MEASURES OF TUBULAR FUNCTION IN NORMOALBUMINURIC INSULIN-DEPENDENT DIABETIC-PATIENTS AND THEIR RELATIONSHIP WITH SODIUM-LITHIUM COUNTERTRANSPORT ACTIVITY

The aim of this study was to assess the relationship between markers o f tubular function, markers of glycaemic control and erythrocyte sodiu m-lithium countertransport activity (SLC) in 40 normotensive, normoalb uminuric insulin-dependent diabetic (IDDM) subjects and 11 normal cont rol subjects. Nine IDDM subjects had SLC >0.40 mmol lithium h(-1) litr e RBC(-1). Glomerular filtration rate (GFR) and the excretion rate of retinol-binding protein (REP), N-acetyl-beta-D-glucosaminidase (beta-N AG) and glucose were significantly higher in IDDM subjects compared to control subjects (Mann-Whitney test, p = 0.02, <0.001, <0.001 and <0. 001, respectively), whilst the two groups had similar SLC and TmPO4 le vels. There was no significant relationship between SLC and the other variables in IDDM subjects, even when comparing IDDM subjects with nor mal and high SLC. beta-NAG excretion rate was correlated to urinary gl ucose (r(s) 0.47, p = 0.001) and, weakly, to the other markers of glyc aemic control (fasting blood glucose r(s) = 0.31, p = 0.03, fructosami ne r(s) 0.28, p = 0.04, HbA(1) r(s) 0.27, p = 0.04). RBP excretion rat e was correlated to the excretion rate of beta-NAG (r(s) 0.38; p = 0.0 07) and albumin (r(s) 0.45; p = 0.002); the excretion rates of beta-NA G and albumin were significantly associated (r(s) 0.37, p = 0.009). Di abetes duration did not correlate to any of the aforementioned variabl es. In this study, beta-NAG and REP overnight excretion rates were hig her in normoalbuminuric IDDM subjects compared to control subjects but no relationship was present between SLC and tubular function in IDDM patients without complications. Excretion rates of different proteins appear to be interrelated and, in IDDM, beta-NAG excretion is associat ed with glycaemic control.