ATP reduces macromolecule permeability of endothelial monolayers despite increasing [Ca2+](i)

We investigated the relationship between the ATP-evoked rise of cytosolic C a2+ concentration ([Ca2+](i)) and barrier function in porcine aortic endoth elial monolayers. ATP (0.01-100 mu M) induced a transient rise of [Ca2+]i a nd reduced permeability in a concentration-dependent manner. In contrast, t he Ca2+ ionophore ionomycin (1 mu M) elicited a rise in [Ca2+](i) comparabl e to that induced by ATP (10 mu M), but it increased permeability. For the reduction of permeability, nucleotides were found to be in the following or der of potency: ATP = ATP gamma S > ADP = UTP. Blockade of adenosine recept ors by 8-phenyltheophylline (10 FIM) did not affect ATP (10 mu M)-induced r eduction of permeability. ATP reduced permeability even in endothelial mono layers that had been loaded with the Ca2+ chelator BAPTA to prevent the ris e in [Ca2+](i). U-73122 (1 mu M), an inhibitor of phospholipase C (PLC), co mpletely abolished the effect of ATP (10 FIM) on permeability. It also abol ished the translocation of protein kinase C (PKC) in response to ATP, which could also be achieved by the PKC inhibitors Go-6976 (100 nM) or bisindoly lmaleimide I (1 mu M). In the presence of PKC inhibitors, however, the perm eability effect of ATP was not affected. The presence of inhibitors of aden ylate or guanylate cyclase (50 mu M SQ-22536 or 20 mu M ODQ) prevented chan ges in cyclic nucleotides but did not affect the permeability effects of AT P. The study shows that ATP reduces macromolecule permeability via a PLC-me diated mechanism that is independent of the concomitant effects of ATP on c ytosolic Ca2+, cyclic nucleotides, or PKC.