MULTIPLE PHOSPHOLIPASE-A2 ACTIVITIES IN CANINE VASCULAR SMOOTH-MUSCLE

Three phospholipase A2 activities from canine vascular smooth muscle were identified and characterized including: (1) a cytosolic calcium-independent phospholipase A2 which is activated by nucleotide di- and triphosphates; (2) a cytosolic calcium-dependent phospholipase A2 which is activated by physiologic increments in calcium ion concentration; and (3) a microsomal calcium-independent phospholipase A2 which was highly selective for plasmenylcholine substrate. Vascular smooth muscle cytosolic calcium-independent phospholipase A2 was activated 338% +/- 11 (X + S.E.; n = 15) by physiologic concentrations of ATP. Similar amounts of activation were also present utilizing other nucleotide di- and triphosphates (e.g., ADP, CTP, GDP and GTP) as well as non-hydrolyzable nucleotide triphosphate analogs (e.g., ATP-gamma-S, AMP-PNP and GTP-gamma-S). Vascular smooth muscle cytosolic calcium-dependent phospholipase A2 was purified 455-fold by sequential DEAE-Sephacel, Phenyl-Sepharose Mono Q, hydroxyapatite and Superose 12 chromatographies. The partially purified calcium-dependent phospholipase A2 was activated by physiologic increments in calcium ion concentration (e.g., 1 muM) and possessed an apparent native molecular weight of 95 kDa, an acidic isoelectric point (pI = 4.8) and a neutral pH optimum (pH 7.0). Vascular smooth muscle microsomal phospholipase A2 activity was predominantly calcium-independent and was over six-fold selective for hydrolysis of plasmenylcholine substrate. Taken together, these results demonstrate the existence of three separate and distinct phospholipase A2 activities in vascular smooth muscle and identify ATP and calcium ion as independent modulators of discrete phospholipase A2 activities in vascular smooth muscle cells.