Effect of lanthanum and reduced temperature on ⁴⁵Ca efflux from rabbit aorta

The ability of lanthanum (La3+) to block calcium efflux from smooth muscle cells of the rabbit aorta has been examined. La3+ promotes the very early phase of 45Ca efflux, which is extracellular in origin, and partially inhibits the latter, cellular portion. Stimulation of 45Ca efflux caused by the release of intracellular 45Ca with either 10-4 M dinitrophenol or 10 mM caffeine was not reduced by pretreatment with 10 mM La for 40 min, whereas stimulation due to norepinephrine was abolished. It was concluded that during the use of the 'La method' for measuring cellular 45Ca there is an underestimation due to unblocked 45Ca loss. This loss can be reduced by processing tissue at 2°C, which inhibits transport processes. The time course of 45Ca uptake and the stimulation of uptake by high K+ are qualitatively but not quantitatively similar if tissues are washed at 37 and 2°C. Tissues washed in La3+ at 2°C for 60 min retain approximately double the cellular 45Ca of those washed at 37°C. This methodology provides an improved correlation between estimates of cellular calcium derived from 45Ca uptake and 45Ca efflux experiments.