Influence of aprotinin and gabexate mesilate on arachidonic acid release by the Ca-ionophore A 23187 in the lung.

In a model of isolated, ventilated rabbit lungs, perfused with Krebs-Henseleit albumin buffer in a recirculating system, increased availability of free AA (arachidonic acid) results in an increase in pulmonary vascular resistance and permeability. The former can be ascribed to cyclooxygenase products of AA, among which thromboxane A2 is mainly responsible. The increase of vascular permeability is, at least partly, due to lipoxygenase products of AA. Availability of free AA for the different oxygenation pathways can be achieved either by direct application of free AA to the perfusion fluid or by stimulation of AA release from the membrane phospholipid pool by the Ca-ionophore A 23187. The serine proteinase inhibitor gebaxate mesilate in a concentration range between 1 microM and 10 microM and aprotinin in a concentration range between 8 and 200 KIE/ml dose-dependently reduce the increase in vascular resistance after stimulation with A 23187. Correspondingly the increase in vascular permeability due to A 23187 is significantly reduced by gabexate mesilate (5 microM) to 52% and by aprotinin (200 KIE/ml) to 73%. On the contrary the increase in pulmonary vascular resistance and permeability after direct application of free AA to the perfusion fluid is not affected by gabexate mesilate and aprotinin. AA metabolism by cyclooxygenase from ram vesicular gland microsomes is inhibited in vitro by gabexate mesilate and by aprotinin only in very high concentrations (greater than 1mM respectively greater than 2130 KIE/ml). Measurements with porcine pancreas and bee venom phospholipase A2 reveal no influence of aprotinin on these enzymes. Gabexate mesilate inhibits pancreas phospholipase A2 in concentrations more than 10-fold higher than those necessary in the isolated lungs (IC50 = 430 microM), bee venom phospholipase A2 not being affected at all. It is thus apparent that the release of AA from the membrane phospholipid pool rather than any particular step in its oxygenation metabolism is the site of action of these proteinase inhibitors in the pulmonary vascular bed. The possible involvement of an intracellular proteinase is discussed.