14CO2 fixation in incubated rat diaphragms

Area of Science:

• Biochemistry
• Physiology
• Metabolic Research

Background:

• Skeletal muscle utilizes various metabolic pathways for energy production.
• Understanding carbon dioxide (CO2) fixation and exchange is crucial for comprehending muscle metabolism.

Purpose of the Study:

• To investigate the time course of label incorporation from sodium bicarbonate (NaH14CO3) into citric acid cycle intermediates and amino acids in rat diaphragms.
• To quantify CO2 exchange and fixation rates mediated by specific enzymes.
• To elucidate the role of malic (iso)enzyme activities in skeletal muscle carbon metabolism.

Main Methods:

• Incubation of isolated rat diaphragms with NaH14CO3 to track label incorporation over 60 minutes.
• Estimation of CO2 exchange and fixation rates using enzymatic assays.
• Utilizing specific inhibitors like 2-cyano-4-hydroxycinnamate and hydroxymalonate to probe metabolic pathways.
• Analysis of cell-free muscle extracts to assess enzyme activity under specific assay conditions.

Main Results:

• Significant CO2 fixation occurs via pyruvate carboxylation and propionyl-CoA carboxylation in rat diaphragms.
• Isocitrate dehydrogenase accounts for substantial CO2 exchange.
• Approximately 90% of CO2-derived C4 compounds are subsequently decarboxylated.
• NADP-dependent malic enzymes are involved in CO2 fixation, as indicated by inhibition studies.
• Mitochondrial pyruvate transport inhibition did not affect NaH14CO3 incorporation, suggesting cytosolic fixation pathways.

Conclusions:

• Rat diaphragms exhibit continuous cytosolic replenishment and mitochondrial depletion of citric acid cycle intermediates.
• NADP-dependent malic enzymes play a significant role in CO2 fixation within skeletal muscle.
• These findings contribute to understanding the dynamic carbon flux in resting skeletal muscle.