Calcium displacement by lanthanum in subcellular compartments of rat ventricular myocytes: characterisation by electron probe microanalysis

Objective: Much of the calcium in cardiac myocytes resides in a kinetic compartment defined by rapid exchange and rapid displacement by La3+. The aim of this study was to ascertain the subcellular location of this calcium pool. Methods: Electron probe microanalysis (EPMA) was used to measure redistribution of total calcium in rat cardiac myocytes after 30 s in 1 mM lanthanum. “Cells”, mitochondria, and myofibrils were separately analysed. The data permitted calculating a third (difference) compartment, containing primarily sarcoplasmic reticulum, sarcolemma, and T tubule lumen. Total calcium levels in junctional sarcoplasmic reticulum and T tubule lumen were also measured directly. Results: Lanthanum decreased total “cell” calcium from 1.4 (SEM 0.3) to 0.5(0.4) mmol·kg−1 dry weight (p