Effects of lanthanum on the heart sarcolemmal ATPase and calcium binding activities

Abstract

The effects of La on Ca2+-ATPase, Mg2+-ATPase, Na+-K+-ATPase, and Ca-binding activities were studied in rat heart sarcolemma. La of 10-100 .mu.M depressed significantly Ca2+-ATPase activity and 50-200 .mu.M lanthanum inhibited Ca binding activity. Lineweaver-Burk plots of Ca2+-ATPase activity showed that inhibition by La was competitive with Ca concentration. Mg2+-ATPase and Na+-K+-ATPase activities were not affected by La when the assay medium contained 1 mM EDTA. In the absence of EDTA, these enzyme activities were significantly decreased by 10-100 .mu.M La. Rat hearts perfused with HEPES [N-2-hydroxy-ethyl piperazine-N''-2-ethane sulfonic acid] buffer containing 0.5 mM La showed electron-dense deposits restricted to the outer cell surface and the sarcolemma obtained from these hearts also had the deposits, indicating that the membrane fraction isolated by the hypotonic shock - LiBr treatment method was of sarcolemmal origin. The Ca2+-ATPase activity of the sarcolemma isolated from La-perfused hearts, unlike the Mg2+-ATPase, Na+-K+-ATPase and Ca binding activities, was significantly less than the control value. La may influence Ca movement across the sarcolemma by affecting sarcolemmal ATPase and Ca binding activities.