Role of Na⁺‐H⁺ and Na⁺‐Ca²⁺ exchange in hypoxia‐related acute astrocyte death

Abstract

Cultured astrocytes do not succumb to hypoxia/zero glucose for up to 24 h, yet astrocyte death following injury can occur within 1 h. It was previously demonstrated that astrocyte loss can occur quickly when the gaseous and interstitial ionic changes of transient brain ischemia are simulated: After a 20-40-min exposure to hypoxic, acidic, ion-shifted Ringer (HAIR), most cells died within 30 min after return to normal saline (i.e., "reperfusion"). Astrocyte death required external Ca2+ and was blocked by KB-R7943, an inhibitor of reversed Na+-Ca2+ exchange, suggesting that injury was triggered by a rise in [Ca2+]i. In the present study, we confirmed the elevation of [Ca2+]i during reperfusion and studied the role of Na+-Ca2+ and Na+-H+ exchange in this process. Upon reperfusion, elevation of [Ca2+]i was detectable by Fura-2 and was blocked by KB-R7943. The low-affinity Ca2+ indicator Fura-FF indicated a mean [Ca2+]i rise to 4.8+/-0.4 microM. Loading astrocytes with Fura-2 provided significant protection from injury, presumably due to the high affinity of the dye for Ca2+. Injury was prevented by the Na+-H+ exchange inhibitors ethyl isopropyl amiloride or HOE-694, and the rise of [Ca2+]i at the onset of reperfusion was blocked by HOE-694. Acidic reperfusion media was also protective. These data are consistent with Na+ loading via Na+-H+ exchange, fostering reversal of Na+-Ca2+ exchange and cytotoxic elevation of [Ca2+]i. The results indicate that mechanisms involved in pH regulation may play a role in the fate of astrocytes following acute CNS injuries.

Astrocytic contributions to bioenergetics of cerebral ischemia

“…Inhibition of the Na þ /H þ exchanger at the onset of reperfusion is also protective (Bondarenko et al, 2004), perhaps because reduction of intracellular Na þ loading reduces Ca 2þ uptake. In contrast, nimodipine, an inhibitor of voltage-dependent L-channels for Ca 2þ , had no protective effect, whereas astrocytic cell death after oxygen deprivation and reoxygenation also could be reduced by pretreatment with a NOS inhibitor (Wang et al, 2002) to minimize NO-dependent inhibition of oxidative metabolism.…”

Ionic Transporter Activity in Astrocytes, Microglia, and Oligodendrocytes During Brain Ischemia

“…16,17 In addition, other transporters, including Na þ /Ca 2 þ exchanger (NCX) and Na In the brain, microglial cells may appear in resting and multiple activated states, including ameboid and phagocytic. Under physiological conditions, microglial cells are in the resting state, characterized by a small cell body with fine, highly branched processes.…”

Decreased Neuronal Death in Na+/H+Exchanger Isoform 1-Null Mice afterIn VitroandIn VivoIschemia

“…Our recent thermodynamic analysis revealed that reversemode operation of Na ϩ /Ca 2ϩ exchange can occur when [Na ϩ ] i increases Ͼ25 mM in astrocytes . Such a mechanism is also involved in astrocyte death after hypoxia (Bondarenko et al, 2005 (Reuter et al, 2002). Together, these data lead us to conclude that stimulation of NHE1 activity in cortical neurons causes intracellular Na ϩ overload and subsequently triggers Ca 2ϩ influx via the reverse mode of Na ϩ /Ca 2ϩ exchange.…”