Calcium Movement in Vascular Smooth Muscle and its Detection Using Lanthanum as a Tool

Abstract

Lanthanum (La³⁺) (2 mM).initially increased efflux of ⁴⁵Ca from rings of rabbit aorta, but also caused an increased retention of ⁴⁵Ca after prolonged washing of the tissue in La³⁺-containing solution for 60 min. La³⁺ was also found to inhibit the uptake of ⁴⁵Ca into this preparation. Exposure to high potassium for 60 min during uptake of ⁴⁵Ca increased the amount of tracer retained after the La³⁺ wash, but the extra amount retained was in excess of that required to activate contraction, and much more than the extra retention after exposure to high potassium for 5 min sufficient to produce maximal tension. Exposure to noradrenaline during ⁴⁵Ca uptake produced a smaller extra retention of isotope following La³⁺ washing. In the presence of high potassium, exposure to noradrenaline had a greater effect on the extra retention of ⁴⁵Ca.The potassium-induced contracture of rabbit aortic rings was prevented and/or relaxed by La³⁺ (2 mM); and while the sustained response to noradrenaline was similarly affected, the initial phasic response to this agent could still be elicited in the presence of La³⁺.These results suggest that La³⁺ can displace Ca²⁺ from extracellular sites and inhibit influx of extracellular Ca²⁺ and part of the efflux of intracellular Ca²⁺. The potassium-induced contractures and the sustained response to noradrenaline involve transmembrane Ca²⁺ movements, while the phasic response to noradrenaline does not. However, results of ⁴⁵Ca uptake studies cannot be interpreted as showing that La³⁺ seals the membrane to Ca²⁺, or that extra Ca²⁺ retained by La³⁺ is related to contraction.