Incomplete and flow dependent extraction of 86Rb in the rat kidney. Errors in local flow estimation

Renal uptake of 86Rb, lasting for 30s after intravenous bolus injection, and uptake of 15 micrometer radioactive microspheres (Ms) simultaneously injected in the left ventricle were measured in rats. Calculated from renal blood flow (RBF) determined by Ms and 86Rb clearance, the renal extraction ratio of 86Rb (ERb) was 0.81 +/- 0.07 (SD). With increasing RBF, ERb decreased significantly. Total body 86Rb extraction ratio nearly equalled ERb as determined 30s after injection. In another group of rats 86Rb was injected into an extracorporal circuit from the carotid artery perfusing the left kidney only. Renal recovery of 86Rb averaged 0.76 +/- 0.13 15-20s after injection. As in the first group, ERb was inversely related to RBF. At increased RBF, calculated local cortical flow is seriously underestimated unless ERb as well as ERb flow dependency are taken into account. However, the relative local flow rates calculated for cortical zones are not appreciably affected by ERb and its flow dependency.