Changes in cerebral blood flow and carbohydrate metabolism during acute hyperketonemia

STEEN G. HASSELBALCH, PETER LUND MADSEN, LARS PINBORG HAGEMAN, KARSTEN SKOVGAARD OLSEN, NIELS JUSTESEN, S0REN HOLM, AND OLAF B. PAULSON Departments of Neurology, Anesthesiology, Nuclear Medicine, and Hospital Pharmacy, Rigshospitalet, National University Hospital, DK-2100 Copenhagen, Denmark

Hasselbalch, Steen G., Peter Lund Madsen, Lars Pinborg Hageman, Karsten Skovgaard Olsen, Niels Justesen, S@ren Holm, and Olaf B. Paulson. Changes in cerebral blood flow and carbohydrate metabolism during acute hyperketonemia. Am. J. Physiol. 270 (EndocrinoZ. Metab. 33): E746-E751, 1996.-During starvation, brain energy metabolism in humans changes toward oxidation of ketone bodies. To investigate if this shift is directly coupled to circulating blood concentrations of ketone bodies, we mea- sured global cerebral blood flow (CBF) and global cerebral carbohydrate metabolism with the Kety-Schmidt technique before and during intravenous infusion with ketone bodies. During acute hyperketonemia (mean P-hydroxybutyrate blood concentration 2.16 mM>, cerebral uptake of ketones increased from 1.11 to 5.60 umol l 100 8-l l min-l, counterbalanced by an equivalent reduction of the cerebral glucose metabolism from 25.8 to 17.2 umol.100 g- l.rninl, with the net result being an unchanged cerebral uptake of carbohydrates. In accordance with this, global cerebral oxygen metabolism was not signifi- cantly altered (144 vs. 135 umol. 100 8-l l min- l). The un- changed global cerebral metabolic activity was accompanied by a 39% increase in CBF from 51.0 to 70.9 ml. 100 gP1*minl. Regional analysis of the glucose metabolism by positron emission tomography- [ IsF] fluoro-2-deoxy-D-glucose indicated that mesencephalon does not oxidize ketone bodies to the same extent as the rest of the brain. It was concluded that the immediate oxidation of ketone bodies induced a decrease in cerebral glucose uptake in spite of an adequate glucose supply to the brain. Furthermore, acute hyperketonemia caused a resetting of the coupling between CBF and metabolism that could not be explained by alterations in arterial CO2 tension or pH.

human brain ketone body metabolism; P-hydroxybutyrate; regulation of cerebral blood flow and brain glucose metabo- lism

UNDER NORMAL CONDITIONS, almost all brain ATP produc- tion is derived from oxidation of glucose (11); however, during starvation, there is a shift in brain metabolism toward oxidation of ketone bodies. In 1967, Owen et al. (15) showed that, after starvation for 5-6 wk in hu- mans, cerebral uptake of ketone bodies (p-hydroxybu- tyrate and acetoacetate) accounted for more than one- half of the cerebral oxygen consumption. Corresponding to the increase in ketone body metabolism, glucose metabolism was reduced. Using the Fick principle, we have recently shown that ketone body metabolism is increased already after 3 days of starvation, accounting for 25% of the oxygen consumption (8).

Little is known about the brain’s capacity to metabo- lize ketone bodies after acute changes in circulating concentrations of ketone bodies. Evidence obtained in mice (19), dogs (4), and humans (1) has suggested that

infusion of ketone bodies may cause an immediate reversal of the effects of hypoglycemia and that acute hyperketonemia increases the cerebral uptake of ke- tone bodies, when infused after prolonged hypoglycu- mia (16). However, the available data are indirect and have been obtained during hypoglycemia, and contradic- tory evidence has been obtained in humans (5).

The purpose of this study was to measure directly if acute infusion with ketone bodies in normoglycemic healthy volunteers will induce alterations in cerebral carbohydrate metabolism and blood flow (CBF). For this purpose, we measured global CBF and global cerebral uptake of oxygen, glucose, ketone bodies, lactate, and pyruvate by use of the Kety-Schmidt technique and arteriovenous differences (a-v) across the brain. Measurements were performed before and during intravenous infusion of P-hydroxybutyrate.

We have previously studied regional glucose metabo- lism (rCMR& with positron emission tomography (PET) using 2-[18F]fluoro-2-deoxy-D-glucose (FDG) as tracer and found a uniform decrease in glucose metabo- lism throughout the brain after 3.5 days of starvation, thus indicating no regional difference in ketone oxida- tion. This finding contrasts with the autoradiographic study in rats by Hawkins et al. (9), who found that neocortical regions in the rat brain were able to utilize ketone bodies to a greater extent than mesencephalic structures. Whether this discrepancy is due to species differences or to methodological differences remains unclear. With PET-FDG, the spatial resolution of previ- ous studies may have prohibited the detection of small regional differences, but the use of scanners w&h higher resolution may allow this point to be clarified. With the use of a PET camera with higher spatial resolution and FDG as a tracer of regional glucose metabolism, we also sought to solve the question of regional differences in this metabolic shift.

MATERIALS AND METHODS

Eight healthy volunteers of normal weight were studied [mean age 24 2 4 (SD) yr, 4 males, 4 females]. Before all investigations, oral and written consent were obtained, and the study was approved by the Central Ethical Committee in Denmark.

ProtocoZ. Each subject was studied two times on 2 dasvs separated by a l- to 2-wk interval. A control study and a study during acute hyperketonemia were performed in random order. On both study days, subjects met in the morning after a light morning meal, and the studies were initiated in the postabsorptive phase 4-5 h after the morning meal. After placement of catheters and a subsequent resting period of 1 h, the subjects were placed physically relaxed in the supine position in a darkened room with eyes closed and padded.

E746 0193-1849/96 $5.00 Copyright o 1996 the American Physiological Society

CEREBRAL METABOLISM AND BLOOD FLOW IN HYPERKETONEMIA E747

Noise was kept at a minimum. In the hyperketonemia study, an infusion of P-hydroxybutyrate was started at time (t) - 140 min and was continued throughout the study. At t = -50 min, subjects were moved to the scanner couch, positioned, and a constant infusion of 133Xe was started at t = -30 min. After 30 min, at t = 0, the 133Xe infusion was terminated, and global CBF was determined by the Kety-Schmidt technique. rCMRo1, was simultaneously determined by PET-FDG after intravenous injection of FDG at t = 0.

P-Hydroxybutyrate infusion. Sodium P-hydroxybutyrate (no. H6501) was supplied by Sigma Chemicals (Poole, UK) and dissolved in sterile water, filtered, and tested for pyro- gens to a final solution of 55 mg/ml. The pH of the final solution was 7.1. P-Hydroxybutyrate was infused in an ante- cubital vein at a rate of 4-5 mgokg body wt-l.min-l, corre- sponding to an infusion of -350 ml/h. The infusion was continued throughout the study, which lasted -3-3.5 h. In the control condition, isotonic saline was infused at the same rate.

Measurement of global CBF and global cerebral metabo- lism. Global CBF and metabolism were measured by the Kety-Schmidt (12) technique in the desaturation mode, using 13”Xe as the flow tracer. Cerebral venous blood was sampled from a catheter inserted percutaneously low on the neck into the internal jugular vein. The catheter tip was advanced to the base of the skull, and the correct placement was verified as described elsewhere (8). Arterial blood was sampled from a catheter in the radial artery. Both catheters had the same dead space volume (1 ml). Measurements of global CBF were performed exactly as previously described in detail (14). In brief, the brain was saturated by an intravenous infusion of 133Xe dissolved in saline at a constant rate of -15 MBq/min for 30 min. A 1.5-ml dead space volume was drawn simulta- neously from both catheters immediately before 1 ml of blood was drawn into preweighed syringes. Blood samples were obtained at exact times as follows: t = -2, - 1, 0,0.5,1,2,3,4, 6, 8, and 10 min, where t = 0 denotes the time when the infusion was terminated. To avoid loss of 133Xe gas by diffusion, the syringes were reweighed immediately after each run and placed in sealed vials for counting in a well counter (COBRA 5003; Packard Instrument, Downers Grove, IL). During the CBF measurement, three paired samples of arterial and jugular venous blood were obtained for paired determinations of arteriovenous differences for oxygen (a-v O& glucose (a-v Glc), and lactate (a-v Lac). Arteriovenous differences for ketone bodies were measured two times and arteriovenous differences for pyruvate once. Arterial carbon dioxide tension (Pa co,) was measured three times during the CBF measurement.

PETFDG. We used the GE 4096+ Pet Camera (General Electric Medical Systems, Milwaukee, WI) yielding 15 consecu- tive slices with a slice thickness of 6.7 mm and a spatial resolution in the image plane of 6.7 mm. Slices were placed parallel to the canthomeatal (CM) line (a line through the lateral canthus of the eye and the external meatus of the ear) with midslice planes from 10 to 104 mm above the CM line. A transmission scan was performed immediately before the activity scan for attenuation correction. At t = 0, 185-210 MBq FDG in 10 ml saline were injected intravenously. Dynamic scanning was started at t = 0 with the following scan sequence: 10 X 6-s scan (O-l min), 3 X 20-s scan (l-2 min), 8 x 1-min scan (2-10 min), 5 x 2 min (lo-20 min), and 5 X 5 min (20-45 min). Blood samples (1 ml) were drawn from the radial artery with increasing time interval. Blood samples were immediately placed on ice, centrifuged, and 500 ul plasma were taken for gamma counting (COBRA 5003, Packard Instrument).

r CMRGlc was calculated from PET by the single scan approach first described by Sokoloff et al. (17) using the last scan (t = 40-45 min) corrected by rate constants and lumped constant determined from the dynamic scan sequence as previously described (8). Rate constants were calculated from six gray and six white matter regions of interest drawn by hand on three slices located -43, 57, and 70 mm above the CM line. Region of interest analysis was performed with the use of the Computerized Brain Atlas (7) and internal ratios were obtained by normalizing regions to the weighted mean of all cortical regions.

Substrate analysis. Blood samples for determination of a-v O2 and Pace were stored on ice and analyzed within 15 min for oxygen saturation, hemoglobin concentration, and Pace with an OSM3 and ABL apparatus (Radiometer, Copenhagen: Denmark). Blood samples for determination of plasma glu- cose and lactate concentrations were drawn into vials contain- ing fluoride-EDTA, stored on ice for 5 min, centrifuged, and analyzed in duplicate within 15 min on a Beckman Glucose Analyzer (Beckman Instruments, Fullerton, CA); the remain- ing portions of theses samples were stored at -80°C until subsequently analyzed in duplicate with a YSI 2300 Lactate Analyzer (Yellow Spring Instruments, Yellow Springs, OH). Whole blood samples for determination of acetoacetate and P-hydroxybutyrate were immediately deproteinized, stored at -80°C and later analyzed as described by Williamson et al. (21). Pyruvate was analyzed by standard enzymatic techniques.

The blood samples used in the PET-FDG and CBF measure- ments contained gamma activity from both 133Xe [decay half-time (TxJ = 5.25 days] and [18F]FDG (decay TI/,= 110 min). Energy windows were set around the energy peak of each tracer (133Xe = 70-90 keV, IsF = 461-561 keV), and correction for cross talk between windows was applied. By extending the time interval from sampling of blood to the gamma counting of the blood samples, cross talk from the IsF window into the 133Xe window decreased rapidly. The blood samples were counted continuously for the next 12 h, and the optimal counting time was found to be 8-10 h after injection, where the cross-talk correction from FDG into the 133Xe window was very small and no detectable diffusion of 133Xe out of the vials had occurred. However, it was found that the time interval between sampling of blood and counting of blood samples had no significant influence on the CBF calculation.

Calculations. CBF was calculated using the Kety-Schmidt (12) equation modified for the application of the method in the desaturation mode. The measured CBF values were corrected for the systematic overestimation of flow values due to incomplete tracer washout at the end of the measuring period (14). Cerebral a-v 02 was calculated using the hemoglobin concentration and oxygen saturation of hemoglobin in arte- rial and internal jugular venous blood. Cerebral a-v Glc and a-v Lac were calculated from glucose and lactate concentra- tions in arterial and venous plasma. It is our experience that glucose and lactate determinations are more precise when performed on plasma than on whole blood, and the lactate and glucose concentrations presented in this study are plasma values corrected to corresponding whole blood values as described previously (13). Global metabolic rates (CMR) were calculated for each substrate according to the Fick principle, e.g., metabolic rate for oxygen (CMRo,) = CBF X a-v OZ. A paired two-tailed Student’s t-test was used in the statistical evaluation of the data.

RESULTS

Arterial concentrations, a-v differences, and meta- bolic rates for all brain metabolites measured are

E748 CEREBRAL METABOLISM AND BLOOD FLOW IN HYPERKETONEMIA

Table 1. Arterial concentrations, arteriovenous differences, and cerebral metabolic rates for brain substrates before and during hyperketonemia

Glucose f3-OHB AcAc Lactate Pyruvate Oxygen

Arterial concentrations, 8 8 6 8 8 8 mmol/l whole blood

Control 4.80 k 0.71 0.31 ?I 0.17 0.02 + 0.02 0.512 0.06 0.06 + 0.02 Hyperketonemia 4.412 0.77 2.16 t 0.42$’ 0.24 + 0.06t 0.65 k 0.19 0.06 5 0.02

Arteriovenous differences, mmol/l whole blood Control 0.512 0.06 0.02 k 0.02 0.00 + 0.01 -0.05 + 0.03 -0.012 0.00 2.85 + 0.28 Hyperketonemia 0.25 f 0.04:‘: 0.08 + 0.03f 0.04 + 0.07 -0.04 + 0.01 -0.02 + 0.01 1.96 2 0.32:‘:

Cerebral metabolic rate, umol ~100 g- 1 l min-l Control 25.83 k 3.06 1.115 1.23 0.00 + 0.01 -2.73 + 0.03 -0.45 + 0.28 144.04 + 21.77 Hyperketonemia 17.22 + 2.99:‘: 5.60 + 2.25$ 2.49 t 4.17 -2.86 + 1.10 - 1.012 0.85 135.33 + 21.75

Values are means + SD; n, no. of subjects. P-OHB, P-hydroxybutyrate; AcAc, acetoacetate. ‘:‘P < 0.0001, TP < 0.01, $P < 0.001: paired 2-sample t-test.

shown in Table 1. During ketone body infusion, blood concentration of P-hydroxybutyrate increased seven- fold from 0.31 to 2.16 mM (P < O.OOOl>, and acetoac- etate concentration also increased from 0.02 to 0.24 mM (P < 0.01) due to the conversion of P-hydroxybutyr- ate to acetoacetate in tissue. Blood concentrations of glucose, lactate, and pyruvate remained constant dur- ing acute hyperketonemia. Global CBF increased by 39% from a control value of 51.0 to 70.9 ml. 100 g -I emin during acute hyperketonemia (P < 0.02). The CBF increase was not due to changes in Pace , which remained constant, whereas pH increased signi 6 - cantly from 7.42 to 7.50 due to an increase in HCO, (P < 0.0002; Table 2). The metabolic rate for P-hydroxybu- tyrate (CMR B-OHB) increased by a factor of 5 from 1.11 to 5.60 umol.100 g-larnin-l (P < 0.001; Table 1). The metabolic rate for acetoacetate increased similarly, but the increase was not statistically significant due to a large variation. Global glucose metabolism decreased by 33% (P < 0.0001) from 25.8 to 17.2 umol*minlgl, whereas the metabolic rate for oxygen remained con- stant (144.0 vs. 135.3 umol.100 g-l*min-l). The net e&x of lactate and pyruvate did not change during hyperketonemia. The molar oxygen-to-glucose uptake ratio was 5.57 in the control condition and increased significantly during hyperketonemia to 7.91 (P < O.OOS>, reflecting an increased oxidation of nonglucose com- pounds (Table 3). The oxygen equivalents of glucose and ketone bodies are also shown in Table 3. In the control condition, 97% of the oxygen taken up by the brain was used for oxidation of glucose. During acute hyperketonemia, however, glucose oxidation accounted for only 74% of the total oxygen uptake, and the

Table 2. Cerebral blood flow, Paco2, pH, and HCO, before and during hyperketonemia

Control Hyperketonemia

CBF, ml. 100 g- l+min l 51.0 2 9.0 70.9 k 16.9:‘: Pac:0,, kPa 5.03 + 0.42 5.04 k 0.60 PH 7.42 + 0.02 7.50 k o.o2’i- HCO,, mmol/l 24.02 + 1.66 29.65 + 2.631-

Values are means i SD for 8 subjects in each group. Arterial CO2 tension (Pace,), pH, and HCO, values are mean values at time of cerebral blood flow (CBF) measurement (3 averaged measurements). ‘:‘P < 0.02, +P < 0.0002: paired 2-sample t-test.

remaining 26% was used for ketone body oxidation. There was a good agreement between the oxygen consumption calculated from oxygen equivalents and the measured oxygen consumption, which differed by +4 and -9% in the control condition and durini hyperketonemia, respectively (Table 3).

As described above, the infusion of P-hydroxybutyr- ate created distinct changes in CBF and in a-v differ- ences for glucose and oxygen (Tables 1 and 2). These changes with time are illustrated in Fig. 1, which shows that there were comparable decreases in a-v Glc and a-v O2 until 60 min after the start of infusion, after which time the decrease in a-v Glc was more pro- nounced than the decrease in a-v OZ. After the start of the infusion (130 min), both a-v Glc and a-v O2 were stable at different relative levels.

Normalized rCMRolc values obtained by PET-FDG are shown in Table 4. The highest increases in the normalized ratio were found in gyrus cingulus and in mesencephalon, but the increase was only significant in mesencephalon, indicating that there was a less pro- nounced decrease in rCMRol, in mesencephalon com- pared with cortex. In the other subcortical and cortical regions, no changes in the ratios were found, indicating no regional change in rCMRGlc during ketone infusion.

DISCUSSION

The present study demonstrates significant changes in brain metabolism and CBF when the blood ketone level is acutely increased in humans. The main findings are as follows: 1) a fivefold increase in P-hydroxybutyr- ate metabolism, 2) a 33% reduction in glucose metabo- lism, 3) a 39% increase in CBF, and 4) a small change in the regional pattern of CMRolc.

Alterations in cerebral carbohydrate metabolism. In- fusion with P-hydroxybutyrate caused a sevenfold in- crease in the blood level of P-hydroxybutyrate and a fivefold increase of CMRP-OHB. The same correlation between the increase in blood ketone levels and the increase in cerebral ketone uptake after prolonged hypoglycemia has previously been observed in experi- mental studies (9, 16) and in humans (6, 8). Indirect evidence for acute cerebral oxidation of ketone bodies during hypoglycemia has been obtained both in ani- mals and humans. In suckling-weaning mice, Thurston

CEREBRAL METABOLISM AND BLOOD FLOW IN HYPERKETONEMIA E749

Table 3. Oxygen-to-glucose molar ratio and O2 equivalents ofglucose and ketone bodies before and during acute hyperketonemia

O2 Equivalents

Molar Ratio Glucose Ketones Total Measured

Control 5.57 + 0.47 145 + 19 (97%) 5*8(3%) 150 + 22 144 2 22 Hyperketonemia 7.912 1.00:‘: 92 + 18-F (74%) 32 + 12* (26%) 124 + 12$ 135 + 22

Values are means -+ SD. Values in parentheses are percentage of total. Molar ratio measured as ratio between oxygen and glucose metabolic rate; Oz glucose (umol.100 8-l. min-‘) = 02 equivalents of total oxidation of glucose corrected for ef?lux of lactate and pyruvate (6 X (CMR respective y); 7

,~ + [(CMRL,, + CMR p,,>/2]\, where CMRG~~, CMRL,,, and CMRP, are metabolic rates for glucose, lactate, and pyruvate, 02 ketones = 02 equivalents of total oxidation of P-hydroxybutyrate (P-OHB) and acetoacetate (AcAc; 4.5 X CMRP-

OHB+~XCMR AcAc) 02 total = 02 glucose + 02 ketones = summed 02 equivalents of glucose and ketone bodies; 02 measured = oxygen consumption measured from CBF X arteriovenous 02 difference. ‘%P < 0.005, +P < 0.0005, $P < 0.01: paired 2-Sample t-test.

et al. (19) reversed hypoglycemia-induced coma and sustained normal brain metabolism by infusion of P-hydroxybutyrate. In humans, ketone body infusion lowered the counterregulatory hormone response to hypoglycemia (1). However, also in humans, by infusing ketone bodies, Frolund et al. (5) found no changes in the hypoglycemia-induced neurohumoral response. Thus, while previous results are not clear and the studies only gave indirect data obtained during hypogly- cemia, the present study, for the first time, provides direct evidence for changes in cerebral metabolism during acute hyperketonemia in normoglycemic hu- mans. The acute hyperketonemia-induced increase in CMRPoHB was counterbalanced by a 33% reduction of CMRclc, but no significant alterations in CMRo, were

observed (144 t 22 vs. 135 t 22 umol.100 g-l.min-l, not significant). Thus, without changing the level of oxidative metabolism, acute hyperketonemia induced a partial substitution of cerebral glucose metabolism with metabolism of ketone bodies. The present study thus indicates that the human brain can use ketone bodies during normoglycemia and emphasizes that brain ketone body uptake is regulated by the availabil- ity of ketone bodies and not by glucose deficiency.

How the shift in brain substrate metabolism is brought about is not entirely known, but experimental studies have found metabolite changes compatible with inhibition of hexokinase, and phosphofructokinase, and thus inhibition of glycolysis (19). Inhibition of glycoly- sis could be brought about by an increase in citrate

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PET-FDG

80 100 120 140 160 180

min Fig. 1. Changes in pH, Pco~, and arteriovenous differences for glucose (a-v Glc) and oxygen (a-v 02) during infusion of P-hydroxybutyrate. Values are mean values of 8 subjects, scaled to 100% at start of infusion (time = 0). Decrease in a-v 02 is presumably due to an increase in CBF, and uncoupling of a-v Glc and a-v 02 60 min after start of infusion indicates increased oxidation of nonglucose substrates. PET-FDG, positron emission tomography-[lsF]fluoro-2-deoxy- D-glucose.

E750 CEREBRAL METABOLISM AND BLOOD FLOW IN HYPERKETONEMIA

Table 4. Regional changes in glucose metabolism after ketone body infusion

Region Control Infusion Whange P

Orbitofrontal 1.03 + 0.05 0.99 t 0.09 -3.7 NS Dorsolateral frontal 1.16 5 0.12 1.15 + 0.07 -0.1 NS Temporal 1.03 -+ 0.06 1.00 + 0.07 -2.6 NS Parietal 0.96 -+ 0.08 1.01 t 0.06 5.4 NS Occipital 0.96 + 0.06 0.93 + 0.11 -2.6 NS Nucleus caudatus 1.09 5 0.04 1.12 + 0.10 2.5 NS Putamen 1.32 I+ 0.17 1.33 + 0.10 1.4 NS Thalamus 1.09 + 0.14 1.15 5 0.12 5.7 NS Hippocampus 0.73kO.09 0.70+0.09 -3.8 NS Gyrus cingulus 1.03 5 0.13 1.10 + 0.11 8.1 NS Pons 0.47+0.09 0.44kO.05 -2.3 NS Mesencephalon 0.69-'0.08 0.75?0.09 8.4 0.008 White matter 0.412 0.05 0.39 5 0.06 -3.0 NS

Values are means 2 SD. Values are regional glucose metabolic rates (rCMRc;I,) in region normalized to weighted mean of all gray matter regions (i.e., nucleus caudatus = rCMRolc in nucleus caudatus/ rCMRo1,. in mean gray matter). Increase in normalized value indi- cates relatively smaller decrease in rCMRolc compared with mean gray matter. P < 0.01 was considered significant due to multiple comparisons. NS, not significant.

concentration and a decrease in ADP concentration induced by the increased P-hydroxybutyrate metabo- lism through the tricarboxylic acid cycle (16). In that study, it was also observed that hyperketonemia caused a marked increase of cerebral lactate efflux, indicating inhibition of pyruvate oxidation, which again inhibits glycolysis. In contrast to these experimental data, the unchanged level of cerebral lactate and pyruvate eMux observed during hyperketonemia in the present study can be taken to signify unaltered cerebral production of lactate and pyruvate. This observation is in accordance with data obtained during hyperketonemia after 3 days of starvation (8), indicating that inhibition of glycolysis is not related to inhibition of pyruvate oxidation. Whether the discrepancy of results is due to species difference or can be explained by methodological differ- ences remains to be clarified.

Analysis of the regional changes in glucose metabo- lism revealed that the mesencephalon-to-cortex ratio increased significantly by 8% during ketone body infu- sion, which indicates a relatively smaller decrease in glucose metabolism in this region, signifying that mes- encephalic structures do not oxidize ketone bodies to the same extent as the rest of the brain. This is in line with autoradiographic data from rats (9), but the reason for this regional difference in ketone body utilization capacity remains to be clarified.

Alterations in CBF. Despite the unchanged overall metabolic rate, infusion with P-hydroxybutyrate in- creased global CBF from a baseline value of 51 to 71 ml l 100 g- 1 l rnin during hyperketonemia. This 39% increase in CBF was not due to alterations in Pace, as this variable was unchanged by hyperketonemia. Likewise, the CBF increase cannot be explained by the minor increase in pH, which, if anything, should lead to a decrease in CBF.

Theoretically, the increase in CBF could be due to a direct action of P-hydroxybutyrate on cerebrovascular

smooth muscle, but no existing literature provides information on this point. Information on blood flow response to hyperketonemia in other organs is also sparse. Acute hyperketonemia increased human fore- arm blood flow by 15% (ZO), but the exact mode of action remains obscure. Because the CBF increase was paral- lel to a reduction of CMRolc, it could seem similar to the inverse relationship between glucose metabolism and CBF, which has been demonstrated in hypoglycemia. Using single photon emission tomography in normal volunteers, Tallroth et al. (18) found a marked increase (-20%) in CBF 10 min after induction of acute hypogly- cemia (mean plasma glucose 2.0 mM). Similar results have been obtained in rats by Bryan et al. (3), who induced a 140-240% increase in CBF by lowering the mean plasma glucose level to 1.5 mM. The CBF in- crease in the present study differs somewhat from the above studies in the sense that inhibition of glycolysis was not mediated by an acute lowering of plasma glucose, since there was ample glucose supply to the brain. Thus the CBF increase seems to be related to the decrease in glycolytic flux, in line with the experimen- tal study of Breier et al. (2>, who demonstrated signifi- cant CBF increases by blocking glycolysis by pharmaco- logical doses of 2deoxy-D-glucose. However, in contrast to studies in which severe glucopenia or blocking of carbohydrate metabolism may induce other and more profound metabolic changes in the tissue, the present study reveals that coupling of glycolysis and CBF is independent of the energy state of the brain or the total cerebral oxidative metabolism, because these did not change during hyperketonemia. This finding points to glycolytic intermediates as regulators of CBF, but the mechanisms behind the coupling of CBF and glucose metabolism remain unknown. Several other putative mediators have been proposed, including cathecola- mines and adenosine (10, 22), but none of these are likely to have played a significant role in the present study performed during normoglycemia, where no neu- rohumoral stress response or adenosine accumulation are expected to occur.

In contrast to the present data, after several days of starvation, no changes in CBF were observed, although glucose metabolism had decreased similarly to what was observed in the present study (8). It may be that a secondary downregulation of CBF takes place with time, but, since the mechanisms of the coupling still remain unclear, these speculations should be clarified in future studies.

We thank Dr. Jens Kondrup and the technical staff at the Dept. of Medicine A, Rigshospitalet, for help with analysis of the blood samples. We also thank Gerda Thomsen, Pia Tejmer, and Karin Stahr for technical assistance.

This study was supported by grants from The Danish Medical Research Council, The Lundbeck Foundation, The Foundation of 17.12.1981, and The Simon F. Hartmann Family Foundation.

Address for reprint requests: S. G. Hasselbalch, Dept. of Neurology N2081, National University Hospital, Rigshospitalet, Blegdamsvej 9, DK-2100 Copenhagen, Denmark.

Received 8 September 1995; accepted in final form 21 November 1995.

CEREBRAL METABOLISM AND BLOOD

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