Feedback-mediated reduction in glomerular filtration during acetazolamide infusion in insulin-dependent diabetic...

1. A sustained high glomerular filtration rate in diabetes mellitus is associated with increased proximal reabsorption, suggesting alterations in the tubuloglomerular feedback system. To test this hypothesis, renal function was studied in eight control subjects and 14 recent-onset euglycaemic insulin-dependent diabetic patients before and after infusion of the carbonic anhydrase inhibitor, acetazolamide (5 mg/kg body weight). 2. Acetazolamide induced a dramatic fall in glomerular filtration rate in both diabetic patients and control subjects (from 138 +/- 5 to 114 +/- 4 and from 127 +/- 3 to 113 +/- 2 ml min-1 1.73 m-2, respectively, P less than 0.0001). This fall in glomerular filtration rate was strongly correlated with the acetazolamide-induced decrease in absolute proximal reabsorption calculated by using lithium clearance. 3. To further assess the potential role of angiotensin II in the acetazolamide-induced tubulo-glomerular feedback response, 11 additional diabetic patients were investigated before and after the administration of acetazolamide plus the angiotensin-converting enzyme inhibitor, enalaprilat (1.25 mg intravenously). Despite the effective blockade of angiotensin II formation and a slight decrease in renal vascular resistance, the glomerular filtration rate fell significantly and by a similar magnitude as seen with acetazolamide alone. 4. These results indirectly suggest that there is an altered basal tubulo-glomerular feedback system in diabetic patients but a normal response to the increase in distal delivery. No convincing role for an angiotensin II-mediated effect on the afferent limb of the tubuloglomerular feedback response could be demonstrated.