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Control of mitochondrial content of adenine nucleotides by submicromolar calcium concentrations and its relationship
Abstract:
Rat liver mitochondria were incubated at 30 degrees C with 4 mM ATP in a medium similar in electrolyte composition to that of hepatic cytosol. Under these conditions, a net increase in mitochondrial adenine nucleotides was observed that was dependent on the concentration of free Ca2+ [( Ca2+]) in the incubation medium. At 0.2 microM [Ca2+] or less, there was no demonstrable uptake of adenine nucleotides; at 0.4 microM [Ca2+], or greater, net uptake occurred. The calcium-dependent accumulation of nucleotides by mitochondria required Mg2+ in the incubation medium and was insensitive to carboxyatractyloside. The uptake of adenine nucleotides was enhanced by the addition of antimycin A or antimycin A together with oligomycin. Accumulation of nucleotides appeared to be associated with a small increase in mean mitochondrial volume, but the membrane potential was not affected. No uptake or loss of NAD-NADH by mitochondria was detected. Ruthenium red failed to inhibit the calcium-dependent uptake of adenine nucleotides by the mitochondria, indicating that stimulation of this process by Ca2+ does not involve transport of the cation into mitochondria by the Ca2+ uniporter. Because glucagon acts to elevate cytosolic [Ca2+] from approximately 0.2 microM to 0.6 microM, the same range affecting nucleotide uptake, it is proposed that the increase in mitochondrial adenine nucleotides that follows treatment with glucagon is mediated by the rise in cytosolic [Ca2+] produced by the hormone. This hypothesis was supported by the observation that epinephrine and A23187, agents that raise cytosolic [Ca2+], increased the content of mitochondrial adenine nucleotides in isolated hepatocytes. Furthermore, cells, incubated under calcium-depleting conditions, had a diminished response to glucagon.
Insights
Calcium ions (Ca2+) stimulate the uptake of adenine nucleotides into rat liver mitochondria, a process crucial for cellular energy regulation. This calcium-dependent mechanism is vital for understanding how hormones like glucagon influence mitochondrial function.
Area of Science:
- Mitochondrial Physiology
- Cellular Metabolism
- Calcium Signaling
Background:
- Mitochondria play a critical role in cellular energy production and adenine nucleotide homeostasis.
- The regulation of mitochondrial adenine nucleotide content is essential for cellular function and response to hormonal signals.
- Cytosolic calcium levels (Ca2+) fluctuate in response to hormones and can impact mitochondrial activity.
Purpose of the Study:
- To investigate the effect of calcium ions (Ca2+) on the uptake of adenine nucleotides into rat liver mitochondria.
- To elucidate the mechanism and physiological relevance of calcium-mediated adenine nucleotide transport.
- To determine if hormonal regulation of mitochondrial adenine nucleotides is mediated by changes in cytosolic Ca2+.
Main Methods:
- Incubation of isolated rat liver mitochondria with ATP and varying concentrations of Ca2+ and Mg2+.
- Assessment of adenine nucleotide content, mitochondrial volume, and membrane potential.
- Use of inhibitors like carboxyatractyloside, antimycin A, oligomycin, and ruthenium red to probe transport mechanisms.
- Experiments with isolated hepatocytes treated with hormones (glucagon, epinephrine) and ionophores (A23187).
Main Results:
- Net increase in mitochondrial adenine nucleotides was observed at Ca2+ concentrations of 0.4 microM and above.
- Calcium-dependent nucleotide uptake required Mg2+ and was independent of the carboxyatractyloside-sensitive transporter.
- Uptake was enhanced by antimycin A and occurred without changes in membrane potential or significant alterations in mitochondrial volume.
- Ruthenium red did not inhibit Ca2+-dependent nucleotide uptake, suggesting the Ca2+ uniporter is not involved.
- Hormones and ionophores that increase cytosolic Ca2+ also increased mitochondrial adenine nucleotide content in hepatocytes.
Conclusions:
- Cytosolic Ca2+ directly stimulates the accumulation of adenine nucleotides in mitochondria.
- This Ca2+-dependent mechanism provides a pathway for hormones like glucagon to rapidly modulate mitochondrial adenine nucleotide content.
- The findings highlight a novel link between calcium signaling and mitochondrial energy metabolism regulation.