Interactions between several rare earth ions and calcium ion in vascular smooth muscle

The effects of lutetium (Lu+++), europium (Eu+++) and neodymium (Nd+++) on 45Ca distribution, 45Ca movements and contractions were examined in rabbit aortic smooth muscle. Tension responses to norepinephrine were markedly inhibited by all three rare earth ions; K+-induced responses and those to histamine were partially blocked. Addition of 1.5 mM Lu+++, Eu+++ or Nd+++ to the bathing solution reduced 45Ca uptake (Nd+++ greater than Eu+++ greater than greater than Lu+++), produced a maintained increase in 45Ca efflux from strips of media-intimal layer washed out in a 0-Ca++ solution (Nd+++ greater than Eu+++ greater than Lu+++), and, when present during 45Ca incubation (before 45Ca washout), altered the release of 45Ca elicited by addition of 1.5 mM Ca++ during the 45Ca washout (inhibition by Nd+++ greater than Eu+++ greater than Lu+++). The 3-fold greater uptake of 45Ca in a 0-Ca++ and 1.5 mM Lu+++ solution than in an equivalent Sr++ solution results from a decreased effect of Lu+++ upon the slower 45Ca washout component. The actions of Lu+++ were quantitatively similar to those of low La+++ concentrations (0.05-0.10 mM), whereas those of Nd+++ and Eu+++ resembled effects obtained with equimolar concentrations of La+++. Thus, the three rare earth ions produce qualitatively similar alterations in 45Ca uptake and superficial binding. The difference observed can be attributed to variations in their affinity for superficial Ca++ binding sites and in their ability to block Ca++ uptake.