A radioisotopic method for the simultaneous quantitation of regional cerebral blood flow and glucose utilization in small dissected samples: Validation studies and values in the nitrous oxide-anesthetized rat

A method is described for the simultaneous determination of the rates of regional cerebral blood flow (rCBF) and regional cerebral glucose utilization (rCMRgl) in 6–7 mg brain samples dissected from multiple areas of interest. The method utilizes [¹³¹I]-iodoantipyrine ([¹³¹l]IAP) to measure rCBF by indicator fractionation, and [¹⁴C]2-deoxyglucose to measure rCMRgl. [¹³¹I]IAP was synthesized with specific activity exceeding 350 Ci/mmol and radiochemical purity greater than 99.5% by the radioiodination of antipyrine with Na¹³¹I. A triple-counting strategy was developed to quantitate¹⁴C activity of the dissected brain samples in the presence of¹³¹I. The factors contributing to the propagated error of the double-label separation strategy were defined and optimal assay parameters were determined. The separation strategy was validated by measuring rCBF simultaneously with both [¹³¹I]IAP (x) and [¹⁴C]IAP (y) in a series of rats. The equation of the regression line was y = 1.025 x −0.065 (correlation coefficient 0.985), denoting excellent agreement. In another series of 5 normocapnic rats anesthetized with nitrous oxide, rCBF and rCMRgl were measured simultaneously. In individual animals, the rates of rCBF within 14–16 brain areas were closely coupled to their respective rates of glucose metabolism. For the group data, the linear regression equation relating rCBF (y) to rCMRgl (x) was y = 1.76 x + 0.13 (correlation coefficient 0.93,P < 0.001). These studies provide direct evidence, based upon data obtained in the same brain, of a close coupling of regional metabolic rate and blood flow.