Ca2(+)-dependent synthesis of prostaglandin I2 and mobilization of arachidonic acid from phospholipids in cultured endothelial cells permeabilized with saponin

The feasibility of using saponin as a permeabilization agent to study the effect of free Ca2+ concentration (Ca2+]f) on prostaglandin I2 (PGI2) synthesis and mobilization of arachidonic acid from membrane phospholipids was investigated in cultured bovine pulmonary artery endothelial cells (BPAEC). Treatment of BPAEC with 20 micrograms/ml saponin caused selective permeabilization of the plasma membrane as determined by measurements of the release of lactate dehydrogenase and beta-hexosaminidase. In cells prelabeled with 3H]arachidonic acid for 22 h, permeabilization with 20 micrograms/ml saponin induced PGI2 synthesis and release of 3H]arachidonic acid from membrane phospholipids. These effects were dependent upon Ca2+]f in the range 72 nM to 5 microM. Release of 3H]arachidonic acid from phospholipid classes was determined in suspensions of BPAEC prelabeled with 3H]arachidonic acid and permeabilized with 20 micrograms/ml saponin. At Ca2+]f optimal for PGI2 synthesis, 16.2% of the total incorporated 3H]arachidonic acid was released from phosphatidylinositol (3.4%), phosphatidylethanolamine (3.5%) and phosphatidylcholine (9.3%). The time course and dependence upon Ca2+]f of 3H]arachidonic acid release from phospholipids correlated with PGI2 synthesis. The amount of PGI2 synthesized in permeabilized BPAEC was similar to that in cell cultures treated with the calcium ionophore A23187. In comparison, however, PGI2 synthesis induced by A23187 was associated with less release of 3H]arachidonic acid from membrane phospholipids, e.g., 2.3% versus 16.2%. The greater loss of 3H]arachidonic acid from phospholipids in saponin-permeabilized BPAEC was most likely due to the loss of cell integrity and/or nonspecific effects of the detergent on phospholipases. Despite these limitations, the Ca2+ dependence observed for PGI2 synthesis and 3H]arachidonic acid mobilization suggest that saponin-permeabilization may provide a useful system for studies of the intracellular events triggered by the rise in intracellular Ca2+ which culminate in PGI2 synthesis.