Excitation-Contraction Coupling in Rabbit Aorta Studied by the Lanthanum Method for Measuring Cellular Calcium Influx

Abstract

Lanthanum inhibits 45 Ca efflux from internally injected squid axons. Evidence for La 3+ blockade of Ca 2+ fluxes across the cell membrane was also obtained for aortic smooth muscle: 1mM La 3+ completely inhibited contractions which otherwise resulted from adding Ca 2+ to calcium-free depolarizing solutions, and 2 mM La 3+ caused a 50% inhibition of the late 45 Ca efflux. Ca 2+ bound extracellularly could be displaced by La 3+ , and the binding sites preferred La 3+ over Ca 2+ . We could thus use La 3+ to eliminate extracellular bound Ca 2+ from our calcium-influx measurements as follows: after exposure of the aortic strips to experimental solutions labeled with 45 Ca, the extracellular Ca 2+ label was displaced by putting the strips in a calcium-free solution containing 2 mM La 3+ for 60 minutes. The loss of intracellular Ca 2+ was minimized during this hour by the La 3+ blockade of Ca 2+ membrane fluxes. Tissue weight, 45 Ca, and total Ca 2+ were then measured using standard techniques. Studies employing this new method showed that depolarization by high K + , Na + replacement, and high pH activate smooth muscle contraction by stimulating Ca 2+ influx. Low pH and La 3+ block Ca 2+ influx. Norepinephrine initiates aortic contractions by release of intracellularly sequestered Ca 2+ . Angiotensin and histamine appear to release Ca 2+ from this same fraction.