The effects of barbiturates on substrate inhibition of Na, K-ATPase activity

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  • Na, K-ATPase活性の基質阻害に対するバルビツール酸系薬物の作用

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Abstract

Reaction mechanism of barbiturates has widely been explored but not as much for GABAA receptors. Na, K-ATPase is the enzyme responsible for the maintenance of neuronal excitability and may be related to the reaction of barbiturates. As there are few reports about the effect of barbiturates on Na, K-ATPase activity, we studied the effects of thiamylal, pentobarbital, and phenobarbital on Na, K-ATPase activity using extracts of rat and rabbit brains. Na, K-ATPase activity increased depending on the ATP concentration and showed the maximum activity at 2.5 mM, but it decreased at 5 mM ATP by substrate inhibition. Pentobarbital and phenobarbital reduced this decrease of activity by substrate inhibition depending on their concentrations at 5 mM ATP, but not at 2.5 mM. This suggests that pentobarbital and phenobarbital suppresses the substrate inhibition of Na, K-ATPase activity by ATP. Thiamylal simply decreased Na, K-ATPase activity depending on its concentration at both 5 and 2.5 mM ATP without any suppression of substrate inhibition. Pentobarbital and phenobarbital increased the affinity of Na, K-ATPase for Na+ and decreased for K+ at 5 mM ATP. These results suggest that under the condition of substrate inhibition by ATP, pentobarbital and phenobarbital increases Na, K-ATPase activity by changing the conformational state to E1 conformation and increased in the affinity for ATP.

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