CA2-STRENGTH OF SARCOLEMMAL VESICLES SHED FROM RABBIT MUSCLE( LOADINGREDUCES THE TENSILE)

1. Sarcolemmal vesicles shed by rabbit muscle were loaded with Ca2+ by means of A23187 or ionomycin. [Ca2+](o) was buffered between 0.8 and 20 mu M. Membrane strength was measured by pipette aspiration. 2. At 2 0 mu M Ca2+ many vesicles underwent autolysis, or were so weak that th ey burst instantly on aspiration. Between 10 and 2 mu M Ca2+ a graded decrease in membrane strength was demonstrable. At 0.8 mu M Ca2+ the m echanical properties of the sarcolemma remained unaltered. 3. Mg2+ car ried by A23187 does not mimic the effect of Ca2+. Tile ionophore itsel f similarly did not cause a decrease in membrane tensile strength. 4. Pre-treatment with BAPTA-AM, so as to buffer internal Ca2+, partly pro tected vesicles against the decrease in membrane strength produced by Ca2+ loading.5. Membrane strength was not restored by adding excess BA PTA to the bathing solution, seas to reverse the Ca2+ gradient. An irr eversible degradation of the membrane consequent upon raised [Ca2+](i) seems indicated. 6. These findings are discussed in relation to the m echanisms which have been advanced to account for the role of elevated [Ca2+](i) in cell death. 7. An attempt to use staphylococcal alpha-to xin as an alternative means to permeabilize the sarcolemma led to the incidental finding that this pore-forming protein itself greatly weake ns the membrane in doses lower than required for effective permeabiliz ation.