Sodium-dependent calcium release from vascular smooth muscle mitochondria

Area of Science:

• Mitochondrial physiology
• Vascular smooth muscle biology
• Ion transport mechanisms

Background:

• Sarcoplasmic reticulum calcium stores were thought to dominate cytoplasmic calcium regulation.
• Recent findings showed significant increases in vascular smooth muscle (VSM) cytoplasmic sodium (Na+) after sodium pump inhibition.
• This prompted investigation into VSM mitochondrial responses to altered Na+ levels.

Purpose of the Study:

• To determine if VSM mitochondria exhibit Na+-dependent Ca2+ release.
• To assess if physiological changes in cytoplasmic Na+ are sufficient to trigger Ca2+ release from VSM mitochondria.
• To compare VSM mitochondrial Ca2+ handling with that of cardiac mitochondria.

Main Methods:

• Isolation of mitochondria from bovine aorta and mesenteric arteries.
• Measurement of Ca2+ using arsenazo III indicator.
• Exposure of mitochondria to varying concentrations of Na+ and Li+.
• Assessment of the effects of calcium antagonists (diltiazem, nifedipine, verapamil) on Na+-dependent Ca2+ release.

Main Results:

• VSM mitochondria accumulated added Ca2+ and released it upon exposure to Na+ in a concentration-dependent manner.
• Significant Ca2+ release occurred between 20-40 mM Na+, reaching equilibrium within 20 minutes.
• Na+-dependent Ca2+ release was inhibited by diltiazem and, to a lesser extent, nifedipine, but not verapamil.
• Lithium (Li+) caused minimal Ca2+ release compared to Na+.
• VSM mitochondrial responses showed similarities to cardiac mitochondria but with greater Ca2+ release and less Li+ influence.

Conclusions:

• VSM mitochondria release significant amounts of Ca2+ in response to physiologically relevant increases in Na+ concentration.
• This Na+-dependent Ca2+ release mechanism in VSM mitochondria may influence intramitochondrial Ca2+ levels.
• These findings suggest a potential role for VSM mitochondrial Ca2+ release in contractile responses following sodium pump inhibition.