Carrageenan-stimulated release of arachidonic acid and of lactate dehydrogenase from rat pleural cells

Cells isolated from the rat pleural cavity consist mainly of macrophages, mast cells, eosinophils, and lymphocytes. Isolated pleural cells labeled with [¹⁴C]arachidonic acid released appreciable amounts (∼12%) of radiolabel upon exposure to pharmacological concentrations of carrageenan (1-100(μg/ml). The release of radiolabel was decreased by an inhibitor of phospholipase A₂ (p-bromophenacyl bromide) but not by an inhibitor of arachidonate cyclooxygenase (indomethacin). The released products were arachidonic acid and, to a much lesser extent, prostaglandin E₂ and leukotriene C₄. The release of radiolabel was associated with release of cytosolic lactate dehydrogenase over the same range of carrageenan concentrations. Time-course studies indicated that release of radiolabel preceded that of lactate dehydrogenase. Since p-bromophenacyl bromide blocked stimulated release of radiolabel but did not prevent release of lactate dehydrogenase, it is unlikely that increase in arachidonate causes carrageenan-induced cell damage. Nevertheless, the question of whether the activation of phospholipase A₂ in the pleural cells, most probably the macrophages, was sufficient to initiate the carrageenan-induced inflammatory response requires further study. Cytotoxicity which was apparent with as little as 5 μg/ml of carrageenan, may have been a significant consequence of carrageenan action.