Closure of a rapidly exchanging calcium compartment in rat cardiac myocytes by lanthanum

Enzymatically isolated myocytes from adult rats were used to measure 45Ca-uptake from salt media of reduced Ca2(+)-content (0.1 mM) with normal (4 mM) or elevated (20 mM) potassium concentrations. Ca2(+)-uptake was interrupted by filtration followed by rapid chasing the filter with salt solutions containing no Ca2+, 2 mM Ca2+ or 2 mM La3+. Rate and extent of 45Ca-uptake of resting cells were found to be 3-fold enhanced when chasing was performed with La3(+)-containing media. In contrast La3+ does not affect Ca2(+)-exchange of depolarized cells, the fluxes of which approximate rates sufficiently high for contraction activation and that are sensitive to Ca2(+)-channel blockers. The effect of La3+ on resting cells suggests the existence of a small rapidly exchanging La3(+)-sensitive Ca2(+)-compartment located adjacent to the plasma membrane. This compartment is though to be either closed or has become La3(+)-insensitive in activated cells. The subsarcolemmal cysternae of the SR, the so-called "peripheral couplings" are most likely the morphological substrate of this compartment.