Modulation of angiogenic endothelial permselectivity by the cAMP pathway

The chorioallantoic membrane (CAM) of the chick embryo provides an accessib le model of normal angiogenesis in vivo. Previously, we reported a rapid re duction in CAM microvascular permeability to macromolecules between Days 4. 5 and 5.0 of the normal 21-day gestation (V. Rizzo et al., 1995, Microvasc. Res. 49, 49-63). Here, we tested the hypothesis that activation of the cAM P signaling pathway at Day 4.5 would acutely increase permselectivity prior to normal differentiation of CAM endothelial barrier properties at Day 5.0 . Changes in interstitial optical intensities due to extravasation of a gra ded series of FITC-dextrans (20, 40, and 70 kDa) were evaluated by computer -assisted image analysis, and endothelial ultrastructure was monitored by t ransmission electron microscopy. The cAMP analogue 8-bromo-cAMP (10(-4) and 10(-3) M) and forskolin (10(-5) and 10(-4) M), an adenylyl cyclase activat or, acutely decreased permeability of the graded FITC-dextran series in a d ose-dependent fashion. In addition, the nonspecific phosphodiesterase inhib itor IBMX (10(-4) M) served to increase basal restriction of the 20- and 40 -kDa tracers. Further, Rp-cAMPS (10(-4) M), a cAMP antagonist for cAMP-depe ndent protein kinase, abolished the effects of both 8-bromo-cAMP (10(-3) M) and forskolin (10(-4) M) on FITC-Dextran 40 restriction. In all cases, ult rastructural presentation of both the endothelial cell junctions and the ve sicles remained unchanged. The present results are consistent with the conc ept that exogenous cAMP activation decreased permeability of the angiogenic CAM endothelium at Day 4.5 without concomitant ultrastructural changes in the transendothelial macromolecular exchange pathways. Whether endogenous a ctivity of cAMP contributes to normal differentiation of CAM endothelial ba rrier properties between Days 4.5 and 5.0 remains to be tested, (C) 1999 Ac ademic Press.