Paradoxical electromechanical effect of lanthanum ions in cardiac muscle cells.
Although lanthanum ions (La+++) block calcium influx in cardiac cells, they may paradoxically accentuate the sodium-free contracture. We have therefore studied the effects of La+++ on the zero sodium response in chick embryonic myocardial cell aggregates. Zero sodium alone causes: (a) A maintained contracture; (b) Asynchronous localized contractions that are selectively inhibited by caffeine or ryanodine, and presumably reflect release of calcium from the sarcoplasmic reticulum; (c) A nonspecific conductance increase that is ascribable to calcium-activated ion channels. Addition of La+++ potentiates the sodium-free contracture, and causes similar potentiation of the localized contractions and the conductance increase. All three phenomena occur 5-10-fold faster in 1 mM La+++ than in sodium-free fluid alone. In contrast, when La+++ is combined with caffeine or ryanodine, the zero sodium response is suppressed. We conclude that the paradoxical effect of La+++ on the contracture is not due to calcium influx, but to enhancement, or disinhibition of intracellular calcium release. Relaxation of normal myocardium may involve control of spontaneous calcium release by lanthanum- and sodium-sensitive calcium transport across the surface membrane.
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002642:Chick Embryo; D004553:Electric Conductivity / Q000187:drug effects; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D007811:Lanthanum / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects; D009200:Myocardial Contraction / Q000187:drug effects*; D009206:Myocardium / Q000378:metabolism; D016276:Ventricular Function
D007811:Lanthanum; D002118:Calcium