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

Abstract

SUMMARY. 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. (Circ Res 54: 217-226, 1984)

Contracture and the calcium paradox in the rat heart.

“…It should be noted that Ca 2+ permeability of sarcolemmal fragments isolated from hearts subjected to the calcium paradox is not increased. 17 The present findings support our earlier hypothesis, 5 and those of Ganote et al, 7 Ashraf et al, 6 and Vander Heide et al 9 concerning the series of events leading to the final manifestation of damage associated with the Ca 2+ paradox (Table 1). Initially, on Ca 2+ repletion, cytosolic Ca 2+ increases, presumably because of a relatively small entry of Ca 2+ , probably through physiologic pathways.…”

The effect of hypothermia during the period of calcium repletion on the calcium paradox

“…The possible routes of the initial gain in calcium include the glyocalyx, the slow channels, the Na+-Ca 2 § exchange mechanism, passive diffusion, and abnormal sites of calcium entry (7). In addition to an increased influx of calcium, a loss in the ability of the sarcolemma to remove calcium from the cells may contribute to the net gain of calcium (11). The raised cytosolic calcium may trigger a number of events leading to damage of the intercalated discs and sarcolemma, and uncontrolled influx of calcium and massive release of enzymes and other intracellular proteins (12).…”

Polyamines mediate uncontrolled calcium entry and cell damage in rat heart in the calcium paradox.

“…Small amounts of polyamines synthesized by Ca2 -activated ODC within seconds after readmission of calcium would cause a rapid massive entry of extracellular Ca2 . In addition, an impaired capacity of calcium-depleted cells to regulate internal Ca2+ by sequestration in the sarcoplasmic reticulum (42,43) and extrusion across the sarcolemma (43,44) via their respective Ca2+ -ATPase pumps has been implicated in the phenomenon of Ca2+-intolerance, and polyamines could contribute to this mechanism. The resulting rise in free myoplasmic and intramitochondrial Ca2+ concentration would cause (a) a loss of contraction, contracture, and cellular ATP depletion resulting from Ca2+ activation ofmyosin ATPase, and (b) uncoupling of mitochondrial oxidative phosphorylation and consequent failure ofATP generation (6,45).…”