(Hypertension. 1996;28:127-132.)
© 1996 American Heart Association, Inc.
Articles |
Insulin Resistance, Elevated Glomerular Filtration Fraction, and Renal Injury
the Division of Nephrology and Clinical Research Unit, Division of Gerontology, Department of Medicine, University of Maryland School of Medicine, and Geriatric Service and the Geriatric Research, Education and Clinical Center (GRECC), Baltimore VA Medical Center, Baltimore, Md.
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Abstract |
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The development of insulin resistance may be an early step in the development of hypertension; however, the mechanism for this process is not known. The worsening of insulin resistance and hypertension could increase both systemic and glomerular capillary pressures and predispose an individual to renal injury. The purpose of this study was to examine the relationship of insulin resistance to glomerular hemodynamics and dietary salt intake in 10 older (68±6 years), obese (body mass index, 31±4 kg/m2), mildly hypertensive (151±8/82±2 mm Hg), sedentary subjects without clinical evidence of diabetes or renal disease. They were studied on separate days with radioisotopic renal clearances (glomerular filtration rate by 99mTc-diethylenetriaminepentaacetic acid urinary clearance; renal plasma flow by 131I-hippuran serum disappearance) and a two-dose (40 and 100 mU/m2 per minute) hyperinsulinemic euglycemic clamp for measurement of glucose disposal after 2 weeks of a 3-g and 2 weeks of a 10-g sodium diet. Glomerular filtration rate (68.1±7.7 to 78.0±6.6 mL/min per 1.73 m2, P=.08) and glomerular filtration fraction (0.21±0.02 to 0.22±0.02, P=.5) did not change significantly after dietary salt was increased. During low dietary salt intake, there was an inverse relationship between glomerular filtration fraction and glucose disposal rate (milligrams per kilogram fat-free mass per minute) at both low (r=-.70, P=.04) and high (r=-.83, P=.006) insulin levels. However, these relationships were attenuated during salt loading. This suggests that a greater degree of insulin resistance, not increased dietary salt, may predispose older mildly hypertensive subjects to renal injury by worsening renal hemodynamics through the elevation of glomerular filtration fraction and resultant glomerular hyperfiltration.
Key Words: insulin resistance • glomerular filtration fraction • renal injury
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Introduction |
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Insulin resistance and the resultant hyperinsulinemia may play a role in the development of hypertension and other components of the insulin resistance syndrome. At least two and possibly three mechanisms favor a causal relationship between insulin resistance and the development of hypertension: the first mechanism is through an alteration of renal sodium handling, wherein hyperinsulinemia may promote sodium reabsorption through an acute direct effect on renal tubules.1 2 Or, as we hypothesize and examine in this article, an elevated glomerular FF associated with the insulin resistance syndrome may create an intrarenal hemodynamic environment3 that favors sodium and water retention, leading to hypertension. A second mechanism is that hyperinsulinemia activates sympathetic nervous system activity in some patients to raise cardiac output and stimulate peripheral vasoconstriction.4 5 6 7 8 This increase in plasma volume resulting from sodium retention, along with the increase in cardiac output and peripheral vasoconstriction, mediated by activation of the sympathetic nervous system raises systemic arterial pressure. A third mechanism is via obesity, a condition common to insulin resistance and hypertension. Obese individuals have decreased vascular compliance, increased sympathetic nervous system activity, and heightened salt sensitivity, conditions that are related to the insulin resistance syndrome, hyperinsulinemia, and hypertension.9 10
In this study, we examine the effect of dietary salt and insulin resistance on renal hemodynamics in a cohort of older, obese, mildly hypertensive subjects with normal fasting glucose and no clinical evidence of renal disease to determine their relationship to the development of renal damage, such as glomerular and systemic hypertension.11 We test the hypothesis that insulin resistance is associated with altered glomerular FF; hence, a worsening of insulin resistance is a risk factor for renal injury because of higher FF, which amplifies salt and water retention and results in greater glomerular capillary and systemic blood pressures. Preliminary results are presented that show a link between insulin resistance and changes in the renal microcirculation in older obese, deconditioned subjects with mild hypertension that are suggestive of a heightened potential for the development of renal injury.
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Methods |
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Subjects
Ten healthy, older, sedentary male and female subjects with mild hypertension and no evidence of renal disease or dysfunction were recruited for study. Before being enrolled in the study, the subjects gave voluntary informed written consent as approved by the Human Volunteers Research Committee of the University of Maryland School of Medicine. Subjects were eligible for the study if their sitting diastolic pressure on an ad libitum sodium diet was greater than or equal to 85 mm Hg and less than or equal to 105 mm Hg after being withdrawn from all antihypertensive medication for 4 weeks. Subjects were excluded from the study if they had clinically significant concomitant medical illnesses such as cardiac, renal (serum creatinine >135 mmol/L), hepatic, or gastrointestinal disease or required concomitant medications. Also excluded were individuals with a recent history of smoking or drug or alcohol abuse, clinically relevant mental disorders, or concomitant use of medications that might interfere with blood pressure or renal function.
Procedures
Subjects were screened before participation by a medical history physical exam, fasting plasma glucose, and routine blood chemistries. Subjects were excluded from participation if they exceeded 140% (body mass index >35 kg/m2) of ideal body weight, had fasting plasma glucose levels greater than 7.8 mmol/L, or the presence of underlying illness shown by medical history and examination of baseline laboratory biochemical analyses. Before metabolic studies, VO2max and body composition (percent body fat, fat-free mass, waist-to-hip ratio) were measured in the subjects who met the criteria for study entry.12
To eliminate the effects of diet on insulin sensitivity and blood pressure, we taught all subjects the principles of an AHA step I diet over 8 weeks before metabolic testing and evaluation of renal hemodynamics. This diet consisted of 50% to 55% calories as carbohydrate, 30% to 35% as fat, 15% to 20% as protein, 300 to 350 mg/d cholesterol, and 3 g/d sodium. Subjects were weight stable on this diet for 4 weeks before research testing. Registered dietitians monitored adherence by reviewing weekly food records and body weight and calculating dietary composition from biweekly 7-day food records (Nutritionist III software, N-Squared Computing). Twenty-four-hour urine collections for sodium verified dietary adherence to the prescribed sodium diet.
Study Design
After completion of the weight-stable 8-week AHA step I diet, the subjects underwent sequential measurement of renal hemodynamics (GFR, RPF, and glomerular FF [GFR/RPF]) with radioisotopic techniques as previously described.13 14 The next day, after completion of the renal hemodynamic measurements, subjects had a multidose hyperinsulinemic euglycemic clamp (40 and 100 mU/m2 per minute insulin infusion rates) with simultaneous measurement of basal and insulin-stimulated glucose disposal. The subjects were then switched to a 10-g sodium diet having the same caloric constituents as the 3-g sodium diet for 2 weeks before being retested with the radioisotopic renal clearances, followed on the next day by the hyperinsulinemic euglycemic clamp.
Measurement of Body Composition
Body weight was measured (±50 g) with a Homms beam balance (Western). The waist-to-hip circumference ratio was measured as the ratio of the minimal circumference of the abdomen to the circumference of the buttocks at the maximal gluteal protuberance. Percent body fat was measured by dual-energy x-ray absorptiometry (DEXA) (model DPX-L, Lunar Radiation Corp), and fat-free mass was calculated as total body mass minus fat mass in kilograms.
Measurement of VO2max
A treadmill VO2max test was performed in each subject on at least 2 separate days as previously described.12 A true VO2max value was considered to be attained if two of the following three criteria were met: (1) respiratory exchange ratio at maximal exercise greater than 1.10, (2) maximal heart rate greater than 90% of age-predicted maximum (220-age), and (3) a plateau in VO2 (<200 mL/min change in VO2) during the last stages of exercise. If a true VO2max was not attained on the second test or the VO2max results for the two exercise tests differed by greater than 200 mL/min, additional VO2max tests were performed until these criteria were met.
Blood Pressure Determinations
Blood pressure was measured in the morning by the same nurse clinician throughout the study with a random-zero mercury sphygmomanometer in the same arm and with the subject in the sitting position. Three consecutive blood pressure measurements were taken at no less than 30-second intervals. Korotkoff sounds I and V were used for recording of systolic and diastolic pressures, respectively.
Renal Hemodynamic Measurements
GFR, RPF, and glomerular FF were determined after an overnight fast as follows: 1 hour before the study, subjects consumed an oral water load of 10 to 15 mL/kg body wt to establish a brisk urine flow. An intravenous bolus injection of 100 mCi 99mTc-diethylenetriaminepentaacetic acid (DTPA) was then given, and after a 60-minute wait, the subjects voided, blood samples were drawn, and three timed sequential 1-hour urine collections were obtained, after which additional blood samples were drawn. The 99mTc-DTPA activity in the samples was determined by liquid scintillation counting. Urinary clearances of 99mTc-DTPA were calculated for each 1-hour collection period as urine activity times urine flow rate divided by average plasma activity. Average plasma activity was calculated as the mean of the plasma values over the interval from the beginning to the end of each urine collection. GFR was expressed as the average of the three 1-hour collection values.13 RPF was determined by measurement of the disappearance from serum of 60 mCi 131I-hippuran at precisely 44 minutes after injection, as previously described.14
Hyperinsulinemic Euglycemic Clamp Protocol
Three days before metabolic testing, subjects were provided with a calculated weight-maintaining AHA step I diet comparable to their home AHA diet. On the morning of the third day, subjects reported to the laboratory after a 12-hour overnight fast. A two-dose hyperinsulinemic euglycemic clamp was performed in each subject with the glucose-clamp technique, as modified by Rizza et al.15 In brief, a polyethylene catheter was inserted into an antecubital vein for infusion of potassium, insulin, and glucose. Another polyethylene catheter was inserted into a dorsal hand vein for blood sampling. This hand was placed in a heated (70°C) box to arterialize blood for sampling. Both catheters were kept patent by slow saline infusions. After a 30-minute equilibration period, three arterialized venous blood samples were obtained 10 minutes apart for measurement of baseline glucose and insulin levels. After the third baseline blood sample was drawn, a primed constant infusion of regular insulin (Humulin-R, Eli Lilly & Co) was started at a rate of 40 mU/m2 per minute. This insulin infusion rate was maintained for 120 minutes. At 120 minutes, a second primed constant infusion of insulin was administered at a rate of 100 mU/m2 per minute, which was also maintained for 120 minutes.
During the hyperinsulinemic euglycemic clamp, blood samples were obtained at 5-minute intervals, and aliquots of plasma were stored at -70°C for subsequent measurement of insulin. Plasma glucose was maintained at baseline levels by means of a variable infusion of 20% glucose that began 4 minutes after the insulin infusion. The glucose infusion rate was adjusted at 5-minute intervals according to a computerized algorithm that calculates required glucose infusion rates based on plasma glucose levels. Potassium chloride was infused at a rate of 10 mEq/h concurrently with insulin to avoid hypokalemia. GDR values were calculated during the last 30 minutes of the first and second doses and normalized for fat-free mass (milligrams per kilogram fat-free mass per minute). Steady-state plasma insulin levels were determined from the blood samples drawn every 10 minutes. Steady-state plasma GDR values were calculated during the last 30 minutes of each dose.
Statistical Analysis
Data were analyzed with a standard statistical software package (StatView, Abacus Concepts). Paired t tests were used for comparison of study variables in relation to changes in dietary salt. Pearson product correlation coefficients were used for determination of the relationships of glucose disposal to GFR, RPF, and glomerular FF during the two different salt diets. All data are expressed as mean±SD.
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Results |
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Table 1
shows subject characteristics. The 10 older (68±6 years), obese (body mass index, 31±4 kg/m2), deconditioned (VO2max, 19.0±3.7 mL/kg per minute), mildly hypertensive (151±8/82±2 mm Hg) subjects were studied on separate days with radioisotopic renal clearances and a two-dose hyperinsulinemic euglycemic clamp after 2 weeks of the 3-g and 2 weeks of the 10-g sodium diet.
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Blood Pressure and Renal Hemodynamics
Table 2 shows changes in the study variables in relation to dietary salt. Mean systolic, diastolic, and arterial pressures increased from low salt to high salt diet, but these changes did not reach statistical significance. Increasing dietary salt tended to increase GFR (P=.08) and RPF (P=.18).
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Hyperinsulinemic Euglycemic Clamps
Glucose values during the basal period and during the 40- and 100-mU insulin infusions were stable, with low coefficients of variation for both the low and high salt diets (Table 3). There was no influence of dietary salt on glucose infusion rates during the low and high salt diets (Fig 1).
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) and high (
) dietary salt interventions. FFM indicates fat-free mass.
Relationship of Renal Hemodynamics to Glucose Disposal
To determine the effects of dietary salt on insulin sensitivity and renal hemodynamics, we studied subjects on 3-g and 10-g sodium diets. There was an inverse relationship between GFR (Fig 2) and GDR during the low-dose insulin clamp (r=-.54, P=.02) and the higher-dose insulin clamp (r=-.68, P=.002) during both high and low salt diets. Similar relationships were found for glomerular FF during the low-dose insulin clamp (r=-.63, P=.005) and the higher-dose insulin clamp (r=-.71, P=.0001) for high and low salt diets (data not shown). RPF was not significantly related to GDR at either the low (r=.10, P=.71) or higher (r=.09, P=.75) rates of insulin infusion (data not shown) during either high or low salt diets.
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To distinguish the effects of high versus low salt diet on renal hemodynamics and glucose disposal, we analyzed data by dietary salt. During low salt diet (3 g sodium/d), there was an inverse relationship between glomerular FF and GDR during both the low-dose (r=-.70, P=.04) and high-dose (r=-.83, P=.006) hyperinsulinemic clamps (Fig 3). After high salt diet (10 g sodium/d), there was a trend toward an inverse relationship between glomerular FF and GDR during the low-dose (r=-.55, P=.13) and high-dose (r=-.59, P=.09) insulin clamps (Fig 4) that did not reach significance (data missing on one subject). This suggests that the high salt diet attenuated the relationship between glomerular FF and GDR.
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Discussion |
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The metabolic and physiological consequences of insulin resistance and hypertension on renal function are important concerns, as they increase the overall risk for renal injury and acceleration of hypertension. The pathogenetic mechanisms for this process are unclear. In this study, we examined the effects of dietary salt on the relationship of GDR, an index of insulin sensitivity, to glomerular hemodynamics in obese, sedentary, presumably insulin-resistant, older hypertensive subjects.
The results of our study, conducted in a small group of older, sedentary, obese, clearly insulin-resistant, mildly hypertensive subjects, demonstrate that higher GFR and glomerular FF are associated with a reduced GDR. An elevated glomerular FF could reflect glomerular hyperfiltration and increased glomerular capillary pressure, conditions identified as risk factors for the development of glomerulosclerosis in experimental models of renal disease and type I diabetes mellitus.16 17 Additionally, these renal hemodynamic changes can facilitate renal salt and water conservation,3 expand blood volume, and may contribute to maintaining a volume-dependent, salt-sensitive hypertensive state in insulin-resistant individuals.
The pathogenesis of diabetic nephropathy in patients with type I diabetes mellitus is related to the development of glomerular hyperfiltration as a consequence of elevated glomerular capillary pressures.16 This leads to the appearance of proteinuria and development of hypertension, a well-accepted explanation for the progression of renal disease in diabetics.16 17 The pathogenesis of diabetic nephropathy in patients with type II diabetes mellitus is not well understood, as several etiologic factors acting simultaneously may promote the development of nephropathy.18 Nevertheless, therapeutic efforts to lower systemic blood pressure and reduce glomerular capillary pressure with angiotensin-converting enzyme inhibitors are effective in delaying renal damage in human hypertensive subjects with type I diabetes and proteinuria.18
It is important to point out that our measures of renal hemodynamics were performed with subjects in the fasting state on the day before the hyperinsulinemic euglycemic clamps. Consequently, there may be a different relationship between renal hemodynamics and GDR if renal hemodynamics are measured during the acute hyperinsulinemic glucose loading procedure. Thus, our correlational observations between renal hemodynamics and GDR should be interpreted in the following manner: In the fasting state, when there is little evidence to suggest that insulin levels are elevated in hypertensive individuals independent of obesity, our measurements of GFR and glomerular FF were higher in the more insulin-resistant subjects, as defined by their decreased GDR at low- and high-dose insulin infusions measured during the euglycemic clamp.
Our subjects manifested a subtle but not statistically significant increase in mean arterial pressure in transition from the low (3 g sodium) to the high (10 g sodium) salt diet. Prior investigation demonstrated that increasing dietary salt in salt-sensitive individuals increases glomerular FF and proteinuria.19 20 21 Consequently, dietary salt sensitivity could be a confounding variable in the interpretation of our results. For this reason, we studied our subjects during both low salt and high salt diets. The results indicate that irrespective of dietary salt consumption, a higher glomerular FF was related to a reduction in GDR during the clamp or to insulin resistance. This was observed consistently during the low salt diet, suggesting that insulin resistance may be an independent factor mediating the deterioration of renal function when the absolute intake of dietary salt is controlled in the low, recommended range for hypertensive patients. However, during higher salt diet, a trend toward higher glomerular FF with impaired GDR was evident, suggesting an interaction between the effects of insulin resistance with high dietary salt on renal hemodynamics. Thus, under conditions of low salt intake, older, obese, sedentary hypertensive subjects with impaired GDR have higher GFR and glomerular FF; however, this relationship is attenuated during high salt loading, perhaps because of the direct effects of dietary salt on renal hemodynamics,19 20 21 the renin-angiotensin system, or insulin resistance.22
We did not randomize the order of salt diets to facilitate the ease and compliance in the dietary changeover from low salt to high salt. In a previous study19 that did randomize the order of low salt and high salt diet over a 2-week period, we did not demonstrate any carryover effect on blood pressure or renal hemodynamics. Additionally, in contrast to younger subjects,22 insulin resistance did not increase during the high salt diet in these older, deconditioned, obese hypertensive subjects.
The explanation for the observed relationship between impaired GDR and elevated GFR and glomerular FF during fasting is unclear. GFR was consistently higher in subjects with impaired glucose disposal, yet RPF was not significantly different. The net result was an increase in glomerular FF. One might speculate that these findings could be the result of either inadequate autoregulation of afferent glomerular arteriolar resistance or excessive efferent glomerular arteriolar vasoconstriction relative to afferent glomerular arteriolar dilation. This could be indicative of the failure of an endogenous renal dilator on the efferent arteriole such as dopamine, kallikrein, nitric oxide, or insulin, as has been proposed in salt-sensitive states,23 or could be an epiphenomenon indicative of another as yet undetermined process occurring in the kidney of hypertensive insulin-resistant individuals. These glomerular hemodynamic changes also could enhance salt sensitivity and elevate systemic arterial pressure by increasing renal tubular sodium reabsorption via previously described mechanisms.3 Additionally, salt loading could reduce the observed "rise" in GFR and glomerular FF by suppressing angiotensin II production and therefore reduce its vasoconstrictive effect on efferent glomerular arteriolar resistance, whereas mild salt restriction could activate the renin-angiotensin system and enhance postglomerular vasoconstriction and elevate glomerular FF. These ideas could be tested in future studies by comparing glomerular hemodynamic responses under fasted and hyperinsulinemic conditions.
In summary, these results suggest that a reduction in glucose disposal, consistent with insulin resistance in older, sedentary, obese, hypertensive subjects, is associated with higher glomerular FF in the fasted state. They may in part explain the propensity of hypertension to accelerate renal injury in these individuals. This relationship was strong during low salt intake but weakened during high salt intake, suggesting that high dietary salt may interact with the renin-angiotensin system in insulin-resistant states. The relationship of the insulin resistance syndrome to glomerular FF in these subjects may provide insight into therapeutic modalities for reducing the risk of chronic renal failure and cardiovascular disease. Since nonpharmacological therapeutic interventions (diet and exercise) as well as pharmacological agents (thiazolidinedione or metformin therapy) can enhance insulin sensitivity and improve glucose disposal, intervention studies are needed to assess their effect on glomerular hemodynamics and determine their efficacy in reducing the risk for progression to renal injury in obese, sedentary, older hypertensive individuals.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This work was supported by an American Heart Association, Maryland Affiliate, Grant-in-Aid (Dr Weir); a grant from the Fraternal Order of Eagles, Maryland (Dr Weir); the Department of Veterans Affairs Geriatric Research, Education and Clinical Center at Baltimore; National Institutes of Health grants (KO1 AG 000655-01 to Dr Dengel, Geriatric Leadership Academic Award 1KO 7AG 00603 to Dr Goldberg); and the Claude D. Pepper Older Americans Independence Center (P60 AG 12583 to Dr Goldberg). We would like to acknowledge the expert secretarial assistance of Jacqueline Brandenburg and the research support of Linda Bunyard, Marilyn Lumpkin, and Jana Dengel in the conduct of these studies.
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Footnotes |
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Reprint requests to Matthew R. Weir, MD, Division of Nephrology, University of Maryland Hospital, 22 S Greene St, Baltimore, MD 21201.
Received September 21, 1995;
first decision November 29, 1995; first decision March 1, 1996;
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