EFFECTS OF LANTHANUM ON CONTRACTION, CALCIUM DISTRIBUTION AND Ca⁴⁵ MOVEMENTS IN INTESTINAL SMOOTH MUSCLE

¹ Department of Pharmacology, Medical College of Virginia, Richmond, Virginia

In the longitudinal smooth muscle layer isolated from guinea-pig ileum, 1.8 mM lanthanum ion (La⁺⁺⁺) inhibited both resting tone and contractions induced by either acetyicholine or 80 mM potassium ion. The equilibrated tissue space occupied by nonradioactive Ca⁺⁺ or by Ca⁴⁵ was reduced by La⁺⁺⁺. Efflux of Ca⁴⁵ was transiently increased by La⁺⁺⁺ only in muscles not previously exposed to Ca⁺⁺-containing solutions during washout. The actions of La⁺⁺⁺ upon muscle contraction and Ca⁴⁵ movements resembled those observed with high concentrations of Ca⁺⁺. Increasing the proportion of bound Ca⁴⁵ by incubating muscles in solutions containing Ca⁴⁵ but no added Ca⁺⁺ resulted in an increased inhibition of Ca⁴⁵ uptake by La⁺⁺⁺. Under these conditions, La⁺⁺⁺ blocked a Ca⁺⁺-induced shift of Ca⁴⁵ from the slow to the fast washout component. La⁺⁺⁺ also prevented a transient ethylenediamine tetraacetic acid-induced increase in Ca⁴⁵ efflux. Thus, La⁺⁺⁺ appears to replace Ca⁺⁺ at superficial binding sites, to decrease the mobility of Ca⁺⁺ located at other less superficial membrane sites and to prevent uptake of Ca⁴⁵ to various cellular sites. Ca⁺⁺ movements and Ca⁺⁺ binding sites important for muscle contraction apparently involve different but interdependent cellular compartments which can be altered either directly or indirectly by La⁺⁺⁺.

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