Kinetic properties of the redox switch/redox coupling mechanism as determined in primary cultures of cortical neurons and astrocytes from rat brain

We investigate the mechanisms underlying the redox switch/redox coupling hypothesis by characterizing the competitive consumption of glucose or lactate and the kinetics of pyruvate production in primary cultures of cortical neurons and astrocytes from rat brain. Glucose consumption was determined in neuronal cultures incubated in Krebs ringer bicarbonate buffer (KRB) containing 0.25–5 mM glucose, in the presence and absence of 5 mM lactate as an alternative substrate. Lactate consumption was measured in neuronal cultures incubated with 1–15 mM lactate, in the presence and absence of 1 mM glucose. In both cases, the alternative substrate increased the Km (mM) values for glucose consumption (from 2.2 ± 0.2 to 3.6 ± 0.1) or lactate consumption (from 7.8 ± 0.1 to 8.5 ± 0.1) without significant changes on the corresponding Vmax. This is consistent with a competitive inhibition between the simultaneous consumption of glucose and lactate. When cultures of neurons or astrocytes were incubated with increasing lactate concentrations 1–20 mM, pyruvate production was observed with Km (mM) and Vmax (nmol/mg/h) values of 1.0 ± 0.1 and 109 ± 4 in neurons, or 0.28 ± 0.1 and 342 ± 54 in astrocytes. Thus, astrocytes or neurons are able to return to the incubation medium as pyruvate, a significant part of the lactate consumed. Present results support the reversible exchange of reducing equivalents between neurons and astrocytes in the form of lactate or pyruvate. Monocarboxylate exchange is envisioned to operate under near equilibrium, with the transcellular flux directed thermodynamically toward the more oxidized intracellular redox environment.B.G.R. was a postdoctoral fellow of C.S.I.C. and I.R.V. received an Erasmus fellowship from the University of Coimbra. M.M.C.A.C. was on a sabbatical visit from the University of Coimbra (Portugal). T.B.R. and L.L.F. received pre-doctoral fellowships from the Portuguese FCT. M.L.G.M. holds a tenure track ‘‘Ramón y Cajal’’ contract from CSIC. S.C. receives a grant from Institute of Health Carlos III, as does M.L.G.M. Portuguese Fundaçäo para a Ciência e a Tecnologia provides funding for T.B.R. and L.L.F. Contract grant sponsor: Spanish Ministry of Education and Science; Contract grant numbers: SAF 2001-2245, SAF 2004-03197; Contract grant sponsor: Institute of Health Carlos III; Contract grant numbers: PI051530, PI051845; Contract grant sponsor: Portuguese Fundaçäo para a Ciência e a Tecnologia; Contract grant numbers: SFRH/BD/5407/2001, PRAXIS XXI/BD/21532/99.Peer Reviewe