Interactions of bradykinin, calcium, G-protein and protein kinase in the activation of phospholipase A2 in bovine pulmonary artery endothelial cells.

Rise in free cytosolic calcium concentrations [Ca2+]i in response to bradykinin and guanosine 5'-O-thiotriphosphate (GTP tau S) was related to the action of phospholipase A/ (arachidonic acid release). At 900 microM extracellular CaCl2, bradykinin induced a typical Ca2+ movement consisting of an initial [Ca2+]i peak at approximately 400 nM followed by a sustained increase in the steady-state cytosolic Ca2+ level at approximately 290 nM. As the extracellular CaCl2 concentration was reduced to 100 microM, the bradykinin induced initial spike was reduced followed by only a marginal increase in steady-state cytosolic Ca2+ levels. Treatment of endothelial cells with saponin (0.002% w/w) did not increase [Ca2+]i and saponin treated cells exhibited a very similar pattern of Ca2+ mobilization in response to bradykinin. However, with saponin treatment, GTP tau S (100 microM) increased [Ca2+]i at an almost identical tracing exhibited with 50 nM bradykinin stimulation (in either the presence or absence of 0.002% saponin). No additive increase in [Ca2+]i was observed in cells stimulated with both 100 microM GTP tau S and 50 nM bradykinin or in bradykinin stimulated cells subsequently exposed to GTP tau S. Pertussis toxin (PTX) did not affect the bradykinin induced Ca2+ mobilization. However, as we showed previously, PTX inhibited bradykinin stimulated arachidonic acid release. These results indicate transduction of the bradykinin signal by G-protein for both phospholipase A2 (PLA2) activation and Ca2+ mobilization but likely by different G alpha subunits, a PTX sensitive and an insensitive subunit. Furthermore, the bradykinin and GTP tau S stimulated release of arachidonic acid appears to be only partially dependent on [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)