ON THE SITE OF ACTION OF LANTHANUM IN FROG SARTORIUS MUSCLE

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

Experiments were performed to elucidate the mechanism by which lanthanum ion (La⁺⁺⁺) acts to alter Ca⁴⁵ movements and tension responses in frog sartorius muscle. La⁺⁺⁺ (1.0 mM) decreased Ca⁴⁵ uptake but did not appreciably change total Ca⁺⁺, Na⁺ or K⁺ contents. The increase in residual Ca⁴⁵ space (after a 120-minute washout) usually induced by 80 mM K⁺ was prevented by La⁺⁺⁺, but an increase elicited with caffeine (5.0 mM) was not blocked. Similarly, tension responses to K⁺ were inhibited by La⁺⁺⁺ but responses to caffeine were not. The height and duration of the tension response to 80 mM K⁺ in a nitrate Ringer's solution was altered by La⁺⁺⁺ in a manner analogous to that seen when the K⁺ concentration was lowered to 32 mM. A transient increase in Ca⁴⁵ efflux was elicited by La⁺⁺⁺ only when the washout solution contained no nonradioactive Ca⁺⁺. These observations indicate that La⁺⁺⁺ acts at some superficial sites located in the sartorius muscle surface and transverse tubule membranes to displace Ca⁺⁺ and to prevent tension responses and ionic movements associated with these membrane sites. Failure of La⁺⁺⁺ to inhibit either increases in residual Ca⁴⁵ space or contractures produced by caffeine indicates that these effects of caffeine are brought about by mechanisms different from those involved in the similar effects of potassium ion.

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