Measurement of glomerular filtration rate and renal plasma flow in the diabetic rat by the single-injection isotopic technique: effects of altered distribution volumes of 51Cr-EDTA and 125I-hippuran.

Following an intravenous injection of an isotopically-labelled clearance marker, glomerular filtration rate (GFR) or renal plasma flow (RPF) can be measured from the rate of plasma isotope disappearance, without collecting urine. The plasma disappearance curve can be closely approximated from a single, timed plasma measurement, if the volume of distribution (V) of the marker is known. Although V is generally predicted from body weight, effects of disease states on the relationship between V and body weight have not been studied. To apply this method of measuring GFR and RPF in streptozotocin-diabetic rats, we investigated the effects of diabetes and insulin treatment on V of 51Cr-EDTA and 125I-orthoiodohippuran (OIH). In untreated diabetic, insulin-treated diabetic and control rats, highly significant linear relationships were found between body weight and V of either isotope (r = 0.68-0.97). However, the slopes and intercepts of these relationships showed that diabetes and insulin treatment significantly altered V of 51Cr-EDTA and 125I-OIH. The greatest change was observed in untreated diabetic rats, in which V of 125I-OIH was increased approximately 12% compared to V in weight-matched control rats. Insulin treatment partially reversed this effect, but V of 125I-OIH in insulin-treated diabetic rats remained increased compared to controls. Using the relationships we derived for V vs body weight, GFR and RPF were measured in the three groups of rats by the single-injection, single-plasma sample method. Severely hyperglycemic, untreated diabetic rats showed reduced RPF (p less than 0.005) but no significant reduction in GFR, compared to age-matched control rats.(ABSTRACT TRUNCATED AT 250 WORDS)