Functional alterations of the sarcolemma in Ca2+-free perfused hearts.

The preservation of sarcolemmal Ca2+, Mg2+-ATPase activity was shown to be dependent on a critical Ca++ concentration (10(-5) - 2 X 10(-5) M) present during both perfusion of the heart muscle and isolation of the plasma membrane. The Ca transport activity of sarcolemmal vesicles isolated from Ca-deprived myocardium decreased by 80% to 24 +/- 6 compared to control values of 148 +/- 12 nmoles Ca/mg X min at 22 degrees C. Coinciding with Ca depletion a change in the tertiary structure of the sarcolemma, i.e. a significant increase in the alpha-helical content of membrane proteins could be observed. The impairment of the Ca transport was elicited during the period of Ca-free perfusion and was not bound to the ultrastructural damage following Ca overload of the cell upon reperfusion with Ca (Ca paradox). Whenever the Ca pump protein was exposed to low concentrations of Ca either during separation over a density gradient or by incubating sarcolemmal vesicles after isolation in Ca-free medium, the Ca transport function was rendered ineffective. It is concluded that the net gain in tissue calcium which occurs during Ca repletion may be associated with a loss of the ability of the cell to extrude Ca actively via a sarcolemmal Ca2+, Mg2+-ATPase.