An in vitro model of the arterial endothelial barrier was established in which transfer of trypan blue-labelled albumin across confluent monolayers of pig aortic endothelial cells grown on polycarbonate membranes was measured. 2. A range of inflammatory mediators, i.e. histamine, bradykinin, platelet activating factor and thrombin, had no effect on the transfer of labelled albumin across aortic endothelial monolayers. 3. Calcium ionophore A23187 and the phorbol ester, phorbol myristate acetate (PMA), each induced concentration-dependent increases in transfer of labelled albumin. These increases were associated with changes in cell shape, consistent with endothelial contraction. Ionophore A23187 caused some detachment of cells. 4. The ability of PMA to increase transfer of labelled albumin probably results from activation of protein kinase C since it was not shared by the inactive analogue, 4 alpha-phorbol 12,13-didecanoate. 5. Neither a combination of superoxide dismutase and catalase nor the cyclo-oxygenase inhibitor, flurbiprofen, affected resting or PMA-induced increases in albumin transfer. Oxygen-derived free radicals and prostaglandins appear not to be involved in the response to PMA. 6. Each of three procedures designed to elevate adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels, i.e. dibutyryl cyclic AMP, forskolin and (+/-)-isoprenaline, reduced the ability of PMA to promote increased transfer of labelled albumin but had no effect on resting transfer. The effect of (+/-)-isoprenaline was abolished by the beta-adrenoceptor blocking agent, propranolol. 7. Elevation of cyclic GMP content by use of 8 bromo cyclic GMP or atriopeptin II had no effect on resting or PMA-induced transfer of labelled albumin. 8. Arterial endothelial barrier function can be compromised by agents that promote endothelial contraction. Agents that increase endothelial cyclic AMP levels, and so reduce entry of high molecular weight substances into the arterial wall, may warrant evaluation as potential anti-atherogenic drugs.
