[Atrial specific granules are an intracellular calcium store]

Area of Science:

• Cardiology
• Cell Biology
• Biochemistry

Background:

• Atrial natriuretic peptide (ANP) secretion is potentially linked to calcium (Ca) release from atrial specific granules (ASG).
• Understanding Ca concentration within ASG and its maintenance mechanisms is crucial for elucidating ANP secretion pathways.

Purpose of the Study:

• To quantify Ca concentration within ASG.
• To investigate the role of Ca(2+)-ATPase in maintaining high Ca levels within ASG.
• To determine if ASG function as a Ca store in atrial cardiocytes.

Main Methods:

• Quantitative electron microscope X-ray microanalysis of ultrathin cryosections from rat auricles.
• Electron microscopic (EM) cytochemical technique for Ca(2+)-ATPase detection.
• Utilized a JEM-1200EX electron microscope with a Link AN 10,000 energy dispersive X-ray spectroscope.

Main Results:

• ASG exhibit a high Ca concentration (81 +/- 15 mmol/kg), comparable to sarcoplasmic reticulum.
• EM cytochemistry revealed Ca(2+)-ATPase activity localized to the ASG membrane.
• Evidence suggests Ca(2+)-ATPase actively pumps Ca2+ into ASG from the cytosol.

Conclusions:

• ASG are confirmed to contain highly concentrated Ca.
• The ASG membrane possesses Ca(2+)-ATPase activity, responsible for maintaining high intra-granular Ca.
• ASG function as a significant Ca store within atrial cardiocytes, potentially influencing ANP secretion.