INFLUENCE OF HYDRATION STATE ON RENAL FUNCTIONS OF DOGS

Clinically normal dogs were evaluated in states of dehydration, euhydr ation, and after fluid administration to determine effects of hydratio n state on renal clearance values. Endogenous creatinine, exogenous cr eatinine, and [C-14]inulin clearances, were determined to measure glom erular filtration rate (GFR); in some experiments p-aminohippurate cle arance was determined to measure renal plasma flow. Dehydration caused significant (P < 0.05) decrease in clearance values, compared with eu hydration, 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 regimen s was evaluated for effects on clearance values (treatment A = 30 ml o f 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 soluti on/kg/h). All regimens of fluid therapy caused significant P < 0.05), progressive increases in GFR, but treatment C resulted in the most sta ble GFR values. Increases in clearance values were associated with pos itive fluid balance; the rate of fluid administration was greater than the rate of urine formation. Data from 285 GFR determinations on 85 d ogs were evaluated retrospectively. For each determination, three 20-m inute urine collections were made beginning 40 minutes after 30 ml of water/kg was given by gavage. Values between collections were signific antly (P < 0.05) different, but varied by < 3%. Comparison of methods for measurement of GFR indicated that endogenous creatinine clearance and [C-14]inulin clearance were highly correlated (R2 = 0.82), but mea n clearance values were markedly different (mean +/- SEM, 28.70 +/- 0. 01 and 37.07 +/- 1.29 ml/min, respectively). Exogenous creatinine clea rance and [C-14]inulin clearance were highly correlated (R2 = 0.95), a nd mean values were 40.54 +/- 0.70 and 41.02 +/- 0.70 ml/min respectiv ely. We conclude that: state of hydration has a marked effect on GFR; rate of fluid administration that exceeds rate of urine production res ults in progressive increases in GFR; a single water gavage of 30 ml/k g gives stable GFR values for three 20-minute collection periods, may avoid subclinical states of dehydration, and facilitates accurate urin e collections; and endogenous creatinine clearance, as conducted in th is study, does not accurately measure GFR.