The importance of Ca2+-mediated phospholipase A2 activation for stimulus-evoked PGE2-release from rabbit splenic capsular strips.

Rabbit splenic capsular strips released PGE2 when contracted by noradrenaline. Contraction as well as PGE2-release were Ca2+-dependent. In the presence of Ca2+ the ionophore A 23187 induced a long lasting contraction and a vigorous PGE2-release. In Ca2+-free medium the ionophore was ineffective. Introduction of Ca2+ to splenic strips preincubated with the ionophore in the absence of Ca2+ reinstalled contracture and PGE2-release. The PGE2-release induced induced by the PG-precursor arachidonic acid was Ca2+-independent. Addition of phospholipase A to splenic strips elicited a strong PGE2-release. These results indicate, that the first step of stimulus evoked PG-release is the Ca2+-mediated activation of phospholipase A2. This enzyme liberates from membrane bound phospholipids C-20-fatty acids, which then serve as substrates for the PG-synthetizing enzymes. Therefore the rate limiting step in stimulus-evoked PG-release seems to be rather the phospholipase A2 activity than the state of activation of one of the PG-synthetizing enzymes.