Erythrocyte sodium-lithium countertransport is not different in type 1 (insulin-dependent) diabetic patients with and without diabetic nephropathy

Na/Li countertransport abnormalities in type 1 diabetes with and without nephropathy are familial

OBJECTIVE

To determine whether there is a familial abnormality in erythrocyte Na/Li countertransport (CT) kinetics in the approximate one-third of type 1 diabetic patients that succumb to a familial predisposition to nephropathy.

RESEARCH DESIGN AND METHODS

Erythrocyte Na/Li CT kinetics were measured in nondiabetic first-degree relatives of type 1 diabetic patients with nephropathy (DNrel) (n = 32) or without nephropathy (DCrel) (n = 22) and normal control subjects ( n = 25).

RESULTS

Increases in outside-site Na ion association rate constant and turnover rate of Na/Li countertransport (CT) in DNrels caused increases in Vmax/Km and Vmax, respectively. Thiol alkylation with N-ethy]maleimide (NEM) modifies these kinetic parameters abnormally in nephropathy. With Na ions at the outside site of the transporter, thiol alkylation causes a large decrease in Vmax; but in their absence, Vmax is decreased in normal control subjects, unchanged in DCrels, or increased in DNrels. The relationship between Vmax values after thiol alkylation with or without Na ions was different in DNrels (P < 0.001). Kinetic parameters with and without thiol alkylation identified 60% of DNrels and 20% of DCrels as abnormal. The single-flux rate assay of Na/Li CT did not give this discrimination, and its use may cause discrepancy between studies.

CONCLUSIONS

Clinically normal untreated DNrels have the same abnormality in Na/Li CT as the affected patients. DNrels had a metabolic syndrome with increased BMI and plasma triglycerides, but no elevation in blood pressure. Na/Li CT can detect those type 1 diabetic patients at risk of nephropathy who have a familial abnormality in a membrane thiol protein.

Progression of diabetic nephropathy

BACKGROUND

Diabetic nephropathy is a major cause of renal failure. The decline in glomerular filtration rate (GFR) is highly variable, ranging from 2 to 20, with a median of 12 mL/min/year. The risk factors of losing filtration power (progression promoters) have not been clearly identified. Furthermore, information on optimal arterial blood pressure, glycemic control, and cholesterol levels are lacking.

METHODS

We measured GFR with (51)Cr-EDTA plasma clearance technique, blood pressure, albuminuria, glycosylated hemoglobin A1c, and serum cholesterol every year for seven years (range 3 to 14 years) in 301 consecutive type 1 diabetic patients with diabetic nephropathy recruited consecutively during 1983 through 1997. Diabetic nephropathy was diagnosed clinically if the following criteria were fulfilled: persistent albuminuria> 200 microg/min, presence of diabetic retinopathy, and no evidence of other kidney or renal tract disease. In total, 271 patients received antihypertensive treatment at the end of the observation period.

RESULTS

Mean arterial blood pressure was 102 +/- 0.4 (SE) mm Hg. The average decline in GFR was 4.0 +/- 0.2 mL/min/year and even lower (1.9 +/- 0.5 mL/min/year) in the 30 persistently normotensive patients, none of whom had ever received antihypertensive treatment (P < 0.01). A multiple linear regression analysis revealed a significant positive correlation between the decline in GFR and mean arterial blood pressure, albuminuria, glycosylated hemoglobin A(1c), and serum cholesterol during follow-up (R(adj)(2) = 0.29, P < or = 0.001). No threshold level for blood pressure, glycosylated hemoglobin A(1c), or serum cholesterol was demonstrated. A two-hit model with mean arterial blood pressure and glycosylated hemoglobin A(1c) below and above the median values (102 mm Hg and 9.2%, respectively) revealed a rate of decline in GFR of only 1.5 mL/min/year in the lowest stratum compared with 6.1 mL/min/year in the highest stratum (P < 0.001).

CONCLUSIONS

The prognosis of diabetic nephropathy has improved during the past decades, predominantly because of effective antihypertensive treatment. Genuine normotensive patients have a slow progression of nephropathy. Several modifiable variables have been identified as progression promoters.

Effects of antihypertensive therapy on progression of diabetic nephropathy

There is a clear relationship between hypertension and the microvascular complications of diabetes. Genetic predisposition to hypertension has been correlated to the risk of diabetic nephropathy in type I diabetes, and hypertension is a well known risk factor for developing nephropathy in patients with type II diabetes. Multiple studies have emphasized the importance of hypertension on renal disease progression, and blood pressure control with conventional antihypertensive drugs slows the rate of renal function loss in diabetic nephropathy. Furthermore, evidence of the role of renin-angiotensin system (RAS) on progression of renal damage has focused much interest on the therapeutic action of the RAS blockade. In patients with type I diabetes, blocking the RAS with angiotensin converting enzyme (ACE) inhibitors prevents progression from microalbuminuria to overt nephropathy, and in overt nephropathy decreases the gradual loss of renal function beyond its blood pressure lowering effect. Less clinical information is available in type II diabetic nephropathy, but our experience and some recent studies suggest that ACE inhibitors also have a renoprotective action in type II diabetes. The role of calcium channel blockers in diabetic nephropathy is not clear. Several short-term studies with the first generation dihydropyridine calcium antagonists showed a lower effect on urinary albumin excretion and a more rapid progression to renal failure than with ACE inhibitors. However, other calcium channel blockers, particularly of the non-dihydropyridine type, have been shown to have a beneficial effect on diabetic nephropathy, decreasing proteinuria and slowing progression.

Prevention and slowing down the progression of the diabetic nephropathy through antihypertensive therapy

Diabetic nephropathy is the major cause of illness and premature death in people with diabetes, largely through accompanying cardiovascular disease and end-stage renal failure. Diabetic patients are several times as prone to kidney disease as nondiabetic people and the accumulative risk of diabetic nephropathy in insulin-dependent diabetes mellitus (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM) is about 30%-50% after 25 years of disease. Diabetic nephropathy is a progressive disease that takes several years to develop, ending in chronic renal insufficiency. Proteinuria heralds the onset of diabetic nephropathy, and the worsening of proteinuria parallels the progression of renal disease. The main risk factors for the frequency, severity, and progression of diabetic nephropathy are the degree of hyperglycemia and associated metabolic disturbances, hypertension, protein overload, cigarette smoking, as well as the duration of diabetes. Interventional strategies for primary, secondary, and tertiary prevention of diabetic nephropathy therefore include meticulous glycemic control, appropriate treatment of associated lipid abnormalities, rigorous control of the blood pressure, reduction in dietary protein intake, in particular animal protein, and of fat intake, and stopping cigarette smoking. Randomized clinical trials indicate that antihypertensive therapy is beneficial in preventing and slowing down the progression of diabetic nephropathy. There is now increasing evidence that angiotensin-converting enzyme inhibitors and certain calcium antagonists produce a more beneficial effect on diabetic nephropathy in terms of reducing proteinuria and slowing the progression to diabetic renal failure. These drugs are attributed nephroprotective capacity beyond their blood pressure lowering capacity and initial clinical trials with combinations have revealed even additive protective effects on end organs.

Can we further slow down the progression to end-stage renal disease in diabetic hypertensive patients?

HYPERTENSION AND DIABETES

Hypertension occurs about twice as frequently in diabetics compared with the general population, with a prevalence of approximately 25% in young patients with insulin-dependent diabetes mellitus (IDDM) and 50% in newly diagnosed non-IDDM (NIDDM) patients. Studies strongly suggest that hypertensions is involved in the progression and perhaps the onset of diabetic nephropathy, which is a major cause of illness and premature death in diabetic patients, largely through accompanying cardiovascular disease and end-stage renal failure.

TREATMENT

A large body of evidence has accumulated which emphasizes the beneficial effects of antihypertensive treatment in reducing proteinuria and preserving renal function in both IDDM and NIDDM. It appears that angiotensin converting enzyme inhibitors and certain calcium antagonists, notably the non-dihydropyridine type and second-generation dihydropyridine calcium antagonists, produce a more beneficial effect on nephropathy in terms of reducing proteinuria and slowing progression in renal failure. These drugs have displayed a nephroprotective capacity beyond their systemic blood pressure-lowering effects, and initial clinical trials with combinations of different antihypertensive drug classes have revealed additive effects in reducing albuminuria together with the lowest rate of decline in glomerular filtration rates with the lowest incidence of adverse effects.

AVAILABLE STUDIES

The studies available on antihypertensive treatment in IDDM and NIDDM patients with incipient or overt diabetic nephropathy are mainly prospective. There have also been some preliminary trials with antihypertensive combinations in diabetic patients.

Kinetic behavior of the erythrocyte sodium-lithium countertransporter in nonnephropathic diabetic twins

Elevated erythrocyte sodium-lithium countertransport activity occurs in diabetes and may be genetically mediated. The relation of this abnormality to the disease and its complications is unclear. To remove confounding genetic factors and the impact of complications, we studied sodium-lithium countertransport activity together with its kinetic components, maximal rate of turnover (Vmax) and external affinity for sodium (kNa), in identical-twin pairs discordant for insulin-dependent diabetes who were normotensive and had no evidence of nephropathy. Fifteen twin pairs were studied along with the same number of healthy control subjects matched with the twins for gender, age, and body mass index. Clinical and laboratory characteristics of the twins and controls were similar, with the exception that whole blood glucose and glycated hemoglobin concentrations were higher in diabetic twins (P < .001). Comparison of countertransport activity between nondiabetic and diabetic twin groups failed to uncover any significant differences (P = .30, Wilcoxon). Similarly, there were no differences in countertransport activity between the nondiabetic twin group and the controls (P = .38, Mann-Whitney). Furthermore, no associations were noted between residual activity values and residual data of any of the other clinical or laboratory characteristics measured. Comparison of Vmax between nondiabetic and diabetic twin groups showed a significant elevation in the diabetic twins (0.515 + 0.220 v 0.439 + 0.229 mmol Li/L RBC x h, P = .049, paired t test). By contrast, no significant differences were noted between the nondiabetic twin group and the controls (P = .15, unpaired t test). Comparison of kNa between nondiabetic and diabetic twin groups found no significant differences in kNa (P = .42, Wilcoxon). Similarly, there were no differences in kNa between nondiabetic twins and controls (P = .14, Mann-Whitney). Neither the residual data for Vmax nor kNa showed any association with the residual data of any of the other clinical or laboratory characteristics measured. When intertwin correlations were examined, all three parameters describing the behavior of the sodium-lithium countertransporter showed significant intertwin correlations (activity, r = .51, P = .04; Vmax, r = .82, P = .001; kNa, r = .76, P = .001). In conclusion, the diabetic state has a small effect on the Vmax of the sodium-lithium countertransporter. Failure to consider the complex nature of the activity measurement is likely to have been partly responsible for earlier confusion with regard to the effect of diabetes on the countertransporter, since experimental conditions varied between studies and individual kinetic components were not measured. The associations between twins in this study with respect to Vmax and kNa indicate a genetic influence on both constants of the countertransporter. Vmax appears also to be sensitive to certain as yet unidentified environmental factors.

Diabetic nephropathy and pregnancy

Knowledge of the pathogenic mechanisms of diabetic nephropathy (by which hyperglycemia, hyperfiltration, and hypertension cause the gradual development of microproteinuria, mesangial expansion, and eventual glomerular closure) provides the basis for effective treatment. Intensified glycemic control and antihypertensive therapy that is safe for the fetus are crucial for success during pregnancy. Considered outcome measures include perinatal survival, size at birth, child development, and long-term maternal renal function.

Diabetic nephropathy in type II diabetes

The incidence of end-stage renal failure in patients with type II diabetes has dramatically increased in recent years, both in the United States and, with some delay, in some European countries. These epidemiologic observations have thoroughly dispelled the mistaken belief that renal prognosis was benign in type II diabetes. Recent interest has focused on the early stages of nephropathy in type II diabetes. With respect to renal hemodynamics, renal morphology, and progression of established diabetic nephropathy, there are no substantial differences between types I and type II diabetes. There is good evidence that preventive measures are effective, ie, glycemic control, blood pressure control, protein restriction, and discontinuation of smoking. The high prevalence of the disease (which in principle is preventable) calls for intense efforts to (1) educate the medical community, (2) substantially improve patient education and medical care, and (3) intensify research in this field.

Genetic susceptibility to diabetic kidney disease: an update

Various types of evidence supporting the hypothesis of genetic susceptibility to diabetic kidney disease (DKD) are reviewed. Three groups of studies were distinguished: (1) epidemiologic and family studies, (2) studies of phenotypic markers/predictors, and (3) studies of DNA markers. Although all of these studies point to the existence of susceptibility to DKD, further research is required. Particularly needed are studies that examine the mechanisms of interaction between genetic susceptibility to DKD and poor glycemic control, and studies to identify specific molecular mechanisms underlying this susceptibility.

Erythrocyte sodium-lithium countertransport activity is related to membrane fluidity in IDDM patients

Sodium-lithium countertransport (SLC) activity at a standard physiological sodium concentration is raised in uncomplicated IDDM, for which the kinetic mechanism is a raised maximum velocity (Vmax). Diabetic patients with nephropathy do not have raised values for Vmax but a low Michaelis constant (km). Transporter activity could be influenced by its membrane lipid environment. This was assessed in 21 control subjects, 32 uncomplicated diabetic patients, 17 patients with diabetic nephropathy and 11 patients with non-diabetic nephropathy by measuring the fluorescence anisotropy of DPH and TMA-DPH to assess different membrane regions. Standard SLC was higher in all the patient groups compared to the control subjects: 0.307 +/- 0.020 mmol Li/h x 1 cells in uncomplicated IDDM; 0.300 +/- 0.032 in diabetic nephropathy patients and 0.276 +/- 0.019 in non-diabetic nephropathy patients vs 0.216 +/- 0.011 mmol Li/h x 1 cells in control subjects (p < 0.001, p < 0.05, p < 0.05, respectively). This was due to raised Vmax values in the uncomplicated group: 0.528 +/- 0.035 vs 0.385 +/- 0.022 mmol Li/h x 1 cells in control subjects (p = 0.001) and low values for km in the diabetic nephropathy group: 58 (27-170) vs 106 (81-161) mmol/l in control subjects (p < 0.001). Raised SLC in the non-diabetic nephropathy group was largely due to raised Vmax: 0.460 +/- 0.030 mmol Li/h x 1 cells; p = 0.053, with no difference in km: 99.5 (74-137).(ABSTRACT TRUNCATED AT 250 WORDS)

Increased red cell sodium lithium countertransport activity, total exchangeable sodium, and hormonal control of sodium balance in normoalbuminuric type 1 diabetes

The relationship between erythrocyte sodium lithium countertransport activity (SLC), total exchangeable sodium (NaE), and hormonal control of renal function was examined in 40 normotensive, normoalbuminuric, non-neuropathic Type 1 diabetic subjects, of whom 8 had elevated SLC (> 0.40 mmol Li h-1l-1 rbc). Eleven health controls with normal SLC, who were of comparable age, body mass, and blood pressure were also studied. By contrast with healthy controls, SLC in Type 1 diabetes was not associated with plasma renin activity (PRA), aldosterone, systolic blood pressure or lean body mass. SLC was also unrelated to atrial natriuretic peptide (ANP) (Type 1 diabetes only) and NaE. NaE was not correlated with any other variables. The relationships between PRA and aldosterone in healthy controls were retained in Type 1 diabetes (R2 0.37 supine, p = 0.00001, and 0.27 ambulant, p = 0.0005), as were respective direct and inverse relations between vasopressin and ANP and both PRA (rs 0.54 to 0.57, rs -0.43 to -0.53), and aldosterone (rs 0.78 to 0.80, rs -0.71 to -0.80). Fasting free serum insulin and vasopressin were both inversely related to ANP (rs -0.91 and -0.71, respectively). In the absence of autonomic dysfunction, hypertension or early nephropathy in Type 1 diabetes, increased SLC or exchangeable sodium were unrelated to each other or with hormonal control of sodium balance, but the homeostatic factors controlling hormonal interaction appear to be maintained. The interaction between insulin and hormonal control of sodium and water balance may be modified by circulating free insulin concentrations.

Pre-diabetic blood pressure predicts urinary albumin excretion after the onset of type 2 (non-insulin-dependent) diabetes mellitus in Pima Indians

Blood pressure was measured in 490 non-proteinuric Pima Indians from the Gila River Indian Community in Arizona at least 1 year before the diagnosis of Type 2 (non-insulin-dependent) diabetes mellitus. Urine albumin concentration was measured in the same subjects 0-24 years (mean 5 years) after diabetes was diagnosed. Prevalence rates of abnormal albumin excretion (albumin-to-creatinine ratio > or = 100 mg/g) after the onset of Type 2 diabetes were 9%, 16%, and 23%, respectively, for the lowest to highest tertiles of pre-diabetic mean blood pressure. When controlled for age, sex, duration of diabetes and pre-diabetic 2-h post-load plasma glucose concentration, higher pre-diabetic mean blood pressure predicted abnormal urinary excretion of albumin after the onset of diabetes. This finding suggests that the higher blood pressure seen in diabetic nephropathy is not entirely a result of the renal disease, but may precede and contribute to it.

Sodium-lithium countertransport and triglycerides in diabetic nephropathy

Elevated erythrocyte sodium-lithium countertransport (SLC) activity is an intermediate phenotype of essential hypertension among Caucasians, and is controversially associated with nephropathy in Type 1 (insulin-dependent) diabetes. Hypertriglyceridemia is a frequent concomitant of elevated SLC in the general population, and may be found in diabetic nephropathy. The present study was designed to investigate the influence of kidney disease, serum triglycerides and blood pressure on the interindividual variability of SLC in Type 1 diabetes. SLC and fasting major serum lipids were studied in 35 Type 1 diabetic patients with persistently elevated urinary albumin excretion and in a group of patients matched for age, sex and duration of diabetes, but with normoalbuminuria. SLC was elevated in patients with clinical nephropathy (N = 10; median: 420 mumol.1RBC-1.hr-1) and in patients with microalbuminuria (N = 25; median: 405 mumol.1RBC-1.hr-1) compared with normoalbuminuric patients (median: 296 mumol.1RBC-1.hr-1; P < 0.01 vs. both groups). Hypertriglyceridemia and hypercholesterolemia were found only among patients with macroalbuminuria. Analysis of covariance indicated that the association of elevated SLC with kidney disease (P < 0.006 in all models) was largely independent of serum triglycerides, but also of total cholesterol, insulin dose and measures of glycemic control. Only diastolic blood pressure was positively associated with SLC (P < 0.02) independently from nephropathy (P < 0.005) also after restricting analysis to the normoalbuminuric patients. Kidney disease and raised blood pressure remain major concomitants of elevated SLC in Type 1 diabetics.

An overview of renal pathology in insulin-dependent diabetes mellitus in relationship to altered glomerular hemodynamics

Clinical diabetic nephropathy in man is the consequence of the development of a specific constellation of glomerular, tubular, vascular, and interstitial structural abnormalities accompanied by highly characteristic immunohistochemical alterations that, together, are unique to diabetes. Because changes resembling the specific pathology of diabetes do not develop in patients with conditions that lead to long-standing glomerular hyperfunction (such as unilateral nephrectomy), it is unlikely that glomerular hemodynamic abnormalities per se can be the cause of diabetic nephropathy. Whether hemodynamic abnormalities represent a risk factor that, in the presence of the diabetic state, can accelerate the rate of development of the basic lesions of diabetic nephropathy is currently unclear. However, there is considerable evidence that when the renal lesions of diabetes are far advanced, factors such as systemic hypertension can determine the rate of renal functional deterioration in diabetes as in other disorders. Although the diabetic rat may be a useful model for the study of aspects of the pathogenesis of diabetic nephropathy, much confusion has resulted from the inclusion of focal segmental glomerular sclerosis as a diabetic lesion. Similarly, the acceptance of all increases in urinary protein excretions in this model as resulting from or reflecting of diabetic nephropathology can be misleading. It is concluded that treatment aimed at manipulating renal hemodynamics in diabetic patients without evidence of renal disease should remain in the realm of clinical research.

Predisposition to hypertension: risk factor for nephropathy and hypertension in IDDM

Less than a quarter of the patients with juvenile-onset IDDM develop diabetic nephropathy during the first 20 years of diabetes. To study the determinants of this complication, we selected patients who had come with newly diagnosed IDDM to the Joslin Clinic between 1967 to 1972, and we examined them in 1986 to 1988, that is, 15 to 21 years after onset of diabetes. Using a case control design we compared three groups of cases, that is, advanced nephropathy (N = 43), only microalbuminuria (N = 41), and hypertension alone (N = 17), with a group of controls who remained normoalbuminuric and normotensive despite the long duration of IDDM (N = 61). In comparison with controls, patients with advanced nephropathy had more parents with hypertension (odds ratio 3.8), higher Vmax values of Na/Li countertransport in red blood cells (odds ratio 10.0 for the highest tertile), and higher mean arterial pressure during adolescence and early adulthood (odds ratio 3.1 for those above the median). They also had significantly poorer glycemic control during their first 12 years of diabetes. Patients with hypertension alone were similar to those with advanced nephropathy with regard to markers of predisposition to hypertension but differed from them with regard to glycemic control, having the best glycemic control of all the study groups. Patients who developed only microalbuminuria during 15 to 21 years of IDDM (some of whom will progress to overt proteinuria later) did not differ significantly from controls with regard to predisposition to hypertension. In conclusion, predisposition to hypertension is a major risk factor for the development of advanced diabetic nephropathy and essential hypertension during the first 20 years of IDDM.

Is increased erythrocyte sodium-lithium countertransport a useful marker for diabetic nephropathy?

Genetic predisposition to essential hypertension has been proposed as a risk factor for the development of diabetic nephropathy in type 1 (insulin-dependent) diabetes mellitus. An increased sodium-lithium countertransport activity (NaLiCT) has been suggested as a genetic marker for essential hypertension. We therefore evaluated NaLiCT in diabetic patients with (N = 39) or without (N = 23) diabetic nephropathy (DNP), patients with non-diabetic renal diseases (N = 42) and in healthy controls (N = 24). The NaLiCT was elevated in both diabetic patient groups compared to healthy controls (median 244; range 134 to 390 mumol.liter cells-1.hr-1), but was not different in patients with DNP (median 314; range 162 to 676), without DNP (median 325; range 189 to 627) and patients with non-diabetic renal disease (median 300; range 142 to 655). The genetic predisposition to DNP is illustrated by the fact that diabetic sibs of probands with DNP showed a higher occurrence of DNP than diabetic sibs of patients without DNP. We analyzed whether familial DNP clustered with an increased NaLiCT. The NaLiCT in sibs concordant for the presence of DNP (N = 10; median 307; range 217 to 428 mumol.liter cells-1.hr-1) was not significantly different from that in sibs concordant for absence of DNP (N = 15; median 279; range 189 to 442). We conclude that erythrocyte sodium-lithium countertransport activity cannot be used as a marker to identify patients at risk for the development of diabetic nephropathy.