Segmental Tubular Sodium-Reabsorption In Type-1 Diabetes

Segmental tubular sodium reabsorption in Type 1 (insulin-dependent) diabetes was measured in 36 patients in a cross-sectional study including one group (n = 13) without significant albuminuria (UalbV < 30 mg 24 h-1), one group (n = 16) with albuminuria in the range from 30 to 300 mg 24 h-1, and a group (n = 7) with nephropathy (UalbV > 300 mg 24 h-1). Lithium clearance was used to measure end proximal delivery. From end proximal delivery, Cr-51-EDTA clearance (GFR) and sodium clearance, segmental tubular reabsorption was calculated. For all patients, GFR was directly correlated with end proximal delivery (r = 0.62, p < 0.0005), while end proximal delivery was inversely correlated to fractional proximal reabsorption (r = -0.71, p < 0.0005). In the subgroup with UalbV less than 30 mg 24 h-1, the direct correlation between GFR and end proximal delivery was also significant (r = 0.77, p < 0.05). In the group with nephropathy (UalbV > 300 mg 24 h-1), mean GFR and end proximal delivery were decreased and fractional proximal reabsorption was increased, but there was still a positive correlation between GFR and end proximal delivery (r = 0.75, p < 0.05) and an inverse correlation between end proximal delivery and fractional proximal reabsorption (r = -0.85, p < 0.05). It is concluded that in these groups of diabetic patients the end proximal delivery is increased while GFR is increased. This finding is an argument against a recent hypothesis explaining intraglomerular hypertension, as well as increased glomerular filtration rate in some patients, as secondary to decreased end proximal delivery and reduced tubuloglomerular feedback activity. Nevertheless, the tubuloglomerular feedback activity may be reduced in Type 1 diabetes due to constant extracellular volume expansion.