Critical evaluation of cerebral blood flow measurements made with 14C-ethanol

Correlation of local changes in cerebral blood flow, capillary density, and cytochrome oxidase during development

Although elevations in cerebral metabolic demand during development may induce angiogenesis, the correlation among ontogenic changes in local cerebral blood flow, cytochrome oxidase activity (an index of oxidative capacity) and capillary density have not been examined previously. We measured these parameters in selected regions of the brains of anesthetized rabbits of various ages. Increases in all three parameters occurred postnatally within the cerebral cortex and striatum, whereas in the medulla, values at birth were similar to those in adults. In newborns, the pattern of distribution of blood flow within the parietal cortex was such that levels were maximal in the outer layers and declined in deeper layers. This distribution correlated closely with that of capillary density, whereas cytochrome oxidase activity was maximal at levels deeper in the cortex. By postnatal day 17, the distribution for all three parameters was similar to that of cytochrome oxidase activity in young animals. A regression analysis of the regional values demonstrated a positive correlation between capillary density and blood flow in young (< or = postnatal day 8) and old (> or = postnatal day 17) animals. In contrast, cytochrome oxidase activity and capillary density were poorly correlated in young animals but positively correlated in older animals, with the slopes being markedly different (P < 0.005). The results suggest that early in postnatal development, the pattern of cytochrome oxidase activity is relatively mature compared with that of capillary density. By postnatal day 17, microvascular anatomy is closely associated to oxidative capacity, likely reflecting a steady state regulation of capillary density to metabolic requirements.

Effect of monoamine oxidase inhibition on the regional cerebral blood flow response to circulating noradrenaline

The effect of an acute i.v. infusion of noradrenaline (NA) on regional cerebral blood flow (rCBF) was investigated in the awake rat using [14C]iodoantipyrine as diffusible tracer. The contribution of vascular monoamine oxidase (MAO) to the efficiency of the enzymatic blood-brain barrier (BBB) to catecholamines was assessed by measuring the multiregional cerebrovascular response to circulating NA given alone or after i.v. administration of the monoamine oxidase inhibitor, clorgyline. Since i.v. infusion of NA elevates blood pressure, the influence of NA on the cerebrovascular bed was first studied by determining the relationship between rCBF and the mean arterial pressure (MAP). When the MAP was only slightly increased (to approximately 130 mm Hg), a trend to flow decrease under NA infusion was observed. Secondly, we compared the effects of NA on rCBF in animals treated or not treated with clorgyline. This was performed under moderate hypertension (within the 'autoregulated' range of MAP) to avoid any risk of mechanical damage to the BBB. Clorgyline administration alone did not significantly modify rCBF, but the subsequent i.v. infusion of NA induced an increase in rCBF (weighted mean 14%) in all structures investigated. The differences being statistically significant (P less than 0.05) in 5 out of 13 structures by up to 20%. Compared to studies involving disruption of the morphological BBB in which plasma NA elicits a widespread important increase in blood flow, the weak cerebrovascular effects we observed provide indirect evidence for the efficiency of the BBB to catecholamines in the conscious rat within the autoregulated range of arterial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Systemically administered adenosine increases caudate blood flow in rabbits

Adenosine has been proposed to be a chemical link between cerebral metabolism and blood flow. In the present study, we investigated whether the intravenous or intracarotid administration of adenosine could influence regional cerebral blood flow in anesthetized rabbits. The study was performed with the [14C]ethanol tissue sampling technique which enables quantitative, instantaneous, multiregional blood flow measurements. With either mode of adenosine administration, no change in cerebral blood flow was observed, except in the caudate nucleus in which a significant vasodilation took place. These data indicate that, in rabbits exogenous adenosine increases blood flow in highly specific brain areas, by mechanisms that are discussed.

Stress and local cerebral blood flow: studies on restrained and unrestrained rats

The degree of stress has been compared between two protocols used for the measurement of local cerebral blood flow (LCBF) in conscious rats. The first method involved acute surgical procedures (cannulation of both femoral veins and arteries) under halothane anesthesia. It was followed by a recovery period (2-3 h) during which the rat was before LCBF measurement. The second method employed chronic cannulation of the abdominal aorta and vena cava, allowing the LCBF assays to be performed on freely moving rats. Plasma corticosterone and a glucose tolerance tests showed that the freely moving rats were less stressed than the gently restrained ones. The LCBF of the two groups were not significantly different except in the frontal and parietal cortex, where it was more elevated in the freely moving rats. LCBF may be sensitive to the environmental conditions in freely moving rats whereas these vascular effects may be reduced after 2-3 h of gentle restraint. The two protocols tested in this study could be considered as good methods for studying LCBF in conscious rats, although some stress remained in gently restrained rats. Freely moving rats can be used for behavioural studies providing that the time lag of the arterial samples is taken into account. Since the basal LCBF values of gently restrained rats are minimally affected by the stress inherent in the preparation, this convenient protocol could be considered as useful for numerous investigations.

Significance of the cerebrovascular effects of immobilization stress in the rabbit

The question of the significance of the cerebrovascular effects of stressful situations in animals is still controversial. In the present article, an experimental model of immobilization stress in the rabbit is described, and its specificity in relation to arterial blood pressure and PaCO2 is investigated. CBF was measured with the multiregional tissue sampling technique using [14C]-ethanol as tracer. After dissipation of althesin anesthesia, the stress reaction was elicited by tactile abdominal stimuli. The response was evidenced by an instantaneous acute hypertension (+33.8% during the CBF measurement period). Within the first minute of the reaction, the CBF was significantly increased in all nine structures studied by 39% (caudate nucleus) to 82% (parieto-temporal cortex). The study of the influence of arterial blood pressure and the PaCO2 on CBF showed that cerebrovascular autoregulation and CO2 sensitivity were differently affected in the various structures during the stress reaction. However, the stress response of the brain circulation could not be entirely ascribed to one or both of these two systemic factors, thus suggesting the contribution of a local intrinsic activation. The model presented here could be useful for long-term studies of cerebrovascular repercussions of repeated acute hypertensions of a stressful nature.

Generalized cerebral vasoconstriction induced by intracarotid infusion of angiotensin II in the rabbit

This study investigated the influence of angiotensin II, perfused into one common carotid artery at a dose of 0.065 micrograms/kg/min, on the cerebrovascular resistance of the anesthetized rabbit by means of complementary in vivo methods. Heat clearance and mass spectrometry measurements indicated that in the homolateral caudate nucleus angiotensin induced a significant decrease in local blood flow (18.2 +/- 9%), a fall in pO2 (14.2 +/- 5.3%) and no significant change in pCO2. The [14C]ethanol tissue sampling technique revealed a significant decrease in flow in all 10 structures sampled in the brain. This decrease was similar in magnitude in both the ipsilateral and the contralateral hemisphere with regard to the site of injection. When expressed in terms of cerebrovascular resistance (CVR) and allowing for a slight increase in blood pressure (less than 10%), these results show that angiotensin II infusion induced an increase in CVR of 18-32%. We conclude that: A unilateral intracarotid infusion of a low dose of angiotensin II induces an increased vascular tone in all cerebral structures. This action, being bilateral, cannot readily be explained by a direct action of angiotensin II on the cerebral vessels in view of the very low recirculating concentration of angiotensin II (less than 10(-9) M). The hypothesis of a cerebral vasomotor influence of angiotensin II by action on a central structure is discussed.

Validity of cerebral blood flow measurements obtained with quantitative tracer techniques

A great number of results for the cerebral blood flow obtained in the animal with quantitative tracer techniques have been collected from the literature. They are exposed in order to compare both normal flow values in different laboratory species, and the characteristics, accuracy and sensitivity of each technique. A dramatic overall dispersion of flow values is observed, allowing neither the flow level particular to each species to be estimated, nor the average value provided by a given technique to be found. The physiological and technological causes of such a dispersion are discussed. Several techniques seem to have limitations which even alter the interpretation of their results, and especially the origin of the local or regional blood flow results. Other techniques may be criticized from the quantitative standpoint, but give more reliable results.

Multiregional cerebral blood flow changes induced by a cholinomimetic drug

The effect of parasympathomimetic drug, carbachol, on regional cerebral blood flow was tested in the rabbit with the 14C-ethanol tissue sampling technique. Intracarotid injection of carbachol significantly increased flow with respect to untreated control animals in 6 out of 10 structures sampled. However, compared with animals to which atropine was also administered, the flow increases were greater with carbachol alone. These effects are discussed in terms of muscarinic and nicotinic activation.