Biochemical properties of membranes isolated from calcium-depleted rabbit hearts.

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Abstract
The purpose of this study was to define the biochemical properties of sarcolemma from the calcium-depleted rabbit heart. Calcium repletion after calcium-free perfusion results in irreversible damage to the heart (calcium paradox). No difference was found in specific activity of the Na+ -Ca++ antiporter in a crude preparation of sarcolemmal vesicles that was isolated from calcium-depleted hearts, compared with control perfused hearts. Likewise, the passive calcium efflux from sarcolemmal vesicles, preloaded with calcium via the Na+ -Ca++ antiporter, showed rates that were identical with control values. This indicates that the sarcolemma calcium permeability is not affected by calcium-free perfusion of the heart. Na+,K+ -ATPase activity in sarcolemma isolated from calcium-depleted hearts was reduced by 75% (P less than 0.005) compared with the control activity. Sarcolemmal phosphoproteins, whether produced by endogenous cyclic AMP- or calcium-calmodulin-dependent protein kinase, were not altered by calcium-free perfusion of the heart. The content of an important calcium-binding site in the myocardial cell, the sialic acid residues, was also estimated. Only a long period (60 minutes) of calcium-free perfusion resulted in a significant decrease (by 68%, P less than 0.025) of sialic acid content in the homogenate but not in the sarcolemma preparation. In hearts that were reperfused for 15 minutes with a normal calcium concentration (1.3 mM), sarcolemmal Na+,K+ -ATPase remained depressed and calcium permeability was still unchanged. It is possible that the sarcolemma isolation method selected a distinct part of the sarcolemma from the calcium-depleted and repleted heart that had no modified glycocalyx and permeability barriers to calcium ions, and that another part of the sarcolemma with altered properties was lost during the isolation procedure. Another possibility is that reconstitution processes during isolation affected membrane permeability properties. The results of the Na+,K+ -ATPase measurements provide evidence that the net calcium gain of the cells after calcium repletion may be associated, in part, with a loss in ability of the sarcolemma to remove calcium from the cytosol.