Bickler P E
Department of Anesthesia, University of California Medical Center, San Francisco, California 94143, USA.
Am J Physiol. 1998 Jul;275(1):R86-91. doi: 10.1152/ajpregu.1998.275.1.R86.
Survival of brain anoxia during months of winter dormancy by the Western painted turtle, Chrysemys picta, may rely on inactivation of neuronal ion channels. During 2 h of anoxia, Ca2+ influx via the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor decreases 30-40%, but it is not known if prolonged anoxic dormancy is associated with even more profound downregulation of this important channel. Because ionized Ca2+ in cerebrospinal fluid (CSF) increases five- to sixfold during prolonged anoxia, the potential for uncontrolled Ca2+ influx and neurotoxicity is increased. To study the regulation of NMDA receptor activity, we measured NMDA-mediated changes in intracellular Ca2+ (NMDA-DeltaCa2+) in turtle cerebrocortical sheets with fura 2. Turtles were kept in N2-bubbled aquariums for 2 h to 6 wk at 2-3 degrees C. NMDA-DeltaCa2+ decreased 60 +/- 14% (P < 0.05) after 2 h of anoxia and did not decrease further for 6 wk. Intracellular Ca2+ increased from 135 to 183 nM (P < 0.05) after 3 wk of anoxia and thereafter returned toward preanoxic levels. When NMDA receptor activity was assessed in artificial CSF containing the ions found in anoxic brain CSF (pH 7. 25, 69 mM lactate, 8.4 mM Ca2+, and 5.1 mM Mg2+), NMDA-DeltaCa2+ was twice control initially but was 21% less than in normoxic artificial CSF after the end of 6 wk, suggesting altered sensitivity of the NMDA receptor to ionized Ca2+ during prolonged anoxia. Regulation of NMDA receptor activity in turtle cerebrocortex during 6 wk of anoxia thus results in depression of NMDA receptor Ca2+ flux, despite a sixfold increase in ionized extracellular Ca2+.
西部锦龟（Chrysemys picta）在冬季数月的冬眠期间脑缺氧的存活可能依赖于神经元离子通道的失活。在缺氧2小时期间，通过谷氨酸受体的N-甲基-D-天冬氨酸（NMDA）亚型的Ca2+内流减少30 - 40%，但尚不清楚长时间的缺氧休眠是否与该重要通道更显著的下调有关。由于在长时间缺氧期间脑脊液（CSF）中的游离Ca2+增加五到六倍，不受控制的Ca2+内流和神经毒性的可能性增加。为了研究NMDA受体活性的调节，我们用fura 2测量了龟脑皮质片中NMDA介导的细胞内Ca2+变化（NMDA-ΔCa2+）。将龟置于2 - 3摄氏度的充氮水族箱中2小时至6周。缺氧2小时后，NMDA-ΔCa2+降低了60±14%（P < 0.05），并且在6周内没有进一步降低。缺氧3周后，细胞内Ca2+从135 nM增加到183 nM（P < 0.05），此后又恢复到缺氧前水平。当在含有缺氧脑CSF中发现的离子（pH 7.25、69 mM乳酸、8.4 mM Ca2+和5.1 mM Mg2+）的人工CSF中评估NMDA受体活性时，最初NMDA-ΔCa2+是对照的两倍，但在6周结束时比正常氧人工CSF低21%，这表明在长时间缺氧期间NMDA受体对游离Ca2+的敏感性发生了改变。因此，在缺氧6周期间，龟脑皮质中NMDA受体活性的调节导致NMDA受体Ca2+通量降低，尽管细胞外游离Ca2+增加了六倍。
