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 < 0.05); loss from the third (difference) compartment was primarily responsible. Simultaneously, junctional sarcoplasmic reticulum increased from 5.1(0.6) to 8.2(0.9) mmol.kg-1 dry weight (p < 0.05), while the T tubule lumen was unchanged.

CONCLUSIONS

These results suggest that lanthanum displaced calcium primarily from sarcolemmal sites and that calcium redistributed to sarcoplasmic reticulum from an intracellular source.