Compartment analysis of the radiorenogram and distribution of Hippuran I¹³¹ in dogs

¹ Mayo Clinic and Mayo Foundation, Rochester, Minnesota

Radioiodinated Hippuran was injected into the inferior vena cava of three unilaterally and three bilaterally nephrectomized dogs. Radioactivity was recorded over the precordium and remaining kidney in the former group and the precordium and renal fossa in the latter. The disappearance of I¹³¹ from plasma was analyzed and interpreted by two-compartment and three-compartment systems. Theoretical curves for cardiac and renal areas (constructed from analysis of arterial plasma samples) agreed well with curves from in vivo counting. Analyzed by either model, the first compartment exceeded the plasma volume severalfold, part seeming to be in the kidney. Extravascular compartments, impossible to sample, were not identified. About 15% of whole-blood radioactivity was in erythrocytes. Biliary excretion of Hippuran I¹³¹ was insignificant unless nephrectomy was bilateral; then up to 60% was excreted in the bile in 24 hr. Protein binding could not be demonstrated. Radiorenograms converted to fairly accurate rate constants may prove more useful than descriptive methods currently employed. Understanding the distribution and excretion of Hippuran appears helpful to accurate interpretation of the radiorenogram.