Regional brain blood flow in the conscious gerbil

Regulation of local cerebral blood flow in normal and hypoxic newborn dogs

Local cerebral blood flow (LCBF) was measured autoradiographically in newborn puppies by an indicator fractionation technique using 4-iodo-[14C]antipyrine as the diffusible indicator. Measurements were obtained in unanesthetized, normotensive animals, and the sensitivity of blood flow to hypercapnia and acute hypoxia was determined in 32 brain structures. LCBF in normal and hypoxic puppies was correlated with local cerebral glucose utilization (LCGU) obtained under the same experimental conditions (Duffy et al, 1982). In normocapnic (PaCO2 33 mm Hg) control animals, highest rates of blood flow were found in gray matter nuclei of the brainstem, in the medulla oblongata, and in the posterolateral nucleus of the thalamus (50 to 77 ml/100 gm/min); far lower flows were recorded among white matter structures (5 to 11 ml/100 gm/min). The vasodilatory response to both hypercapnia and hypoxia was greatest among brainstem gray matter structures, intermediate among cortical and diencephalic gray matter structures, and least in white matter. When LCBF was plotted as a function of LCGU for control animals, a positive linear correlation was obtained for all structures (p less than 0.001), implying that in newborns, as in adults, cerebral blood flow and metabolism are physiologically coupled. In hypoxic puppies, no consistent relationship between LCGU and LCBF could be demonstrated; however, there was suggestion that the two measurements correlated inversely, presumably reflecting enhanced anaerobic glycolysis in structures (e.g., hemispheric white matter) that were not adequately protected by compensatory hyperemia. White matter damage, a frequent complication of perinatal hypoxia-asphyxia, may be a consequence in part of the limited capacity of white matter to vasodilate in response to te chemical "signals" of hypercapnia and lactic acidosis.

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 ([131I]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 quantitate 14C activity of the dissected brain samples in the presence of 131I. 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 less than 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.

11C-Iodoantipyrine for the measurement of regional cerebral blood flow by positron emission tomography. Validation studies

Positron emission tomography (PET) makes it possible to employ an in vivo autoradiographic paradigm to measure regional cerebral blood flow (rCBF) in man. In this study, we synthesized the positron-emitting radiopharmaceutical 11C-iodoantipyrine (11C-IAP) and validated its suitability as a CBF tracer. 11C ( T and one-half 20.4 min) was produced by the (p,alpha) nuclear reaction on 14N. 11C-methyl iodide was used to methylate 3-methyl-1-phenyl-2-pyrazolin-5-one to form 11C-antipyrine, which was iodinated. Radiochemical purity of the 11C-IAP product was 93-98% except as described below. rCBF was measured with 11C-IAP in nitrous oxide-anesthetized Wistar rats by the method of indicator fractionation, and values were compared with rCBF values measured with simultaneously administered commercially produced 14C-IAP. rCBF was studied over a range of arterial Pco2 values (31-58 mm Hg, mean 43.0 +/- 3.5). Mean rCBF data for the 2 tracers agreed to within 4.8% for cerebral hemispheral samples, 3.8% for cerebellum, and 5.3% for brainstem. Mean values (+/- SEM) for rCBF using 11C-IAP were 1.67 +/- 0.20 ml gm-1 min-1 for cerebral hemispheres; 1.32 +/- 0.17 for cerebellum; and 1.50 +/- 0.21 for brainstem. When chromatographic analysis revealed tracer impurity, rCBF, as measured with 11C-IAP, fell consistently below values obtained with 14C-IAP. The data indicate that 11C-IAP, when properly synthesized and submitted to batch-by-batch quality control, may be suitable for measuring rCBF in man by emission tomography.

Comparison of radio-labeled butanol and iodoantipyrine as cerebral blood flow markers

Cerebral blood flow (CBF) techniques based on the principle of indicator fractionation rely upon free diffusibility of the blood flow indicator into brain (i.e. complete cerebral extraction). Extraction of two commonly-used indicators, iodoantipyrine and n-butanol, was evaluated in rats by measuring torcular venous efflux after systemic injection of the indicator under conditions of normal and high CBF. The extraction of n-butanol was found to be virtually complete at all blood flows examined; iodoantipyrine, on the other hand, was completely extracted only at flows under 180 ml/100 g/min, despite the fact that the oil: water partition coefficient for iodoantipyrine exceeds that for n-butanol. Brain uptake indices for the two indicators were also measured: brain uptake of n-butanol was greater than that of iodoantipyrine, and the difference was more marked if the indicator entered brain mixed with blood than if it entered as a bloodless bolus. Blood components may thus interact with iodoantipyrine to retard its movement across the blood-brain barrier and thereby limit extraction of this lipid-soluble substance. Inasmuch as iodoantipyrine is diffusion-limited at blood flows above 180 ml/100 g/min, butanol is a more accurate CBF indicator above the normal flow range in the rat.

Development of a tomographic myelin scan

The principle that myelin can be imaged noninvasively using the emission tomographic distribution of a lipophilic radioactive tracer was investigated. Properties of agents suitable for noninvasive myelin scanning are discussed with specific reference to blood-brain barrier permeability, metabolism, and tracer lipophilicity. The brain distributions of inert tracers are correlated with their partitioning between octanol and saline. A test probe, iodobenzene, was labeled with iodine 125 for preliminary invasive studies in the rabbit. The equilibrium brain distribution, determined either autoradiographically or by regional dissection, corresponded closely to that of myelin. 123I-labeled iodobenzene, a gamma-emitting analog, was then administered to a monkey, and tomographic reconstruction revealed a pattern of brain uptake corresponding to white matter.

Medical therapies for mood disorders alter the blood-brain barrier

The effects of amitriptyline, lithium, and electroconvulsive shock on cerebral permeability and blood flow were tested. These three treatments share in common (i) the ability to influence the functional activity of central adrenergic neurons by way of effects on the release, reuptake, or metabolism of norepinephrine and (ii) therapeutic efficacy in mood disturbances. Under control conditions, cerebral permeability increases linealy with increasing arterial partial pressure of carbon dioxide and hence cerebral blood flow. All three treatments altered this relationship in a manner consistent with their adrenergic effects. Amitriptyline potentiated this increase in cerebral permeability whereas lithium and electroconvulsive shock blunted this phenomenon. These results support the hypothesis that one function of central adrenergic neurons is regulation of the blood-brain barrier and raise the possibility that a related effect may underlie the clinical usefulness of such treatment.

Simultaneous measurement of cerebral blood flow and unidirectional movement of substances across the blood-brain barrier: theory, method, and application to leucine

The uptake of compounds by the brain depends upon cerebral blood flow. To determine the normal blood flow-cerebral extraction relationship, a method for rapid, simultaneous measurement of cerebral blood flow and brain extraction was developed and applied to blood-brain leucine transfer. Awake rats were injected intravenously with a mixture of n-[(14)C]butanol and [(3)H]leucine. The quantities of indicators accumulated over the following 5-12 s in brain and in a sample of arterial blood withdrawn at a know rate were used to determine the flux of butanol and leucine into brain. Butanol extraction was assessed independently by measuring arterial and cerebral venous concentrations of the indicator after a bolus injection. Cerebral blood flow was equal to the ratio of butanol flux into brain to butanol extraction by brain; leucine extraction was then calculated as the ratio of leucine influx to cerebral blood flow. Leucine extraction by brain and cerebral blood flow were shown to be related exponentially. The maximum velocity of active leucine transport was virtually the same at flows of 150 and 400 ml/100 g/min. The present method is theoretically applicable to the measurement of the extraction of any compound from blood by brain. By measuring the normal blood flow-extraction relationship, one can differentiate changes in extraction secondary to altered flow from changes intrinsic to pathologic conditions with inconstant cerebral blood flow.

Impaired synthesis of acetylcholine by mild hypoxic hypoxia or nitrous oxide

The effect of mild hypoxic hypoxia on brain metabolism and acetylcholine synthesis was studied in awake, restrained rats. Since many studies of hypoxia are done with animals anesthetized with nitrous oxide (N2O), the effects of N2O were evaluated. N2O (70%) increased the cerebral cortical blood flow by 33% and the cortical metabolic rate of oxygen by 26%. In addition, the synthesis of acetylcholine in N2O-anesthetized animals, measured with [U-14C]glucose and [1-2H2,2-2H2]choline, decreased by 45 and 53%, respectively. Consequently, mild hypoxia was studied in unanesthetized rats. Control rats breathing 30% O2 (partial pressure of oxygen, PaO2 = 120 mm Hg) were compared with rats exposed to 15% O2 (PaO2 = 57 mm Hg) or 10% O2 (PaO2 = 42 mm Hg). The synthesis of acetylcholine, measured with [U-14C]glucose, was decreased by 35 and 54% with 15% O2 and 10% O2, respectively; acetylcholine synthesis, measured with [1-2H2,2-2H2]choline, was decreased by 50 and 68% with 15% O2 and 10% O2, respectively. Animals breathing either 15% or 10% O2 had normal cerebral metabolic rates of oxygen but had increased brain lactates and increased cortical blood flows compared with animals breathing 30% O2. These results show that even mild hypoxic hypoxia impairs acetylcholine synthesis, which in turn may account for the early symptoms of brain dysfunction associated with hypoxia.

Blood-brain glucose transport in the conscious rat: comparison of the intravenous and intracarotid injection methods

The unidirectioal transfer of D-glucose from blood to parietal cortex tissue of the brain of awake rats was measured by single intravenous injection of tracer glucose, as well as by single intracarotid injection according to the method of Oldendorf. The maximal unidirectional blood-brain glucose transfer rate (Tmax) was 407 mumol (100 g)-1 min-1 when measured by intravenous injection, and 352 mumol (100 g)-1 min-1 when measured by intracarotid injection. The half-saturation constants (Km) were 7.8 mM and 16.8 mM, respectively. The comparison shows that the two methods give similar results when cerebral perfusion is assessed accurately.

Cerebral blood flow and edema following carotid occlusion in the gerbil

A technique for measuring focal cerebral blood flow (CBF) and brain specific gravity (SG) in gerbils is described; CO2 reactivity and autoregulation were tested. The mean CBF was 29.5 +/- 4.5 ml/100 gm/min and brain SG 1.0500 +/- 0.0004. Unilateral carotid occlusion resulted in a reduction of flow to 12.8 +/- 5.8 ml/100 gm/min in the ipsilateral hemisphere with little change in the contraleteral hemisphere; there was also a decrease in brain SG. One hour after occlusion, brain edema, as judged by decreased SG, developed at CBF less than 20 ml/100 gm/min and reached maximal levels at 7 +/- 2 ml/100 gm/min. The amount of edema appeared to be related chiefly to the residual post-occlusion flow. With bilateral occlusion, CBF was close to zero and there was no change in SG, indicating that in the "no flow" situation, there is no edema.

Rapid simultaneous determination of regional blood flow and blood-brain glucose transfer in brain of rat

A new method was developed and used in rat to measure regional and whole-brain blood flow and blood-brain glucose transfer simultaneously and in 20 s. This simple method consisted of i.v. bolus injection of labeled butanol and tracer glucose, determination of the average arterial tracer concentration and subsequent assay of cerebral tissue activity 20 s after bolus injection. The whole-brain blood flow rate averaged 129 ml (100 g)-1 min-1. The unidirectional blood-brain glucose transfer was twice as high as previously estimated in similar studies on rat, or 144 mumol (100 g)-1 min-1 at 10 mM glucose in plasma. The magnitude is sufficient to explain the high cerebral glucose consumption rates recently determined by means of autoradiographic 2-deoxy-D-glucose method of Sokoloff et al. (1977).

Critical appraisal of cerebral blood flow measured from brain stem and cerebellar regions after 133 Xe inhalation in humans

Validity of regional blood flow (rCBF) measurements recorded over the human posterior fossa after 133Xe inhalation was tested. Recording of counts from both brain stem and cerebellum (BSC) was reproducible and contamination by counts derived from surrounding anatomical structures was low and no greater than that found over hemispheres. BSC values were F1 = 99 +/- 19 ml/100g brain/min, F2 = 17 +/- 4 in reasonable agreement with data reported from experimental animals. BSC flow values showed significant correlation with the state of awareness as judged by clinical and EEG evaluation with lowest F1 values in semicoma and step increases in stupor, non-REM sleep, drowsiness, rest, activation, REM sleep and highest values during focal and generalized epileptic seizures.