Atrial-specific granules in situ have high calcium content, are acidic, and maintain anion gradients.
The composition and pH of atrial-specific granules of rat heart were determined by electron probe x-ray microanalysis and fluorescence microscopy, respectively. The high (75 mmol/kg of dry weight) calcium content and higher than cytoplasmic concentration of chloride in atrial-specific granules were visualized in high-resolution x-ray maps. The Cl- content of granules and cytoplasm decreased and a bromide gradient (granule greater than cytoplasm) was established during incubation in low-chloride, NaBr-containing solutions. Scanning confocal fluorescence light microscopy of live atria incubated with acridine orange demonstrated dye accumulation, indicative of low intragranular pH. We conclude that the granules represent a hitherto unrecognized intracellular store of cardiac calcium and can develop and maintain an anion gradient, presumably through cotransport by means of a proton-pumping ATPase.
Somlyo
A V
AV
Pennsylvania Muscle Institute, University of Pennsylvania School of Medicine, Philadelphia 19104-6083.
Broderick
R
R
Shuman
H
H
Buhle
E L
EL
Somlyo
A P
AP
D000818:Animals; D002118:Calcium / Q000032:analysis*; D002712:Chlorides / Q000032:analysis; D003594:Cytoplasmic Granules / Q000032:analysis*; D006325:Heart Atria; D006863:Hydrogen-Ion Concentration; D008297:Male; D009206:Myocardium / Q000032:analysis* / Q000648:ultrastructure; D051381:Rats; D011919:Rats, Inbred Strains
D002712:Chlorides; D002118:Calcium