In vivo brain phosphocreatine and ATP regulation in mice fed a creatine analog

Abstract

Mitochondrial and cytosolic creatine kinase (CK) isozymes are active in cells with high and variable ATP metabolic rates. beta-Guanidinopropionic acid (GPA), a competitive inhibitor of creatine transport, was used to study the hypothesis that the creatine-CK-phosphocreatine (PCr) system is important in regulating brain ATP metabolism. The CK-catalyzed reaction rate and reactant concentrations were measured in vivo with 31P nuclear magnetic resonance spectroscopy during energy deficit (hypoxia) or high-energy turnover (seizures) states in urethane-anesthetized mice fed GPA, creatine, or standard chow (controls). Brain phosphagen (i.e., cellular energy reserves) or PCr plus phosphorylated GPA (GPAP) concentrations were equal. The phosphagen-to-NTP ratio was lower than in controls. In vivo CK reaction rate decreased fourfold, whereas ex vivo CK activity that was biochemically measured was doubled. During seizures, CK-catalyzed fluxes increased only in GPA-fed mice. Phosphagen increased in GPA-fed mice, whereas PCr decreased in controls. Survival was higher and brain phosphagen and ATP losses were less for hypoxic GPA-fed mice than for controls. In contrast to mice fed GPA, hypoxic survival and CK reactant concentrations during hypoxia and seizures were the same in creatine-fed mice and controls. Thus GPA, GPAP, or adaptive changes in ATP metabolism stabilize brain ATP and enhance survival during hypoxia in mice.

Brain ATP Metabolism in Hypoxia Resistant Mice Fed Guanidinopropionic Acid

“…The GPA-fed mice are resistant to hypoxic mortality and during seizures show increased brain phosphocreatine [3]. The hypothesis for this study was that altered mitochondrial respiration is the mechanism for resistance to hypoxia.…”

“…The duration of GPA feeding was sufficient to reach plateau brain concentrations, that is, approximately equal concentrations of phosphorylated GPA and phosphocreatine, decreased phosphagen:NTP, and a 65-70% decrease in the CK reaction rate [3,4]. The GPA-fed mice are resistant to hypoxic mortality and during seizures show increased brain phosphocreatine [3].…”

“…Feeding this compound to mice markedly decreases mortality and stabilizes brain ATP during hypoxia [3]. Compared to creatine, phosphorylated GPA is a poor substrate for CK with lower V max and higher K m [1].…”

“…These include increases in total CK, adenylate kinase (ADK, EC 2.7.4.3), and succinic dehydrogenase. In contrast to heart and skeletal muscle, GPA feeding resulted, however, in depletion of only half the brain phosphocreatine, while the total phosphagen:nucleoside triphosphate (NTP) ratio decreases, as seen in vivo by 31 P magnetic resonance spectroscopy [3,4]. The in vivo CK-catalysed reaction rate for exchange of phosphoryl groups from phosphocreatine to ATP decreases by 70-80%.…”

Cerebral 31P Magnetic Resonance Spectroscopy and Systemic Acid-Base Balance during Hypoxia in Fetal Sheep

“…Compartmentation of PCr in brain has been suggested by Holtzman et al (29); in the brain of mice, only half of the PCr content could be substituted by creatine analogs, in contrast to skeletal muscle. One PCr compartment is stable, whereas the other compartment is used during hypoxia (30). At the point when cerebral damage is expected to be present (arterial pH Ͻ 7.00) (5), the NTP/P tot and pH i were decreased compared with control values.…”