Use of La3+ to distinguish activity of the plasmalemmal Ca2+ pump from Na+/Ca2+ exchange in arterial myocytes

La3+ was tested for its ability to distinguish external Na+ (Na-0)-independent Ca2+ efflux via the plasma membrane (PM) Ca2+ pump from Na-0-dependent Ca2+ efflux via Na+/Ca2+ exchange. Fura-2 loaded cultured rat aortic myocytes were used with digital imaging to measure the cytosolic free Ca2+ concentration ([Ca2+](cyt)) and to monitor La3+ entry. At a La3+ concentration ([La3+](0)) of 0.25 mM, but not at lower concentrations, La3+ entered the cells; 0.01 mM verapamil blocked this entry. Transient increases in [Ca2+](cyt) were evoked by unloading the sarcoplasmic reticulum with cyclopiazonic acid (CPA) + caffeine (CAF) in Na,Ca-free medium (to inhibit Ca2+ extrusion via Na+/Ca2+ exchange and Ca2+ influx). La3+ (0.03-0.25 mM with verapamil) augmented the Ca2+ transients and slowed Na-0-independent [Ca2+](cyt) recovery in a dose-dependent manner (IC50 similar to 0.01 mM La3+). This La3+-sensitive recovery was apparently mediated by the PM Ca2+ pump. The effects of La3+ were reversible: [Ca2+](cyt) returned promptly toward base line when La3+ was washed out in Na,Ca-free medium containing CPA + CAF. Reintroduction of extracellular Na+ during [Ca2+](cyt) recovery ([La3+](0) = 0.06-0.25 mM) significantly speeded recovery, indicating that the Na+/Ca2+ exchanger was not inhibited by [La3+](0) less than or equal to 0.25 mM. The La3+-sensitive (Na-0-independent) and Na-0-dependent [Ca2+](cyt) recovery rates were comparable. In Na+-loaded cells, less than or equal to 0.25 mM La3+ also did not affect Na+/Ca2+ exchange mediated Ca2+ influx. In medium containing Na+ and Ca2+, 0.125 mM La3+ abolished the serotonin (5-HT) evoked plateau responses that resulted from Ca2+ entry via Ca2+ channels. In Na,Ca-free medium, but not Ca-free medium, however, La3+ converted 5-HT evoked Ca2+ transients into sustained responses. We conclude that low [La3+](0) (0.06-0.25 mM) inhibits the PM Ca2+ pump, but spares Na+Ca2+ exchanger mediated Ca2+ influx and efflux in arterial myocytes.