Kinetic Behavior of the Erythrocyte Sodium-Lithium Countertransporter in Nonnephropathic Diabetic Twins Timothy C. Hardman, Simon W. Dubrey, R. David G. Leslie, and Ariel F. Lant 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 • 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 Vrnax 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, • = .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, Copyright © 1996 by W.B. Saunders Company BNORMAL BEHAVIOR of the erythrocyte mem- brane sodium-lithium countertransporter is associ- ated with diabetes. 1,2 However, problems have arisen in the interpretation of this abnormality, which could be linked either to the disease itself and/or to its complications) The failure of many studies to consider or separate the confound- ing influences of genetics and disease processes has caused confusion. 3 Part of the reason for this may be the complex mix of genetic and environmental influences on the counter- transporter. 4,5 Another reason may lie within the measure- ment of sodium-lithium countertransport activity itself. Until recently, the activity value was thought to represent the maximal turnover rate (Vm~) of the countertransporter. However, there is a basic weakness in the reliance on counter- transport activity as anything other than a rate of lithium efflux determined under specified conditions, since there is good evidence that countertransport activity is a composite expres- sion of the Vm~x and the external affinity for sodium (kNa).6 The present study was performed to determine whether the diabetic state has an effect on the sodium-lithium countertransporter that is distinct from influences of an environmental or genetic nature? To control for environ- mental and genetic influences, the subjects chosen for study were a cohort of twins discordant for diabetes who were free of complications of the disease at the time of study. SUBJECTS AND METHODS Subjects We studied 15 identical-twin pairs discordant for insulin- dependent diabetes mellitus (one twin had diabetes and the other did not) who fulfilled the following inclusion criteria: age 20 to 65 years, white ethnicity, availability of both twins for study, absence of hypertension or microalbuminuria (albumin excretion rate > 20 I~g albumin/rain in a timed overnight urine sample), and absence of treatment with prescribed antihypertensive drugs. The diabetic twin in each pair had been treated from the time of diagnosis (mean _+ SD, 16 _+ 10 years) with either highly purified porcine or human insulin. We also studied 15 healthy control subjects who were individu- ally matched with the discordant twin pairs for sex; in each individual case, control subjects were also matched with nondia- betic twins to within 5% with respect to age and body mass index. These control subjects were selected from a cohort of 100 subjects who were employees of local retail firms screened for the purpose of this study. None of the healthy subjects studied had any known From the Department of Clinical Pharmacology and Therapeutics, Charing Cross and Westminster Medical School, Chelsea and Westmin- ster Hospital, London, the Department of Diabetes, St Bartholomew's Hospital, London, UK, and the Division of Cardiology, Boston Universi~ Medical Centre Hospital, Boston, MA. Submitted July 26, 1995; accepted Apri125, 1996. Supported in part by grants from the Joint Trustees of Westminster and Roehampton Hospitals, London, the Lant Trust for Medical Research, the British Heart Foundation, the Wellcome Trust, and the Diabetic Twin Research Trust. Address reprint requests to Ariel F. Lant, PhD, Department of Clinical Pharmacology and Therapeutics, Charing Cross and Westmin- ster Medical School, Chelsea and Westminster Hospital, 369 Fulham Rd, London, SWIO 9NH, UK. Copyright © 1996 by WB. Saunders Company 0026-0495/96/4510-000353.00/0 Metabo/ism, Vo145, No 10 (October), 1996: pp 1203-1207 1203