PHOSPHOLIPASE A(2) AND PROTEIN-KINASE-C CONTRIBUTE TO MYOFILAMENT SENSITIZATION TO 5-HT IN THE RABBIT MESENTERIC-ARTERY

1. Calcium (Ca2+, 0.1-100 mu M) stimulated concentration-dependent con tractions in small strips from the rabbit mesenteric artery in which t he smooth muscle cells had been permeabilized with Staphylococcus aure us alpha-toxin. 2. 5-Hydroxytryptamine (5-HT) and phenylephrine, each in the presence of 10 mu M guanosine 5'-triphosphate (GTP), concentrat ion-dependently stimulated additional contractions in strips sub-maxim ally contracted by the presence of a buffered concentration of calcium (0.3 mu M). All the additional contraction was abolished with the sel ective inhibitor of protein kinase C, Ro 31-8220 (10 mu M). 3. Quinacr ine (10-50 mu M), inhibiter of phospholipase A(2), selectively inhibit ed the sensitization to 5-HT, but did not alter the sensitization to e ither phenylephrine or GTP. 4. Myofilament sensitization to calcium ma s mimicked by exogenous arachidonic acid (300 mu M, in the presence of indomethacin, miconazole and BW755c) and the stable analogue of arach idonic acid, 5,8,11,14-eicosatetrayonic acid (ETYA, 100 mu M), and in both cases did not require the additional presence of GTP. Ro 31-8220, but not quinacrine, reduced the sensitization to arachidonic acid by around 30%. 5. These results indicate that G protein-linked myofilamen t sensitization to calcium in the mesenteric artery that follows the a ctivation of 5-HT receptors, but not alpha(1)-receptors, involves phos pholipase A(2). The sensitization stimulated by each of these differen t receptors, and a component of the response to arachidonic acid, also appears to involve the activation of protein kinase C.