Kinetics of changes in PO2 and extracellular potassium activity in stimulated rat sympathetic ganglia

Lactate accumulation following concussive brain injury: the role of ionic fluxes induced by excitatory amino acids

During the first few minutes following traumatic brain injury, cells are exposed to an indiscriminate release of glutamate from nerve terminals resulting in a massive ionic flux (e.g., K+ efflux) via stimulation of excitatory amino acid (EAA)-coupled ion channels. The present study was undertaken to elucidate the causal relationship between these ionic shifts and lactate accumulation in the injured brain, by examining the effects of ouabain (an inhibitor of Na+/K+-ATPase), Ba2+ (an inhibitor or non-energy-dependent glial K+ uptake) and kynurenic acid (KYN; a broad-spectrum EAA antagonist) on lactate accumulation. Two microdialysis probes were placed bilaterally in the rat parietal cortex. One was perfused with a test drug (1.0 mM ouabain, 2.0 mM Ba2+ or 10 mM KYN) and the other with Ringer's solution (control) for 30 min prior to injury. Following a 2.2-2.7 atm fluid-percussion injury, lactate levels in the dialysate increased (up to 116.6% above baseline) for the first 16 min and returned to baseline levels within 20 min after injury. This lactate accumulation was attenuated by preinjury administration of ouabain and KYN and was prolonged by Ba2+ administration. These findings indicate that lactate accumulations following concussive brain injury is a result of increased glycolysis which supports ion-pumping mechanisms, thereby, restoring the ionic balance which was disrupted by stimulation of EAA-coupled ion channels.

Administration of excitatory amino acid antagonists via microdialysis attenuates the increase in glucose utilization seen following concussive brain injury

Immediately following concussive brain injury, cells exhibit an increase of energy demand represented by the activation of glucose utilization. We have proposed that this trauma-induced hypermetabolism reflects the effort of cells to restore normal ionic balance disrupted by massive ionic fluxes through transmitter-gated ion channels. In the present study, changes in local CMRglc following fluid-percussion concussive injury were determined using [14C]2-deoxy-D-glucose autoradiography, and the effects of in situ administration (via microdialysis) of excitatory amino acid (EAA) antagonists [kynurenic acid (KYN), 2-amino-5-phosphonovaleric acid (APV; 100 microM, 1 mM, and 10 mM), and 6-cyano-7-nitroquinoxaline-2,3-dine (CNQX; 300 microM, 1 mM, and 10 mM] on glucose utilization were investigated. Animals that did not receive dialysis showed a remarkable increase (up to 181% of normal control) in cortical glucose utilization following injury. In contrast, this high demand for glucose was reduced in areas infiltrated with KYN, APV, and CNQX. These results indicate that EAA-activated ion channels are involved in the posttraumatic increase in glucose utilization, reflecting the energy demand of cells required to drive pumping mechanisms against an ionic perturbation seen immediately following the concussive injury. The effects of KYN, APV, and CNQX suggest that although all subtypes of the glutamate receptor appear to be involved in this phenomenon, N-methyl-D-aspartate-activated channels may play a major role.

Intracellular free sodium and potassium, post-carbachol hyperpolarization, and extracellular potassium-undershoot in rat sympathetic neurones

Double-barrelled ion-sensitive microelectrodes were used to record the free intracellular Na+- and K+-concentrations [( Na+]i, [K+]i) and to determine their relation to changes in membrane potential and extracellular K+ [( K+]e) in rat sympathetic ganglia. The application of 50 mumol/l carbachol resulted in an elevation of [K+]e followed by a post-carbachol [K+]e-undershoot. The membrane depolarization of the sympathetic neurones was associated with an increase in [Na+]i and a decrease in [K+]i. A membrane hyperpolarization and a recovery of [K+]i and [Na+]i to their baseline levels were observed during the [K+]e-undershoot. The time course of the [K+]e-undershoot correlated exactly with the duration of the rise in [Na+]i and decrease of [K+]i. No K+-reuptake occurred in the presence of ouabain. These data confirm, by direct measurements of intracellular ion concentration changes, the contribution of the Na+,K+-pump to the post-carbachol membrane hyperpolarization and [K+]e-undershoot.

Extracellular K+ concentration during electrical stimulation of rat isolated sympathetic ganglia, vagus and optic nerves

Recordings of extracellular potassium concentration ( [K+]e) were made in rat isolated sympathetic ganglia, vagus and optic nerves using ion sensitive microelectrodes. Repetitive orthodromic stimulation of ganglia resulted in [K+]e increases of up to 7 mmol/l above resting level (6 mmol/l), which were followed by post-stimulus undershoots. Activation of vagal A and B fibres did not significantly alter [K+]e but C-fibre activity induced rises of up to 5 mmol/l. Repetitive stimulation of the predominantly myelinated optic nerve resulted in [K+]e rises of up to 2.5 mmol/l. In the ganglion and vagus nerve, application of ouabain (30-1000 mumol/l) led to a raised baseline [K+]e concentration, an increase in the peak achieved during stimulation and a reduced undershoot amplitude. The amplitude of the undershoot in normal solution was shown to be dependent on the duration of the preceding stimulation period as well as the amplitude of the preceding [K+]e rise. In ganglia and vagus nerves, bath application of gamma-aminobutyric acid (10-100 mumol/l) and carbachol (10-100 mumol/l) also elevated [K+]e. It is concluded that repetitive activity in rat peripheral and central nerve fibres leads to significant changes in extracellular K+ ion-concentration and that the restoration of these levels is strongly dependent on the intact activity of the membrane Na+/K+ pump.