Clearance studies were performed in 16 dogs with surgically formed hemibladders after the left kidney had received a single dose of 2000 rad of X-irradiation. The dogs were studied under conditions of water and osmotic diuresis 1 day and at weekly intervals postexposure, for a period of 2 to 8 weeks. In water diuresis, urine flow (V) was significantly greater from the irradiated kidney (IK) than from the control kidney (CK). These differences became greater over the interval of the study and were independent of changes in glomerular filtration rate (GFR), but were positively correlated with differences in free water clearance ($C_{{\rm H}_{2}{\rm o}}$), indicating that the reabsorption of sodium in the proximal tubule was suppressed. When vasopressin was infused, no differences in the ability to reabsorb$C_{{\rm H}_{2}{\rm o}}$ were evident until 28 days after exposure. At this time, negative free water clearance ($T^{c}{}_{{\rm H}_{2}{\rm o}}$) was significantly reduced, averaging 0.30 ± 0.08 ml/min (SE) and 0.52 ± 0.08 ml/min from the IK and CK, respectively (P < 0.01). This defect was also seen when the tubular maximum for$T^{c}{}_{{\rm H}_{2}{\rm o}}$ ($Tm^{c}{}_{{\rm H}_{2}{\rm o}}$) was determined (1.18 ± 0.1 ml/min from CK and 0.015 ± 0.13 ml/min from IK, P < 0.001) and when urine osmolality was determined 18-24 hr after complete food and water deprivation (930 ± 148 and 1898 ± 259 mosmol/liter for IK and CK, respectively, P < 0.05). These studies suggest that renal tubular injury is the major early effect of radiation on renal function, with the earliest effects seen in the proximal tubule followed by marked impairment of the concentrating segment.

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