Synaptosomal Calcium Influx in the Cortex of the Hypoxic Guinea Pig Fetus. 258

Background: Hypoxia modifies the N-Methyl-D-Aspartate (NMDA) receptor ion-channel in the brain of the guinea pig fetus and newborn piglet. Hypothesis: Agonist stimulation of the NMDA receptor with glycine, NMDA and glutamate will result in a higher increase of intrasynaptosomal free calcium concentration ([Ca²⁺]i) in hypoxia as compared to normoxia. Subjects: 58 days guinea pig fetuses: normoxic (N,n=6) and hypoxic(H,n=6). Method: Tissue hypoxia was documented by measuring ATP and phosphocreatine levels. Cortical synaptosomes were prepared from fetal brain tissue. [Ca²⁺]i was measured fluorometrically, with the dye Fura-2, after the addition of glycine, NMDA and glutamate (100μM each).Results: The [Ca²⁺]i at baseline was 305 ± 132 nM (N) and 481 ± 100 nM (H). Addition of glycine, NMDA and glutamate raised the [Ca²⁺]i to 387 ± 142 nM (N) and 630 ± 84 nM (H); 448± 139 nM (N) and 729 ± 98 nM (H); 493 ± 138 nM (N) and 815 ± 132 nM (H), respectively. [Ca²⁺]i at baseline was significantly higher in H vs N (p<0.05). Similarly, following addition of the agonist the [Ca²⁺]i, was significantly higher in H vs N(p<0.01 for each agonist). Furthermore, the agonist dependent increase in [Ca²⁺]i was significantly higher in N vs H(p<0.05 for each agonist). Conclusions: Hypoxia-induced modification of the NMDA receptor results in a higher increase in[Ca²⁺]i, presumably leading to Ca²⁺-mediated activation of both phospholipase A2 and nitric oxide synthase, resulting in free radical-induced neuronal injury. (Funded by NIH-HD-20337).