CAMP PROTECTS ENDOTHELIAL BARRIER FUNCTION INDEPENDENT OF INHIBITING MLC20-DEPENDENT TENSION DEVELOPMENT

Exposure of cultured human umbilical vein endothelial cells to the cAM P agonists theophylline and forskolin decreased constitutive isometric tension of a confluent monolayer inoculated on a collagen membrane, b ut it did not prevent increased tension in cells exposed to thrombin. The inability of cAMP agonists to prevent tension development correlat ed with an inability of cAMP stimulation to prevent increased 20-kDa m yosin light chain (MLC20) phosphorylation in response to thrombin. Alt hough cAMP did not prevent tension development or increased MLC20 phos phorylation, cAMP attenuated the effect of thrombin on transendothelia l electrical resistance across a confluent monolayer inoculated on a g old microelectrode. Activation of cAMP-dependent signal transduction d id not prevent a decline in resistance in thrombin-treated cells, but it more promptly restored transendothelial resistance to initial basal levels (10 min) compared with thrombin only (60 min). ML-7, an MLC ki nase antagonist, at doses that attenuate increased MLC20 phosphorylati on and tension development, did not prevent a decline in resistance in thrombin-treated cells. Yet, ML-7 also restored transendothelial resi stance more rapidly than thrombin alone (20 min) but at a slower rate than cAMP. These data demonstrate that activation of cAMP-dependent si gnal transduction protects barrier function independent of inhibition of MLC20-dependent tension development.