The transport of ketone bodies into the brain of the rat (in vivo)

Monocarboxylate transporters in temporal lobe epilepsy: roles of lactate and ketogenic diet

Epilepsy is a serious neurological disorder that affects approximately 1 % of the general population, making it one of the most common disorders of the central nervous system. Furthermore, up to 40 % of all patients with epilepsy cannot control their seizures with current medications. More efficacious treatments for medication refractory epilepsy are therefore needed. A better understanding of the mechanisms that cause this disorder is likely to facilitate the discovery of such treatments. Impairment in cerebral energy metabolism has been proposed as a possible causative factor in the pathogenesis of temporal lobe epilepsy (TLE), which is one of the most common types of medication-refractory epilepsies in adults. In this review, we will discuss some of the current hypotheses regarding the possible causal relationship between brain energy metabolism and TLE. Emphasis will be placed on the role of energy substrates (lactate and ketone bodies) and their transporter molecules, particularly monocarboxylate transporters 1 and 2 (MCT1 and MCT2). We recently reported that the cellular distribution of MCT1 and MCT2 is perturbed in the hippocampus in patients with TLE. The changes may be an adaptive response aimed at keeping high levels of lactate in the epileptic tissue, which may serve to counteract epileptic activity by downregulating cAMP levels through the lactate receptor GPR81, newly discovered in hippocampus. We propose that the perturbation of MCTs may be further involved in the pathophysiology of TLE by influencing brain energy homeostasis, mitochondrial function, GABA-ergic and glutamatergic neurotransmission, and flux of lactate through the brain.

The postmortem distribution of ketone bodies between blood, vitreous humor, spinal fluid, and urine

The distribution of the ketone bodies: acetone, acetoacetate, and D-beta-hydroxybutyrate, between blood, vitreous humor, spinal fluid, and urine was examined in 105 medico-legal autopsies. The ketone body concentration in the body fluids was determinated by head-space gas chromatography. The correlation between blood and the body fluids could be described with regression lines on the logarithmic-transformed results. The correlation is dependent on the ketone body concentration. The ketone bodies in spinal fluid show the best correlation to blood, followed by vitreous humor, and last urine. The concentration dependence in spinal fluid is mainly due to ketone bodies being metabolized in the brain. The human brain utilizes ketone bodies during normal nutritional state. In vitreous humor, the dependence is mainly due to protein bindings of acetoacetate and beta-hydroxybutyrate in blood and the difference in dry matter between blood and vitreous humor.

Brain uptake and metabolism of ketone bodies in animal models

As a consequence of the high fat content of maternal milk, the brain metabolism of the suckling rat represents a model of naturally occurring ketosis. During the period of lactation, the rate of uptake and metabolism of the two ketone bodies, beta-hydroxybutyrate and acetoacetate is high. The ketone bodies enter the brain via monocarboxylate transporters whose expression and activity is much higher in the brain of the suckling than the mature rat. beta-Hydroxybutyrate and acetoacetate taken up by the brain are efficiently used as substrates for energy metabolism, and for amino acid and lipid biosynthesis, two pathways that are important for this period of active brain growth. Ketone bodies can represent about 30-70% of the total energy metabolism balance of the immature rat brain. The active metabolism of ketone bodies in the immature brain is related to the high activity of the enzymes of ketone body metabolism. Thus, the use of ketone bodies by the immature rodent brain serves to spare glucose for metabolic pathways that cannot be fulfilled by ketones such as the pentose phosphate pathway mainly. The latter pathway leads to the biosynthesis of ribose mandatory for DNA synthesis and NADPH which is not formed during ketone body metabolism and is a key cofactor in lipid biosynthesis. Finally, ketone bodies by serving mainly biosynthetic purposes spare glucose for the emergence of various functions such as audition, vision as well as more integrated and adapted behaviors whose appearance during brain maturation seems to critically relate upon active glucose supply and specific regional increased use.

Lactate as a pivotal element in neuron-glia metabolic cooperation

Lactate has been considered for a long time as a metabolic waste and/or a sign of hypoxia in the central nervous system. Nevertheless, clear evidence that lactate can constitute an adequate energy substrate for brain tissue has been provided as early as in the 1950s with the pioneering work of McIlwain in brain slices. Over the years, several studies using different approaches have confirmed that lactate is efficiently oxidized by brain cells in vitro. Moreover, lactate has been shown under certain circumstances to have a neuroprotective effect and support neuronal activity. Similar confirmation of lactate utilization in vivo as well as putative neuroprotection in various excitotoxic models has been provided. Lactate was even shown to restore cognitive performance upon an hypoglycemic episode in humans. More recently, it was proposed that lactate could be produced by astrocytes and released in the extracellular space to form a pool readily available for neurons in case of high energy demands. Several elements support the concept of a lactate shuttle between astrocytes and neurons in the central nervous system. Among them, the description of specific monocarboxylate transporters found on both astrocytes and neurons is an important observation consistent with this concept. Interestingly, lactate shuttles between different cell types within the same organ have been described outside the central nervous system, notably in muscle and testis. Thus, lactate is emerging as a valuable intercellular exchange molecule in different systems including the brain where it might be an essential element of neuron-glia metabolic interactions.

Transport of 3-hydroxybutyrate by cultured rat brain astrocytes

It is well established that 3-hydroxybutyrate can serve as an energy source for the brain. Since substrate utilization may be regulated in part by transport across the cellular membrane, we investigated the uptake of 3-hydroxybutyrate by primary cultures of rat brain astrocytes. Measurement of the net uptake indicated a saturable system and a Lineweaver-Burke type plot was consistent with a single carrier-mediated mechanism with a Km of 6.03 mM and a Vmax of 32.7 nmol/30 seconds/mg protein. The rate of uptake at pH 6.2 was more than ten times the rate at pH 8.2, with the rate at pH 7.4 being intermediate between these values, suggesting the possibility of cotransport with H+ or exchange with OH- (antiport). Mersalyl had only a slight effect on the transport of 3-hydroxybutyrate, suggesting that sulfhydryl groups are not involved in the transport of this monocarboxylic acid. Phenylpyruvate and alpha-ketoisocaproate also attenuated the transport, but lactate had only a marginal effect. These results suggest that the utilization of 3-hydroxybutyrate as an energy source by astrocytes is regulated in part by carrier-mediated transport and that the uptake system is different from the lactate transport system.

Influence of early chronic phenobarbital treatment on cerebral arteriovenous differences of glucose and ketone bodies in the developing rat

The influence of an early chronic phenobarbital treatment on cerebral arteriovenous differences of glucose, lactate, pyruvate, beta-hydroxybutyrate and acetoacetate was studied in suckling rats. The animals were treated from day 2 to 21 after birth by a daily injection of 50 mg/kg phenobarbital or by saline and were studied at 10, 14 and 21 days. Phenobarbital treatment induced a decrease in cerebral arteriovenous difference of glucose at P14 and no change at P10 and P21. The barbiturate did not have any influence on cerebral arteriovenous difference of lactate and pyruvate at the three stages studied. Cerebral uptake of beta-hydroxybutyrate was unchanged at P10 and increased by two-fold at P14 and by threefold at P21 by phenobarbital. Cerebral arteriovenous difference of acetoacetate was low and did not change with the pharmacological treatment. At P14 and P21, the calculated amount of oxygen used by the brain for the oxidation of ketone bodies was twice as high in barbiturate- as in saline-treated rats and reached values of 47 and 16% respectively in phenobarbital-exposed animals. In addition, the barbiturate seemed to affect the carrier process of beta-hydroxybutyrate from blood to brain. The results of the present study are in good agreement with previous data from our laboratory showing that an early chronic phenobarbital treatment is able to induce a shift in the cerebral energy metabolism balance in favor of ketone bodies.

The transport of leucine and aminocyclopentanecarboxylate across the intact, energy-depleted rat blood-brain barrier

The transport across the blood-brain barrier of the large neutral amino acid leucine and the nonmetabolised aminocyclopentanecarboxylate (ACPC), of similar molecular size, was studied in the perfused, energy-depleted rat brain. It was found that when both leucine and ACPC were perfused for periods of up to 10 min their accumulation in the brain increased in a linear fashion. The ratio of perfusate radioactivity per milliliter and tissue radioactivity per gram (Rt/Rp) rose to above unity for both leucine and ACPC, indicating continued uptake against a concentration gradient of the radiolabel within the CNS. When the effect of increasing the concentration of the amino acid upon its influx into the brain was studied, it was found that under these conditions the kinetics of transport for both leucine and ACPC were of a similar order of magnitude to those reported previously in vivo. The values for the Michaelis constant for transport (Km), maximum rate of transport (Vmax), and the constant for the apparently linear, nonsaturable component (Kd) for leucine into the cerebrum were 84.5 +/- 29.0 microM, 45.5 +/- 1.5 nmole/min/g, and 2.62 +/- 0.15 microliters/min/g, respectively, and for ACPC 381 +/- 64 microM, 54.0 +/- 1.5 nmole/min/g and 0.35 +/- 0.10 microliter/min/g, respectively. Comparing this data with previously reported values it is suggested that the transport of leucine into the central nervous system from a perfusate or bolus where no other competing amino acids are present, is flow dependent. Furthermore, ACPC enters the brain almost entirely by a carrier-mediated process, with little or no nonsaturable influx despite a similar oil/water partition coefficient as leucine.

Regional kinetic constants for blood-brain barrier pyruvic acid transport in conscious rats by the monocarboxylic acid carrier

The present investigation using labeled pyruvate describes the regional distribution and kinetics of the monocarboxylic acid carrier at the blood-brain barrier of conscious rats. The experimental procedure involved the arterial injection of a single bolus of 200 microliter containing [1-14C]pyruvate, [3H]water, and varying concentrations of unlabeled pyruvate into the common carotid via an indwelling externalized catheter. The hemisphere ipsi-lateral to the injection and rostral to the midbrain was removed and dissected into five regions. A kinetic analysis revealed no significant regional differences in Km values with an overall average of 1.37 mM. However, there was regional variation in the density of the monocarboxylic acid carrier as indicated by varied levels of the kinetic constant Vmax. The cortex showed the highest Vmax value of 0.42 +/- 0.08 mumol/min/g whereas values for the caudate/putamen, thalamus/hypothalamus, and remaining portion of hemisphere ranged significantly lower at 0.22-0.27 mumol/min/g. The Vmax for the hippocampus was intermediate at 0.37 +/- 0.12 mumol/min/g. The nonsaturable carrier described kinetically by KD had an overall average of 0.034 ml/min/g. The present study confirms quantitatively previous results suggesting a variable regional distribution of the monocarboxylic acid carrier.

Modulation of substrate transport to the brain

Variations of substrate transport across the cerebral capillary endothelium were examined in response to variations of the substrate demand of the brain tissue, and to variations of substrate concentration in the blood. The substrates examined included glucose and ketone bodies. The transport changes were measured in rats, using an indicator fractionation method modified by the reviewer. Four mechanisms appeared to contribute to the adjustment of substrate transport to variations in substrate demand. The first and least important mechanism was the change of concentration gradient across the endothelium that occurred when the substrate consumption rate changed. The second mechanism was the flow-dependency of the average capillary substrate concentration: the higher the perfusion rate, the higher the average capillary concentration. This mechanism failed to account for the changes of substrate transport observed during marked increases of the metabolic rate. The third and most important mechanism was a change of the capillary diffusion capacity, probably associated with a change of the number of perfused capillaries. The fourth mechanism, not previously described, was an adaptation of transport to permanent changes of substrate concentration in the blood. This mechanism appeared to reflect changes of the concentration (and affinity?) of transport proteins in the plasma membranes of endothelial cells, possibly in association with changes of cellular protein synthesis and gene expression.

Inhibition, by 2-oxo acids that accumulate in maple-syrup-urine disease, of lactate, pyruvate, and 3-hydroxybutyrate transport across the blood-brain barrier

Data are presented in support of the transport of (-)-D-3-hydroxybutyrate across the blood-brain barrier (BBB) being a carrier-mediated process. The kinetic parameters in 21-day-old pentobarbital-anaesthetized rats were Vmax 2.0 mumol.g-1.min-1, Km 29 mM, and KD 0.024 ml.g-1.min-1. The value for Vmax was the same as that for L-lactate and pyruvate transport in animals of the same age. The transport of all three substrates was sensitive to inhibition by low concentrations of either 2-oxo-3-methylbutanoate or 2-oxo-4-methylpentanoate, the 2-oxo acids that can accumulate in patients with maple-syrup-urine disease. The Ki values for the 2-oxo acids were severalfold lower than the respective Km values. 2-Oxo-3-phenylpropionate was a poor inhibitor. The relative affinities of the various monocarboxylic acids for the transport system of the BBB distinguished it from similar systems described in brain, heart, and liver mitochondria; human erythrocytes; and Ehrlich ascites-tumour cells.

Blood-brain glucose transfer: repression in chronic hyperglycemia

Diabetic patients with increased plasma glucose concentrations may develop cerebral symptoms of hypoglycemia when their plasma glucose is rapidly lowered to normal concentrations. The symptoms may indicate insufficient transport of glucose from blood to brain. In rats with chronic hyperglycemia the maximum glucose transport capacity of the blood-brain barrier decreased from 400 to 290 micromoles per 100 grams per minute. When plasma glucose was lowered to normal values, the glucose transport rate into brain was 20 percent below normal. This suggests that repressive changes of the glucose transport mechanism occur in brain endothelial cells in response to increased plasma glucose.

Factors affecting the supply of glucose to the heart of the rat, in vivo

1. The influx of glucose into the heart of intact, living, anaesthetized rats was measured when the levels of insulin the blood were (a) low (as a result of fasting), (b) normal, and (c) high (as a result of injecting insulin). The findings showed that the transport of glucose into cardiac cells is carrier-mediated and is strongly insulin-independent. 2. The major barrier to the supply glucose to the heart from the circulating blood is at the surface membrane of the cardiac cells, rather than at the endothelium of the cardiac capillaries. 3. The extracellular space of the heart was measured and was found to be approximately 25% of the cardiac tissue. 4. During life, glucose, as well as its analogue, 3-O-methylglucose passes across the membranes of the cells of the heart by means of a transport system which is strongly dependent upon insulin and appears to be carried-mediated. A likely explanation for the effect of insulin is that it increases considerably the affinity of the transport carrier for glucose. Saturation of the carrier takes place when the levels of insulin and of glucose in the blood are high. However, when the concentration of insulin is low, e.g. during a fast, the affinity of the carrier for glucose is reduced so that saturation cannot be demonstrated. 5. It is suggested that the low level of insulin that is found in the blood in the early morning, which is due to the night fast, may lead to the cardiac dysfunction which often develops at that time.

The effect of age upon the entry of some amino acids into the brain, and their incorporation into cerebral protein

The way in which the influx of amino acids into the brain changes as the age of rats increases from one to 25 weeks is reported. Most of the amino acids have a very high rate of influx during the first weeks of life, which falls progressively until an adult is reached by about 8 to 10 weeks of age. The high influx in early life is due to carrier-mediated transport systems which act more rapidly in the young than in the adult. These systems can be saturated by raised levels of amino acid in the blood, both in young and adult animals. The high influx in early life is necessary to provide abundant supplies of amino acids for cerebralprotein formation during the period of rapid brain growth. In adult life, influx declines to a level that is adequate to support the synthesis of cerebral protein, which is still being continuously broken down and renewed at a remarkably high rate.