Effects of Phospholipase C on the Na + - Ca 2 + Exchange and Ca 2 + Permeability of Cardiac

We have examined the effects of phospholipase C pretreatment on Ca2+ transport in highly purified canine cardiac sarcolemmal vesicles. Na+-Ca2+ exchange, measured as Nai+-dependent Caz+ uptake, is stimulated when 10-70% of the membrane phospholipid has been hydrolyzed. Although the phospholipase C treatment also increases sarcolemmal passive Ca2+ flux, the membrane maintains a sufficient permeability barrier for enhanced transport (via Na+-Ca2+ exchange) to be observed. These effects can be obtained with phospholipase C from either Clostridium perfringens or Bacillus cereus. We find that the phospholipase C (C. perfringens) preferentially hydrolyzes phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin leaving the negatively charged phospholipids, phosphatidylserine and phosphatidylinositol, intact. This suggests that the presence of negatively charged phospholipids is sufficient to ensure Na+-Ca2+ exchange activity. Thin section electron microscopy reveals gross morphological changes in phospholipase C-treated sarcolemma. The vesicles are aggregated and diacylglycerol droplets are visible. Fusion occurs such that adjacent vesicles share a common bilayer. It is surprising that vesicles displaying severe structural damage can demonstrate enhanced transport activity.