilled: (1) central chest pain more than 15 minutes in duration and with onset within the last 48 hours, or pulmonary edema for no other obvious reason; (2) two consecutive determinations of serum creatine kinase and creatine kinase isoenzyme B showing elevated values and a raised creatine kinase isoenzyme B to creatine kinase ratio, with a maximum elevation occurring 10 to 16 hours after the onset of symptoms; (3) ECG series with development of pathological Q waves and/or development or disappearance of localized ST-segment elevation followed by T-wave inversion in two or more of 12 routine ECG leads. Metabolic and angiographic studies were performed 4 to 6 months after the acute event, when it was expected that acute-phase reactions caused by myocardial damage had subsided. A total of 102 men fulfilled the criteria for participation in the study. Of the 40 patients not studied, 3 died during the acute stage or in the early postinfarction period and 10 declined to participate. Nine patients were excluded because of concomitant diseases, such as manifest diabetes mellitus (n=2), heterozygous familial hypercholesterolemia (n=1), severely impaired renal function (n=4), or large cerebral infarction (n=2). Another 13 patients were not studied for other reasons, such as unavailability of laboratory facilities, and 5 patients were either referred more than 6 months after their infarctions or were unavailable to the research team. None of the patients eligible for the study had hypothyroidism or were on treatment with lipid-lowering drugs, but all had been informed about a lipid-lowering diet in connection with the first visit to the outpatient clinic 6 weeks after admission to the referring intensive care units. The dietician's standardized written instructions aimed at a diet low in fat, rich in complex carbohydrates, and with a limited intake of alcohol. At the time of the metabolic investigation, all but 3 patients were on medication with a cardioselective ß-adrenergic blocker and salicylates (aspirin, 75 mg).

Insulin and insulin propeptides were determined in all patients and in 41 healthy male control subjects. These were the first consecutively recruited members of a group of 96 healthy, age-matched, population-based men (mean age±SD, 39.6±2.7 years). Control subjects were recruited by random selection of individuals born between 1947 and 1956, extracted from a database of all inhabitants of Stockholm County (response rate, 69%). The control subjects completed a program that included clinical examination, medical history, blood sampling for determination of plasma lipoproteins, and an OGTT.

All subjects gave their fully informed consent to the study, the protocol of which had been approved by the regional Ethics Committee.

Laboratory Methods and Procedures
Blood samples for lipoprotein analyses were taken between 8 and 9 AM after 12 hours of fasting, during which time smokers were asked to refrain from smoking. All subjects were free of symptoms of infectious disease at the time of blood sampling. Venous blood for lipoprotein fractionation was drawn into precooled evacuated tubes containing Na₂ EDTA (1.4 mg/mL) and placed in an ice bath. Plasma was then recovered by use of low-speed centrifugation (1400g, 20 minutes) at 1°C and kept at this temperature throughout the preparation procedures. On a separate visit, after 12 hours of fasting, glucose was ingested in a dose of 1.75 g/kg body weight in 150 to 200 mL of water flavored with lemon extract.¹¹ Venous blood samples for determination of blood glucose, insulin, and insulin propeptides were obtained before and 15, 30, 45, 60, 90, and 120 minutes after glucose intake through an indwelling cannula inserted into an antecubital vein, with subjects remaining semirecumbent throughout the test. Fasting samples of glucose, insulin, and insulin propeptides were drawn on two separate occasions spaced 5 minutes apart. Blood was collected in evacuated tubes containing heparin (143 USP units) for the determination of blood glucose. Plasma samples for analyses of insulin and insulin propeptides were then prepared by low-speed centrifugation (1400g, 15 minutes) within 30 minutes and kept at -70°C until they were analyzed.

Blood glucose was measured by a glucose oxidase method (Kodak Ektachem). Total immunoreactive insulin and C peptide were determined by radioimmunoassays (RIAs) with polyclonal antisera supplied by Guildhay Ltd. Immunoradiometric assays were used to measure intact proinsulin and (des 31,32)proinsulin.¹² Murine monoclonal antibodies A6 and 3B1 were obtained from Serono Diagnostics. Murine monoclonal antibody PEP-001 was obtained from Novo Nordisk. The concentration of (des 31,32)proinsulin was calculated by subtracting the cross-reactivity of proinsulin (89%) in the (des 31,32)proinsulin assay. (Des 31,32)proinsulin cross-reacted (0.5%) in the intact proinsulin assay. True insulin was obtained from the insulin RIA by subtraction of intact proinsulin and corrected (des 31,32)proinsulin. Specificity and precision of the analytical methods and their reliability have been described in detail previously.¹³

VLDL, LDL, and HDL cholesterol levels were determined by a combination of preparative ultracentrifugation and precipitation.¹⁴ Subfractions of VLDL, LDL, and IDL were separated by density-gradient ultracentrifugation and subjected to compositional analysis as previously described in detail.¹⁵ Cutoff limits for the lipoprotein phenotyping were set to the 90th percentiles of VLDL triglyceride (1.90 mmol/L) and LDL cholesterol (4.75 mmol/L) concentrations in the control group of 96 age-matched, healthy men. In the present study, lipids and lipoproteins were only considered for lipoprotein phenotyping and in the multivariate evaluation of factors relating to severity of coronary atherosclerosis in the group of patients.

The area under the curve (AUC) for the OGTT concentration profile was calculated for glucose, insulin, insulin propeptides, and C peptides.

Hyperglycemic responses to the oral glucose challenge were defined as a glucose AUC exceeding the 90th percentile of the distribution observed in the 96 control subjects.

Coronary Angiography
Coronary angiography was performed in patients by use of the percutaneous transfemoral technique according to a standard protocol and recorded on 35-mm cine film with cesium iodide–activated image intensifiers. All cineangiograms were assessed by one angiographer (C.L.) blinded to the patient's clinical characteristics and biochemical profiles. The presence and severity of lesions were determined in 15 proximal coronary arterial segments by use of separate semiquantitative classification systems for diffuse coronary atherosclerosis and number and severity of distinct stenoses.^{15 16} The global coronary atherosclerosis score included both diffuse lesions and distinct, hemodynamically significant stenoses, whereas the coronary stenosis score only included lesions that reduced the lumen diameter by 25% or more. Patients were also categorized as having zero-, single-, double-, or triple-vessel disease with regard to the presence of hemodynamically significant lesions (diameter stenosis 50%) in the three major coronary arteries or their branches. The method of determining the coronary atherosclerosis and stenosis scores and the statistical analysis of the reliability of this system have been described in detail elsewhere.¹⁶

Statistical Analysis
Logarithmic transformation was performed on all skewed variables to obtain a normal distribution before statistical computations and significance testing were undertaken. Insulin resistance and ß-cell function (percent of normal) were calculated from the fasting blood glucose and plasma insulin concentrations with the Homeostasis Model Assessment (HOMA) method.¹⁷ Differences in continuous variables between groups were tested either by Student's unpaired two-tailed t test, the Mann-Whitney U statistic, or ANOVA with Scheffé's test used as a post hoc test. ANCOVA was performed with body mass index (BMI) as the covariate. Categorical variables were analyzed with the ² test with Yates' technique. The relations of biochemical variables to coronary scores were estimated by calculating partial correlation coefficients with age and BMI alone or with age, BMI, fasting insulin, and VLDL triglyceride levels used as forced variables. Multiple stepwise linear regression analysis was performed to analyze the independent relations of glucose, insulin, proinsulin-like molecules, C peptide, and plasma lipoproteins to the global coronary atherosclerosis score. Biochemical variables were included in the multivariate analysis when they correlated significantly (P<.05) with angiographic scores in the univariate analysis.

	Results

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Basic Characteristics of Control Subjects and Patients
Basic characteristics of control subjects and patients are shown in Table 1. Patients were shorter and heavier than control subjects and consequently had higher BMIs, and hypertriglyceridemic lipoprotein phenotypes predominated.

View this table: [in this window] [in a new window]   Table 1. Characteristics of Control Subjects and Patients

Immigrants were overrepresented in the patient group (35% versus 2% in the control group). All patients were Caucasians. Eleven immigrants were of Scandinavian origin, and the remainder were non-Scandinavian Europeans (n=4), Middle Eastern (n=5), or Latin American (n=2). None of the ethnic groups that have been characterized by insulin resistance were represented among the patients. There were no differences in clinical or metabolic characteristics between immigrants and native Swedish patients, except that immigrants were shorter and weighed less than the native Swedes (171.4±7.6 versus 179.9±7.3 cm, P<.001, and 82.5±11.9 versus 91.1±12.7 kg, P<.05). BMI, however, did not differ between the two groups.

A majority of the patients had zero- (20%) or single-vessel (38%) CAD, whereas only a small proportion had triple-vessel CAD (15%).

Measurements of Insulin and Insulin Propeptides
Basal concentrations and AUCs for insulin, proinsulin, (des 31,32)proinsulin, and C peptide were elevated to a similar extent in the patients, especially among those with hyperglycemic responses during the OGTT (Tables 2 and 3 and the Figure) in whom the global coronary atherosclerosis score was higher. Thus, insulin and insulin propeptides were proportionately elevated in patients during the OGTT compared with control subjects. However, patients with a hyperglycemic response during the test exhibited a disproportionate increase in insulin propeptides at 15 minutes during the OGTT (Table 3). All case control and within-patient group differences for glucose, insulin, insulin propeptides, and lipoproteins remained statistically significant after adjustment for BMI in ANCOVA, except for the difference in basal insulin concentration between patients with normal and hyperglycemic responses to the OGTT (data not shown). Table 4 provides case control data for 28 pairs of patients and control subjects matched for BMI. Basal and postload glucose and insulin concentrations did not differ significantly between patients and control subjects matched for BMI, as opposed to insulin propeptide concentrations. In addition, the case control differences observed in the entire study group for plasma concentrations of VLDL cholesterol, VLDL triglycerides, and HDL cholesterol and for the measure of insulin resistance remained after matching for BMI. It is notable that the predominance of hypertriglyceridemic lipoprotein phenotypes observed in the entire patient group was as apparent among the patients who were matched for BMI. The ß-cell function was calculated to be normal in absolute terms in all patients. However, in glucose-intolerant and insulin-resistant patients, the ß-cell capacity was obviously not sufficient to maintain normoglycemia.

View this table: [in this window] [in a new window]   Table 2. Basal Values and Areas Under the Oral Glucose Tolerance Test Concentration Profiles for Blood Glucose and Plasma Insulin, Proinsulin, (des 31,32)proinsulin, and C-peptide

View this table: [in this window] [in a new window]   Table 3. Basal Values and Areas Under the Oral Glucose Tolerance Test Concentration Profiles for Blood Glucose and Plasma Insulin, Proinsulin, (des 31,32)proinsulin and C-peptide in Patients With Normal or Hyperglycemic Responses During the Test and in Control Subjects

View larger version (21K): [in this window] [in a new window]   Figure 1. Line plots of insulin, intact proinsulin, (des 31,32)proinsulin, and C-peptide concentrations after oral glucose challenge in young postinfarction patients with normal (X) and respectively hyperglycemic (closed circles) responses and in control subjects (open circles).

View this table: [in this window] [in a new window]   Table 4. Plasma Lipoproteins, Basal Values, and Areas Under the Oral Glucose Tolerance Test Concentration Profiles for Blood Glucose, Insulin, and Insulin Propeptides, Insulin Resistance, and ß-Cell Function in Control Subjects and Patients Matched for Body Mass Index

Neither the coronary stenosis score nor the number of major coronary arteries with hemodynamically significant lesions differed between patients with normal or hyperglycemic OGTT results.

Relations to Angiographic Scores
Significant positive partial correlation coefficients were noted between global coronary atherosclerosis and basal proinsulin (r=.40, P<.01), proinsulin AUC (r=.34, P<.05), basal insulin (r=.31, P<.05), basal C peptide (r=.30, P<.05), and the glucose AUC (r=.30, P<.05) when we controlled for age at the time of the angiography (r=.29, P<.05). When BMI (r=.23, P=.06) was also included as a forced variable (Table 5), the relations weakened, although only the insulin and C-peptide correlations became statistically insignificant. Furthermore, only the HDL cholesterol correlation with the global atherosclerosis score became statistically insignificant when basal insulin and VLDL triglycerides were added to age and BMI as forced variables (Table 5). Hypertension was related to the global coronary atherosclerosis score (P<.05) (² analysis between tertiles of the global coronary atherosclerosis score), but smoking history was not. BMI correlated significantly with the global coronary stenosis score (r=.40, P<.01), whereas no glucose- or insulin-related variables were associated with the number and severity of coronary stenoses (data not shown).

View this table: [in this window] [in a new window]   Table 5. Multiple Stepwise Regression Analysis of the Relations of Insulin, Insulin Propeptides, and Plasma Lipoproteins to the Coronary Atherosclerosis Score

Multiple stepwise regression analysis was used to study the independent relations of insulin-like molecules to the coronary atherosclerosis score, along with major plasma lipoproteins and subfractions of apolipoprotein B–containing lipoproteins (Table 5). In the first model, age and BMI were entered as forced variables. In the second model, basal insulin and VLDL triglycerides were also included as forced variables, because these variables are correlated with insulin propeptides and dense LDL triglycerides. In both models, basal intact proinsulin and dense LDL triglycerides were selected for the final equation and together predicted 19% to 20% of the variation of the global coronary atherosclerosis score. Inclusion of major plasma lipoproteins or other lipoprotein subfractions did not significantly increase the value of R².

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
Our study demonstrates that the hyperinsulinemic responses found during an OGTT in young male survivors of myocardial infarction are caused by "true" insulin. The fraction of all insulin-like molecules that was accounted for by intact proinsulin or (des 31,32)proinsulin did not differ between patients and control subjects in the fasting state or when the entire response to the glucose challenge was considered. However, during the early phase of the OGTT, insulin propeptides constituted a higher proportion of the total amount of insulin-like molecules in patients with hyperglycemic responses during the OGTT compared with control subjects or patients with a normal response to the glucose load. This is in agreement with the findings of others in subjects with impaired glucose tolerance¹⁰ or non–insulin-dependent diabetes⁹ and has been interpreted as an early sign of ß-cell dysfunction. Furthermore, proinsulin levels were found to correlate significantly with the global severity of coronary atherosclerosis.

Speculation that the hyperinsulinemic responses to glucose challenge seen in earlier studies of nondiabetic subjects with manifest CAD^{5 6} are confounded by cross-reaction of conventional radioimmunoassays for insulin with proinsulin-like molecules, as previously indicated in type II diabetes,⁹ can now be terminated. We cannot exclude the possibility that alterations were present among the patients in the circulating levels of other insulin propeptides, such as (des 64,65)proinsulin. However, previous studies of healthy individuals with normal or decreased glucose tolerance have shown that intact proinsulin and (des 31,32)proinsulin together comprise 90% or more of the fasting proinsulin immunoreactivity.¹⁸

Insulin resistance was a characteristic feature of the patients, irrespective of the degree of glycemia during the OGTT. The ethnic heterogeneity in the patient group does not explain the case control differences in insulin resistance or insulin propeptide responses to the OGTT, as these measurements did not differ between immigrants and native Swedish patients. Furthermore, none of the ethnic groups that have been characterized by insulin resistance were represented among the patients.

The HOMA model was used for determining insulin resistance and ß-cell function. The quantitative measurements obtained with this model have been validated with the use of euglycemic and hyperglycemic clamp studies and intravenous glucose tolerance tests.¹⁷ Although highly correlated with the direct measurements by use of clamp techniques, the indirect HOMA predictions of insulin resistance and ß-cell function have definite limitations. However, the parameters were used only for group comparisons in the present study, and correlations were not attempted with insulin and insulin propeptide measurements or with angiographic estimates of coronary atherosclerosis.

It remains unknown whether a disproportionate increase in insulin propeptides during the early phase of the OGTT, as seen in the patients with a hyperglycemic postload response, represents a true ß-cell dysfunction. The case control differences for insulin or insulin propeptides remained after controlling for BMI in covariance analysis. However, when carefully matched patients and control subjects were compared, the group differences for insulin or glucose concentrations disappeared, whereas the differences for insulin propeptides and insulin resistance remained statistically significant. Obesity itself is associated with insulin resistance and reduced insulin elimination and is thus likely to have contributed to the elevated plasma insulin concentrations, whereas the hyperproinsulinemia observed in the patients would be determined, at least to some extent, by other factors.

Numerous studies have demonstrated consistent relations between cardiovascular risk factors (including hypertriglyceridemia, low concentrations of HDL cholesterol, plasma plasminogen activator inhibitor-1 [PAI-1] activity elevation, obesity, and hypertension) and hyperinsulinemia or insulin resistance.^{7 8 19} However, the mechanisms by which hyperinsulinemia, itself an independent risk factor for CAD, is implicated in coronary atherosclerosis in humans are unclear. Experimental studies suggest that hyperinsulinemia, besides having a direct growth-promoting effect on smooth muscle cells, is associated with changes in plasma levels and composition of lipoproteins as well as with perturbations of the interactions between lipoproteins and cellular receptors (reviewed by Stout²⁰ ). The present study suggests that proinsulin levels relate to coronary atherosclerosis as reflected by angiography. Biological effects of proinsulin-like molecules have been studied mainly in relation to carbohydrate metabolism. Insulin and proinsulin have different kinetics, the metabolic clearance rate of proinsulin being less than one third that of insulin. Proinsulin exerts its effects mainly on the liver²¹ and has insulin-like effects on lipid metabolism.²² However, some recent studies have also examined the effect of insulin and insulin propeptides on the synthesis and secretion of fibrinolytic proteins by hepatocytes^{23 24} and endothelial cells^{25 26} in culture. Both intact and split proinsulins are able to induce an increase in PAI-1 secretion from these cultured cells at concentrations seen in the plasma of type II diabetic patients. This suggests that insulin propeptides not only stimulate hepatic synthesis of PAI-1 (by mechanisms partly involving the insulin receptor) but also exert direct effects on endothelial cells in the arterial wall through insulin and insulin-like growth factor-1 independent pathways. It may therefore be speculated that, in addition to insulin, insulin propeptides might be directly implicated in the atherosclerotic and thrombotic processes. In addition, a prolonged half-life of proinsulin-like molecules will add to any potential atherogenic and/or thrombogenic effects that these molecules might have.

Insulin resistance with increased plasma levels of insulin and insulin propeptides is also associated with alterations of plasma lipoprotein metabolism leading to an atherogenic lipoprotein profile (reviewed by Frayn²⁷ ). In particular, the normal physiological inhibitory effect of insulin on hepatic VLDL triglyceride secretion is lost, resulting in an increased hepatic output of VLDL triglycerides. This in turn affects the composition and metabolism of other apolipoprotein B–containing lipoproteins through lipid transfer processes and contributes to a low HDL cholesterol concentration.

The findings that proinsulin seemed to be correlated a little more strongly with the coronary atherosclerosis score than insulin was and that proinsulin and not insulin was included in the multivariate model should be interpreted with caution. The intra-individual variability of plasma insulin concentration is greater than that of proinsulin [and (des 31,32)proinsulin], since the half-life of proinsulin is longer. This gives proinsulin an advantage over insulin in univariate correlations with angiographic scores as well as in the multivariate analysis of independent relationships to disease severity. Accordingly, the potential involvement of proinsulin and insulin in atherogenesis might not differ significantly. However, the lack of (des 31,32)proinsulin associations with the coronary atherosclerosis score speaks against this line of reasoning. Previous angiographic studies, admittedly using nonspecific assays for plasma insulin, have also consistently failed to demonstrate a statistically significant association between plasma insulin level and severity of CAD.^{28 29} Interestingly, the basal C-peptide concentration, an indicator of insulin secretion, has proved to be the most powerful discriminator between men surviving a first myocardial infarction and control subjects in various ethnic groups.³⁰ The basal C-peptide level also correlated with the coronary atherosclerosis score in the present study. However, direct influences of C peptide on the atherosclerotic process seem unlikely.

The possible influence of ongoing ß-blocker medication on carbohydrate metabolism could not be controlled for, since almost all of the patients were taking a cardioselective ß-blocker as part of the routine postinfarction regimen. Although the case control differences in basal or postload insulin levels might have been enhanced by ß-blocker medication, our own earlier observations indicate that differences in postload insulin concentrations between young postinfarction patients and control subjects are not accounted for by effects of ß-blocker medication.³¹ Of note, Pollare et al³² reported a 13% increase in plasma insulin during the late phase of an intravenous glucose tolerance test after 4 months of treatment with metoprolol (200 mg/d) in hypertensive patients. The increase in postload insulin concentration in our patients was two- to threefold compared with the control subjects. The magnitude of this hyperinsulinemia cannot be explained to any considerable extent by metoprolol treatment at a daily dose of 50 to 100 mg. Recently it has also been shown that short-term medication with metoprolol (100 mg/d) or atenolol (50 to 100 mg/d) does not alter insulin sensitivity.^{33 34} However, ß-blocker treatment significantly decreases insulin clearance^{35 36} and might well have accounted for at least part of the elevated insulin to C-peptide concentration ratio in the patients (data not shown).

In conclusion, this study shows that the hyperinsulinemic response to an OGTT found in young, male, postinfarction patients is not accounted for by disproportionately elevated intact and partially processed proinsulins. The novel finding that basal and postload proinsulin concentrations are associated with severity of global coronary atherosclerosis, independent of major lipoproteins and lipoprotein subfractions, needs to be tested in future studies. Whether this association reflects a direct effect of proinsulin is currently unknown.

	Acknowledgments

 
This study was supported by the Swedish Heart-Lung Foundation, the Swedish Medical Research Council (grant 8691), the Nordic Insulin Foundation, the Marianne and Marcus Wallenberg Foundation, the Thuring Foundation, the King Gustaf V 80th Birthday Fund, the Heart Disease and Diabetes Research Trust, and the Cecil Rosen Foundation, London. We are grateful for the valuable advice given by Professor Michael Oliver, MD, David Crook, PhD, and John Stevenson, FRCP, at the Wynn Institute for Metabolic Research, the National Heart and Lung Institute, London, UK. Insulin, C peptide, and insulin propeptide assays were carried out at the Cecil Rosen Research Laboratories at the Wynn Institute.

Received December 12, 1994; revision received March 6, 1995; accepted March 19, 1995.

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