Biochemical Properties of Membranes Isolated from Calcium-Depleted Rabbit Hearts

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