Lactate Uptake and Release in the Presence of Glucose by Sympathetic Ganglia of Chicken Embryos and by Neuronal and Nonneuronal Cultures Prepared from These Ganglia

Abstract

Uptake and output of lactate were measured in lumbar sympathetic chains excised from embryos of white leghorn chickens, 14-15 days old. The chains, typically containing 30-40 micrograms of protein, were incubated in Eagle's minimum essential medium containing bicarbonate buffer, 6-17 mM glucose, various concentrations of lactate, and either [U-14C]lactate, [1-14C]glucose, or [6-14C]glucose. The average rate of uptake of labeled lactate was measured with incubations of 5-6 h, starting with various external lactate concentrations. From these data the instantaneous relation between lactate uptake rate and concentration was deduced with a simple computerized model. The instantaneous uptake rate increased with the concentration according to a relation that fit the Michaelis-Menten equation, with Vmax = 360 mumol/g protein/h and Km = 4.8 mM. Substantial fractions of the lactate carbon were recovered from tissue constituents and in several nonvolatile products in the medium, as well as in CO2. Glucose uptake averaged about 108 mumol/g protein/h and did not vary greatly with external lactate concentration, although the metabolic partitioning of glucose carbon was considerably affected. Regardless of initial concentration, the lactate concentration in the medium tended to change towards approximately 0.6 mM, showing that uptake equaled output at this level, with rates at about 40 mumol/g protein/h. With the steady-state concentration of 0.6 mM lactate, about 20% of the glucose carbon was shunted out into the medium before it was reabsorbed and metabolized into various products. Lactate uptakes by neuronal and nonneuronal cultures prepared from the ganglia did not differ consistently from one another or from uptake by undissociated ganglia. The neuronal cultures tended to oxidize a greater fraction of the consumed lactate to CO2 and to convert a smaller fraction of the lactate to products in the medium than did the nonneuronal cultures. Computer modeling, using known parameters for blood-brain transport of lactate in the adult rat and data on uptake by the ganglia, suggests that lactate may supply substantial fuel to the brain, even in the presence of abundant glucose, when the lactate concentration in the blood is raised to levels commonly observed in exercising humans, such as 10-20 mM. This is in agreement with the findings of several investigators in hypoglycemic humans and in animals with intermediate blood lactate concentrations.

In vivo Identification and Quantitative Evaluation of Carrier-Mediated Transport of Lactate at the Cellular Level in the Striatum of Conscious, Freely Moving Rats

“…The value found for the apparent K m of lactate transport is slightly smaller than that found for sympathetic ganglia in vitro (2.4 vs. 4.8 mM; Larrabee, 1983). This is very encouraging, since diffusional processes are likely only to impede the apparent kinetics (i.e., increase K m or decrease V max) .…”

“…The use of a kinetic model for reversible lactate transport across the cell membrane was prompted by the observation that lactate levels always re turned to a steady-state value of � 1.1 mM in ECF after electroconvulsive shock or chemical stimula tion (Kuhr and Korf, 1988b). Larrabee (1983) has demonstrated similar behavior for lactate transport in sympathetic ganglia in vitro. This transport was saturable and showed distinct characteristics of an equilibrium system-there was a consistent ap proach to the same steady-state level, regardless of whether initial extracellular levels were higher or lower than final steady-state values.…”

“…Recent studies suggest, however, that a monocarboxylic acid carrier exists at the cellular level in heart (Mann et aI., 1985) and in sympathetic ganglia (Larrabee, 1983). Carrier-mediated trans port at the cellular level would tend to stabilize concentrations at a steady-state level under resting conditions, but could result in severely restricted transport when membrane function is impaired (e.g., during ischemia).…”

Astrocytic Glycogen Influences Axon Function and Survival during Glucose Deprivation in Central White Matter

“…The model asserts that astrocytes contain glycogen, which is converted into lactate for transport to axons during glucose deprivation. Lactate is the most likely fuel to be transferred from astrocytes to axons for the following reasons: (1) astrocytes, but not neurons, are known to extrude large amounts of lactate (Walz and Mukerji, 1990); (2) astrocytes, but not neurons, can survive for a limited time on anaerobic metabolism alone (Goldberg and Choi, 1993;Pappas and Ransom, 1995;Ransom and Fern, 1996) and could "afford" to export large amounts of lactate; (3) lactate, but not glucose, is released from astrocytes when glucose is removed (Dringen et al, 1993); (4) lactate has been shown to be an effective fuel in numerous types of CNS tissue (Larrabee, 1983;Schurr et al, 1988;Izumi et al, 1994;Izumi et al, 1997), including RON axons (Wender et al, 1999); (5) lactate from Müller cells fuels photoreceptors in guinea pig retina (Poitry-Yamate et al, 1995). According to the model, axons import lactate for subsequent oxidative metabolism to generate ATP.…”

Lactate released by Muller glial cells is metabolized by photoreceptors from mammalian retina

“…Although glucose is the preferred substrate for brain and retina, there is accumulating evidence that lactate is an alternative substrate for energy metabolism in vertebrate nervous tissues (see, e.g., Larrabee 1983Larrabee , 1992Schurr et al, 1988). Recent evidence additionally points to the possibility of a cellular compartmentation of lactate metabolism and its trafficking from astroglia to neurones McKenna et al, 1993;Wiesinger et al, 1993;Pellerin and Magistretti, 1994).…”