Lanthanum Inhibition of ⁴⁵Ca Efflux from the Squid Giant Axon

Abstract

LETTVIN et al.¹ predicted that lanthanum ions, because of their relatively small size and high charge density, should have a much greater electrostatic attraction for any negative calcium binding site than calcium ions themselves. This was borne out for the membranes of the barnacle muscle fibres² and the lobster axon³. If negatively charged sites located in the membrane also serve to transport calcium ions across it, then lanthanum ions would be expected to displace calcium from these sites, and thereby inhibit the calcium ion flux. It has been shown that this is true for an artificial membrane with negative groups of phospholipids lining aqueous pores⁴. Data obtained by Weiss and Goodman⁵ and Van Breemen⁶ indicate that lanthanum ions also block the calcium transport across smooth muscle membranes. In smooth muscle physiology there is much uncertainty about the relative contribution of calcium influx to the initiation of contraction, and a calcium transport blocking agent would be very useful. In addition, information regarding the nature of the lanthanum blockade should contribute to the understanding of the cell membrane calcium transport system itself.