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Am J Physiol Heart Circ Physiol 273: H1994-H2000, 1997;
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Vol. 273, Issue 4, H1994-H2000, October 1997

NO contributes to neurohypophysial but not other regional cerebral fluorocarbon-induced hyperemia in cats

B. P. Wagner1, R. Stingele1,2, M. A. Williams1,2, D. A. Wilson1, R. J. Traystman1, and D. F. Hanley1,2

Departments of 1 Neurology and 2 Anesthesiology/Critical Care Medicine, The Johns Hopkins Medical Institutions, Baltimore, Maryland 21287

    ABSTRACT
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

The large increase in cerebral blood flow (CBF) after fluorocarbon (FC)-exchange transfusion is thought to be caused by low oxygen content, decreased viscosity, or direct vasodilatory effect of the FC perfusate. The aim of this study was to determine whether nitric oxide (NO)-mediated vasorelaxation is increased in FC-perfused hemoglobin (Hb)-free cats because NO is not scavenged by Hb. We measured regional CBF with radiolabeled microspheres in three groups of anesthetized mechanically ventilated cats. The first group [FC + Nomega -nitro-L-arginine methyl ester (L-NAME); n = 7] underwent a complete FC-exchange transfusion with FC-43 and subsequent nitric oxide synthase (NOS) inhibition with L-NAME (10 mg/kg iv) followed by L-arginine (100 mg/kg iv). A second group (FC + saline; n = 6) underwent an identical protocol, but NOS was not antagonized (saline iv). In a third group (blood + L-NAME; n = 7), cats were not FC exchanged but NOS was inhibited. In a separate cohort of four FC-perfused cats, NOS activity in brain tissue samples was reduced to 26% of control after NOS inhibition. FC-exchange transfusion nearly doubled hemispheric blood flow in both FC-exchanged groups, whereas it was constant in the blood + L-NAME group. These increases in regional CBF (hemispheres, brain stem, cerebellum, thalamus, and white matter) were not reversed by inhibition of NOS, except in the neurohypophysis, where L-NAME reduced blood flow to levels comparable to values in the blood + L-NAME group. In summary, increases in regional CBF after total FC-exchange transfusion are not caused by a lack of NO scavenging, with the exception of neurohypophysis. These findings suggest an increased vasorelaxation in neurohypophysis of FC-perfused and Hb-free cats caused by unscavenged NO, but this mechanism does not play a major role in FC-related CBF increases in the rest of the cerebral circulation.

nitric oxide synthase inhibition; nitric oxide synthase activity; fluorocarbons; regional cerebral blood flow; neurohypophysial blood flow

    INTRODUCTION
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

IN PREVIOUS EXPERIMENTS we found that complete exchange transfusion with the fluorocarbon (FC) FC-43 doubled cerebral blood flow (CBF) in cats (see Ref. 7). A similar elevation of CBF has been observed after fluorocarbon exchange in rats (20, 24, 29). A priori, the explanation of CBF doubling would seem to be maintenance of cerebral oxygen delivery (DO2) given the low oxygen content of FC-43 even at an inspiratory oxygen fraction (FIO2) of 1.0 (4-5 vol%) (20). Low arterial oxygen content (CaO2) is a well-documented stimulus for increasing CBF (2), whether by hypoxic hypoxia, carbon monoxide inhalation, or hemodilution (18).

However, there are other factors involved in FC-perfused animals that may also explain the increased CBF. First, the viscosity of FC emulsions is lower than that of blood (20). From the Hagen-Poiseuille relationship, one would expect a lower vascular hindrance and a higher blood flow without changes in vascular diameter. Second, FC can cause direct vasodilation (6, 24, 31). Third, FC-exchange transfusion eliminates the nitric oxide (NO)-scavenging protein hemoglobin (Hb) from the circulation. The physiological importance of Hb as an elimination pathway for the potent vasodilator NO has been demonstrated. Oxy- and deoxyhemoglobin have been shown to instantly bind NO in vitro (32) as well as in vivo (26). Total FC-exchange transfusion and Hb elimination might therefore lead to decreased NO scavenging. Accumulation of "unscavenged" NO would result in vasodilation and increased CBF. FC-perfused animals provide the opportunity to look at a new Hb role in the regulation of CBF. The aim of this study was to determine the contribution of NO-binding Hb to CBF by comparing CBF changes in Hb-free animals with CBF changes in blood-perfused animals caused by inhibition of NO production.

    MATERIALS AND METHODS
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

Anesthesia. Details of the animal surgical preparation and monitoring procedures employed have been previously described (7). Twenty adult cats (1.7-3.8 kg) were studied. Anesthesia was induced with intramuscular ketamine (20-30 mg/kg) and acepromazine (2-3 mg/kg) and subsequently maintained by inhalation of 30% oxygen-70% nitrous oxide and intravenous fentanyl at 10-35 µg · kg-1 · h-1 plus 25-µg bolus as needed to maintain a constant mean arterial blood pressure (MAP) of 100 ± 25 mmHg. Starting after the end of surgery, animals breathed 100% O2 to maintain adequate O2 transport (DO2) to the brain with FC exchange. Pancuronium bromide (0.1-0.3 mg/kg iv every hour) was given for muscle relaxation. Body temperature was monitored by rectal and epidural probes and maintained at 37-39°C with a water-perfused warming blanket, warmed FC emulsions, and a heat lamp. Animals were intubated with a 4.0 cuffed endotracheal tube (Portex) and ventilated with a positive-pressure respirator (Harvard Respiration pump model 665, South Natick, MA). Tidal volume (13-25 ml/kg) and ventilation rate (12-25 breaths/min) were adjusted to keep arterial PCO2 (PaCO2) between 35 and 40 mmHg. End-expiratory CO2 concentration was continuously monitored by a CO2 gas analyzer (model LB-2, Beckman Instruments, Fullerton, CA).

Surgical preparation. The right brachial artery was catheterized for blood pressure recording. Radiolabeled microsphere injections were made into the left ventricle (LV) via a catheter inserted through one femoral artery. The other femoral artery was catheterized for microsphere reference sampling and blood gas sampling and for blood withdrawal during FC-exchange transfusion. One femoral vein was cannulated for continuous central venous pressure (CVP) recording, the other for FC-exchange transfusion. The brachial vein was cannulated for drug infusion. The splenic vascular bundle was ligated to prevent release of sequestered red blood cells. The head was positioned in a stereotaxic frame. After a midline scalp incision using electrocautery, the temporalis muscles were retracted and the periosteal surfaces were scraped. The sagittal sinus was exposed after the bone was removed with a high-speed drill and was catheterized for blood gas sampling.

Experimental protocol. All animals were allowed 30-60 min to stabilize after surgery (Fig. 1). Blood was then withdrawn from the femoral artery while Ringer lactate solution was infused intravenously to maintain MAP and CVP. Once Hb was reduced to ~4 g/dl, FC-exchange transfusion was performed for 90-120 min with 1,000-1,200 ml FC-43 (Oxypherol, Green Cross, Osaka, Japan) until arterial hematocrit dropped below 1%. The FC-exchange transfusion was achieved using a single roller pump (Gilson Miniplus 2, Middleton, WI) for simultaneous withdrawal and infusion at a speed of ~10-11 ml/min. MAP, CVP, and LV end-diastolic pressure (LVEDP) were kept within 3-5 mmHg of control values by controlling the infusion rate of FC-43. Despite these efforts, many animals required additional boluses of FC during or after FC-exchange transfusion to maintain MAP at a normotensive level. In some animals, transient increases in CVP and LVEDP required additional withdrawal of perfusate. After FC exchange was completed, the animals were given 10 mg/kg body weight of Nomega -nitro-L-arginine methyl ester (L-NAME) intravenously in 1.5 ml normal saline over 5 min. Experimental measurements were made up to 30 min later (see Measurements). L-Arginine (100 mg/kg iv in 1.5 ml normal saline) was then given over 5 min, and experimental measurements were made 15 min later. Experimental measurements were obtained at six times during the protocol: 1) after surgery (baseline), 2) after FC exchange but before L-NAME (control, 0 min), 3-5) 10, 20, and 30 min after L-NAME, and 6) 15 min after L-arginine.


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  		Fig. 1.   Diagram of experimental protocol. FC, fluorocarbon; L-NAME, Nomega -nitro-L-arginine methyl ester.

We studied three groups. In the first group (FC + L-NAME, n = 7 cats), the animals were FC exchanged and given L-NAME and L-arginine to determine NO contribution to increased CBF in FC-perfused animals. In the second group (FC + saline, n = 6), the animals were FC exchanged and given 0.9% saline placebo instead of L-NAME and L-arginine to control for time effects. In the third group (blood + L-NAME, n = 7), the animals were not FC exchanged but were given L-NAME and L-arginine to determine NO contribution to resting CBF in blood-perfused animals.

Measurements. Intravascular pressures were continuously measured with Isotec pressure transducers (Healthdyne Cardiovascular, Irvine, CA) and recorded on a Gould Brush recorder (Cleveland, OH). Blood samples were analyzed with a Radiometer ABL 3 blood gas analyzer (Copenhagen, Denmark). Hb content, oxygen content, and oxygen saturation were determined with a Hemoximeter (Model OSM3, Radiometer). Oxygen content in FC-perfused animals was calculated from arterial PO2 (PaO2) and venous PO2, taking into account the known oxygen content of Oxypherol under 100% FIO2 (20). Cerebral metabolic rate of oxygen consumption (CMRO2) was calculated as the product of CBF times the arterial-venous oxygen content difference (CMRO2 = CBF × [CaO2 - CCVO2], where CaO2 and CCVO2 are arterial and cerebral venous oxygen contents, respectively). Oxygen delivery (DO2) was computed as the product of CBF and CaO2. Fractional oxygen extraction (OEF) was calculated as the ratio of the CaO2 - CCVO2 difference to CaO2.

Regional CBF was measured with radiolabeled microspheres (153Gd, 113Sn, 103Ru, 95Nb, 46Sc, 114In, 15 ± 0.5 µm diameter; DuPont NEN, Boston, MA) using the reference sample method (13). Approximately 7 × 105 microspheres in 0.3 ml were injected into the LV over 20 s, followed by a 15-s flush with 3.5 ml saline (or FC in FC-perfused animals). The reference sample was withdrawn from the femoral artery catheter using a withdrawal syringe pump (Harvard Infusion/Withdrawal Pump model 940; Harvard Apparatus) set at 1.94 ml/min starting 30 s before the injection and continuing for 2 min after the LV catheter was flushed. The brain was removed and placed in a 10% buffered Formalin solution for 3 days. The neurohypophysis was dissected whole, and samples from brain stem, cerebellum, thalamus, caudate nuclei, and white matter were taken. The remaining cerebral tissues were pooled for measurement of total hemispheric blood flow. The reference blood sample and weighed tissue specimens were counted for gamma radioactivity (Minaxi Model 5530, Packard Instrument, Downers Grove, IL) and corrected for overlap of activity among the isotopes. Regional CBF was calculated as the ratio of corrected tissue and reference sample counts multiplied with the ratio of reference blood sample withdrawal rate to the tissue sample weight.

Somatosensory evoked potential. A multichannel signal averager (Nicolet Spirit, Madison, WI) was used to measure somatosensory evoked potential (SEP) (23). Stimulating needle electrodes were placed percutaneously in the palmar surface of the left forelimb, and a silver ball electrode with shielded cable was placed in a burr hole in the right parietal skull. At selected times, 128 stimuli were delivered at a rate of 5.9/s and the evoked responses were time-averaged after bandpass filtering between 30 and 1,500 Hz. The P1-N1 amplitude was measured from the peak of the first positive deflection (P1, 6- to 9-ms latency) to the peak of the first major negative wave (N1, 11- to 15-ms latency). Central conduction time (CCT) was measured by subtracting P1 latency from N1 latency.

Nitric oxide synthase activity and inhibition. To ascertain that L-NAME alters nitric oxide synthase (NOS) activity after FC exchange, we determined brain NOS activity in four cats. These animals underwent identical anesthesia and surgery followed by a complete FC-exchange transfusion. A parietal cortex biopsy was taken after FC exchange. L-NAME was then administered (10 mg/kg iv), and a contralateral parietal cortex biopsy was taken 30 min later. NOS activity was measured by a well-established isotopic conversion assay according to the method of Bredt and Snyder (1, 30). The pre- and post-L-NAME NOS activity results were compared by paired Student's t-test with the level of significance set at P < 0.05.

Statistics. Data are represented as means ± SE. Data were analyzed by one-way analysis of variance (ANOVA; SigmaStat version 1.01, Jandel Scientific). If the F-ratio indicated that a significant effect (P < 0.05) existed between groups at equivalent time points, then a post hoc Student-Newman-Keuls test was performed to isolate the source of the difference. Effects within a group were analyzed by one-way ANOVA for repeated measures followed by a post hoc Dunnett's test.

    RESULTS
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

NOS activity and inhibition. NOS activity decreased from 2.2 ± 1.2 pmol · min-1 · mg protein-1 after FC exchange to 0.44 ± 0.21 pmol · min-1 · mg protein-1 30 min after L-NAME (26 ± 9% of control). Brain NOS activity in FC-43-perfused cats is similar to that measured in blood-perfused cats (30). Similarly, L-NAME reduces NOS activity to the same degree as in blood-perfused cats (30). Therefore, brain NOS activity seems to be maintained in vivo in the absence of intravascular Hb, and L-NAME seems to be efficient in blocking NOS activity after FC exchange. Nonselective inhibition of NOS by L-NAME and the sampling technique used cannot separate the relative contributions of endothelial and neuronal NOS in the parietal brain biopsy (30). Therefore, we cannot exclude that FC might selectively affect endothelial NOS activity. Nevertheless, the immediate systemic hypertensive effect of L-NAME in blood-perfused animals and very recent data (14) suggest that mainly endothelial NOS is responsible for cerebral hemodynamics. Because NOS activity and NOS inhibition were similar in blood-perfused and FC-perfused animals, one would think that endothelial NOS was also inhibited in FC-perfused animals. In fact, two other studies also suggest intact endothelial functions in FC-perfused animals. One study showed an intact blood-brain barrier in FC-perfused brains (12), and another study demonstrated a perfusion rate-dependent release of endothelium-derived vasoactive substances such as ATP, substance P, endothelin, and arginine vasopressin (3).

Physiological effects of Hb washout, NOS blockade, and L-arginine. Total FC-exchange transfusion and administration of L-NAME/L-arginine did not change MAP, arterial pH, or PaCO2 with the exception of a transient increase in MAP 10 min after L-NAME in the blood + L-NAME group (Table 1). After FC exchange, PaO2 increased, CaO2 decreased, DO2 decreased, and OEF increased as expected in the FC + saline and FC + L-NAME groups (Table 1). There was no change in these variables after treatment with L-NAME, saline, or L-arginine. In the blood + L-NAME group, L-NAME reduced DO2 from 13.6 ± 1.6 to 8.4 ± 0.7 ml · 100 g-1 · min-1 at 30 min. Measures of neuronal function were unchanged throughout FC exchange and after administration of L-NAME/L-arginine (Table 2). There was no difference in cerebral metabolic rate among the three groups at baseline, nor was there any treatment effect, with the exception of an isolated reduction 30 min after L-NAME in the blood + L-NAME group.

                              
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  		Table 1.   Physiological data before and after fluorocarbon exchange and after L-NAME and L-arginine administration

                              
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  		Table 2.   Cerebral metabolic rate of oxygen consumption, somatosensory evoked potential, and central conduction time before and after FC exchange and after L-NAME and L-arginine administration

Regional CBF after FC-exchange transfusion, NOS blockade, and L-arginine. Regional CBF values are shown in Table 3. To better quantify FC-induced hyperemia and the effect of NOS inhibition on FC-induced hyperemia, we analyzed the difference of CBF from baseline (Delta CBF) for each time point. For example, comparison of CBF values between FC-perfused and blood-perfused animals at the control time point demonstrates the effect of FC-exchange transfusion on CBF. Comparison of CBF after L-NAME should demonstrate NO-mediated components of FC-induced hyperemia. These Delta CBF values are shown in Fig. 2 for the control time points and 30 min after L-NAME.

                              
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  		Table 3.   Regional cerebral blood flow before and after FC exchange, after L-NAME and L-arginine administration


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  		Fig. 2.   Effect of nitric oxide synthase inhibition with 10 mg/kg L-NAME on FC-induced hyperemia. Each data point represents difference of cerebral blood flow (Delta CBF) from blood-perfused baseline. Control, after complete FC-exchange transfusion (or no exchange transfusion); L-NAME, 30 min after administration of L-NAME (or saline placebo). All values are means ± SE. Changes of blood flow to hemispheres (top) is representative for most brain regions examined. Neurohypophysis (bottom) was only region examined in which FC-induced hyperemia could be completely reversed by L-NAME.

FC-exchange transfusion increased CBF in most brain regions such as hemispheres and neurohypophysis (Fig. 2) and cerebellum, thalamus, brain stem, and white matter (not shown). L-NAME did not reverse the CBF increase; there is no difference between the FC + L-NAME and FC + saline groups at 30 min in hemispheres (Fig. 2) or cerebellum, thalamus, brain stem, and white matter (not shown), suggesting that the downward trend is an effect of time. As expected, CBF was reduced at 30 min in the blood + L-NAME group. In the neurohypophysis a different response to L-NAME was seen; L-NAME significantly reduced neurohypophysial blood flow (NHBF) at 30 min (Fig. 2). Not only did it reduce NHBF to baseline values, but it reduced NHBF to values similar to those seen after L-NAME in the blood + L-NAME group. We further looked for CBF changes caused by residual basal NOS activity within each treatment group by giving L-NAME for NOS inhibition and L-arginine for reversal (Table 3). L-Arginine did not affect CBF except for hemispheric CBF in the FC + L-NAME group. The FC + saline group showed reduction of blood flow in many brain regions (except for white matter and neurohypophysis), suggesting a time influence on CBF throughout the experimental protocol.

The reduction of NHBF after L-NAME as well as hemispheric CBF recovery after L-arginine suggest that FC-exchange transfusion does not affect the endothelial mechanisms necessary for NO-mediated vasoregulation.

    DISCUSSION
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

These data show that 1) L-NAME inhibits brain NOS activity in FC-perfused cats, 2) NOS inhibition with L-NAME does not reverse FC-exchange transfusion-induced CBF increases, and 3) NOS inhibition with L-NAME reverses the increase in NHBF that occurs after FC-exchange transfusion. These results suggest that increased blood flow after FC-exchange transfusion is related to unscavenged NO-mediated vasodilation in the neurohypophysis but that the remainder of the brain is unaffected by this mechanism.

Physiological data and SEP. FCs are known to be much better solvents for oxygen molecules than human plasma. Henry's law states that the concentration of gas dissolved in a liquid is proportional to its partial pressure; there is a linear relationship between PaO2 and oxygen content in FC fluids and in human plasma. At 760 mmHg PaO2 and 37°C, plasma contains 2.3 vol% O2, whereas FC-43 contains ~7 vol% O2 (25). This linear relationship explains the relatively low oxygen content of FC-exchanged cats at very high PaO2. Increased PaO2 after a small intravenous FC bolus and after total FC-exchange transfusion has been seen by many investigators, including previous studies from our group (7), and is mainly explained by increased amounts of dissolved oxygen in FC containing plasma.

Increased OEF has been noted after FC hemodilution (6, 21). This phenomenon has been attributed to increased plasma solubility and oxygen diffusion facilitation, especially because tissue PO2 was increased at the same time (4). Other groups (5) could not reproduce increased OEF when using an FC not inducing hypotension. Lee et al. (20) noticed increased OEF (from 0.4 to 0.72) with stable CMRO2 and stable electroencephalograph in FC-perfused rats. He concluded that OEF and CBF increase to maintain adequate oxygen availability. Our findings and interpretations are similar to Lee et al; during FC-exchange transfusion, OEF and CBF both increase without changing CMRO2, SEP amplitude, or CCT. In a previous study (8), we discovered that graded hypoxia impaired SEP amplitude despite high intravascular PaO2, suggesting an alteration in oxygen distribution throughout the cerebral tissue of FC-perfused cats. Simultaneous measurement of tissue PO2 and redox state of cytochrome-c oxidase copper (28) have supported this hypothetical mechanism. We conclude that OEF is probably increased in FC-43-perfused cats to maintain oxygen availability and is not caused by increased oxygen diffusion facilitation.

Regional CBF. Regional CBF in all brain regions, except neurohypophysis, showed similar behavior to the hemispheric CBF. Regional CBF increased by 75% after FC-exchange transfusion, and this was not reversed by NOS inhibition by 10 mg/kg L-NAME. Thus NO does not seem to be responsible for hemispheric CBF increase after FC-exchange transfusion in cats. From the Hagen-Poiseuille relationship (22), decreased viscosity of the intravascular FC perfusate (2.7 ± 0.1 cP) contributes to the increase in CBF. Decreased viscosity was seen at all shear rates, therefore exhibiting Newtonian behavior (28). Other mechanisms responsible for this CBF increase include low oxygen content or alteration of oxygen affinity/solubility of the intravascular perfusate (19). Alterations in Hb concentration are well known to result in alterations of CBF (15). The relative contributions of viscosity, oxygen content, and oxygen affinity to CBF remain very difficult to elucidate. Lee et al. (20) could demonstrate similar CBF in three groups of rats with similar CaO2 but different viscosities, i.e., FC perfusion, hemodilution, and hypoxic hypoxia. The most likely mechanism explaining CBF increase in FC-43-perfused cats is therefore the extremely low oxygen content resulting in CBF increase to maintain DO2/CMRO2 and neuronal function. These findings are consistent with several studies demonstrating that NO does not mediate hypoxia (low CaO2)-induced increase in CBF [see review by Iadecola et al. (16)]. The low DO2 found after FC-exchange transfusion points to the fact that the animals were stressed by perfusion with FC because of its borderline CaO2 and therefore were slowly deteriorating over time. This would explain why CBF values dropped over time in the FC + saline group in most brain regions. Using a new-generation FC with CaO2 similar to normoxic blood-perfused animals might give us a better answer concerning the role of viscosity, CaO2, oxygen affinity, Hb, and unscavenged NO in FC-perfused animals.

In neurohypophysis, NOS inhibition in the FC + L-NAME group reduced blood flow to values as low as that in the blood + L-NAME group after NOS inhibition. This finding suggests an NO-mediated NHBF increase after FC-exchange transfusion. Previous studies showed that PaO2, sensed in peripheral chemoreceptors, correlates closely with NHBF. Hypoxic hypoxia, but not carbon monoxide-induced hypoxia, increased NHBF, whereas peripheral chemoreceptor denervation prevented hypoxic hypoxia-induced NHBF increase (11). Increased PaO2 after FC exchange and stable PaO2, MAP, and CVP after L-NAME suggest another cause for NHBF alterations after FC-exchange transfusion, namely a NO-mediated NHBF increase. This interesting finding of increased NO contribution to NHBF in FC-perfused and Hb-free cats needs a new explanation. Excess unscavenged NO causing increased NHBF might be a probable mechanism, because intravascular Hb as an avid NO scavenger (26) is lacking. Larger NO contribution to NHBF compared with CBF in other brain regions has also been noticed in blood-perfused animals in other studies (27, 33). The 2.5- to 3-fold higher NHBF compared with any other brain region examined suggests a much greater ratio of blood volume per time to tissue volume. This "luxurious" NHBF might allow detection of the true contribution of NO to blood flow in FC-perfused animals. The different ratio of blood volume per time to tissue volume might be the best single explanation for the different contributions of unscavenged NO detected in different tissues. We have measured blood flow in many organs and have noticed that in other high-blood-flow tissues such as kidneys or heart there is a significant contribution of unscavenged NO (data not shown). Another possible mechanism might be shear stress-induced endothelial NO release, as has been shown in cell cultures (9). In vivo shear stress is defined by viscosity × shear rate (shear rate = velocity/vessel diameter) (22). After FC exchange, viscosity decreased at all shear rates (28), but we did not measure velocity or vessel diameter. We can therefore only speculate that FC exchange might influence vascular shear stress and influence endothelial release of NO. Other possible mechanisms leading to regional differences of NO contribution might be different structure of the blood-brain barrier, complex neural control of both dilator and constrictor activities in the neurohypophysis (10) with different relative contributions of endothelial and neuronal NO, or regional differences in NOS inhibition. Direct in vivo measurements of neuronal and endothelial NOS activity as well as NO might give clear answers.

Recently it has been proposed that Hb plays an important role in regulating arterial blood pressure via S-nitrosylation reactions in the lung when red blood cells are oxygenated (17). If this hypothesis is correct, then one would anticipate that FC-exchange perfusion should be accompanied by a sustained hypertension. For reasons of clarity we point out that our study was designed to determine whether an NO-scavenging role for Hb could be demonstrated at the organ level. We focused attention on one organ only, the brain. To achieve this goal, arterial blood pressure was treated as an independent variable, i.e., arterial blood pressure was controlled by infusing or withdrawing FC solution. Thus the fact that arterial blood pressure is not different between control and FC-perfused animals does not contradict a role for Hb in arterial blood pressure regulation.

In summary, increased NO contribution to NHBF in FC-perfused cats might be caused by a lack of intravascular Hb scavenging NO, whereas in the rest of the cerebral circulation very low CaO2 might dictate FC-related CBF changes and prevent increased NO contribution to CBF.

    FOOTNOTES

Address for reprint requests: D. A. Wilson, Dept. of Anesthesiology and Critical Care Medicine, 600 N. Wolfe St., Blalock 1404-B, Baltimore, MD 21287-4965.

Received 2 December 1996; accepted in final form 27 June 1997.

    REFERENCES
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References

1.   Bredt, D. S., and S. H. Snyder. Isolation of nitric oxide synthetase, a calmodulin-requiring enzyme. Proc. Natl. Acad. Sci. USA 87: 682-685, 1990[Abstract/Free Full Text].

2.   Brown, M. M., J. P. Wade, and J. Marshall. Fundamental importance of arterial oxygen content in the regulation of cerebral blood flow in man. Brain 108: 81-93, 1985[Abstract/Free Full Text].

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12.   Harvey, S. A. K., M. L. Trankina, M. S. Olson, and J. B. Clark. Fluorocarbon perfusion of the isolated rat brain: measurement of tissue spaces, EEG and oxygen uptake. Biochim. Biophys. Acta 1073: 486-492, 1991[Medline].

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AJP Heart Circ Physiol 273(4):H1994-H2000
0363-6135/97 $5.00 Copyright © 1997 the American Physiological Society


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