Summary
Clinically normal dogs were evaluated in states of dehydration, euhydration, and after fluid administration to determine effects of hydration state on renal clearance values. Endogenous creatinine, exogenous creatinine, and [14C]inulin clearances, were determined to measure glomerular filtration rate (gfr); in some experiments p-aminohippurate clearance was determined to measure renal plasma flow.
Dehydration caused significant (P < 0.05) decrease in clearance values, compared with euhydration, and clearance values during euhydration were significantly (P < 0.05) less than values obtained after a single gavage with water (30 ml/kg of body weight).
Sustained administration of 3 fluid regimens was evaluated for effects on clearance values (treatment A = 30 ml of lactated Ringer’s solution/kg/h; treatment B = 30 ml of water/kg by gavage hourly; treatment C = 10 ml of glucose:lactated Ringer’s solution/kg/h). All regimens of fluid therapy caused significant P < 0.05), progressive increases in gfr, but treatment C resulted in the most stable gfr values. Increases in clearance values were associated with positive fluid balance; the rate of fluid administration was greater than the rate of urine formation.
Data from 285 gfr determinations on 85 dogs were evaluated retrospectively. For each determination, three 20-minute urine collections were made beginning 40 minutes after 30 ml of water/kg was given by gavage. Values between collections were significantly (P < 0.05) different, but varied by < 3%.
Comparison of methods for measurement of gfr indicated that endogenous creatinine clearance and [14C]inulin clearance were highly correlated (R2 = 0.82), but mean clearance values were markedly different (mean ± sem, 28.70 ± 0.01 and 37.07 ± 1.29 ml/min, respectively).
Exogenous creatinine clearance and [14C]inulin clearance were highly correlated (R2 = 0.95), and mean values were 40.54 ± 0.70 and 41.02 ± 0.70 ml/min respectively.
We conclude that: state of hydration has a marked effect on gfr; rate of fluid administration that exceeds rate of urine production results in progressive increases in gfr; a single water gavage of 30 ml/kg gives stable gfr values for three 20-minute collection periods, may avoid subclinical states of dehydration, and facilitates accurate urine collections; and endogenous creatinine clearance, as conducted in this study, does not accurately measure gfr.
