The role of angiotensin II and prostaglandins in arcade formation in a developing microvascular network.

There are basically two types of branching patterns in the terminal part of the arteriolar tree. On the one hand, in a number of tissues, including the developing chick embryo chorioallantoic membrane (CAM), the pattern is dichotomous, whereas in other tissues many arteriolar-arteriolar connections, arcades, are found. The structure of the branching pattern depends on the local physical and chemical environment. The goal of this study was to investigate whether substances with an effect on vascular growth influence the vascular branching pattern. We treated chick embryo CAMs daily from day 7 to day 14 postfertilization with 0.9% NaCl, angiotensin II (ANG-II), ANG-II in combination with different angiotensin receptor subtype antagonists, i.e., losartan and CGP 42112A, or the prostaglandin synthesis inhibitor acetylsalicylic acid (ASA). Arcade formation was quantified by counting the number of arcades per cm2 treated area, the branch-node ratio and mean surface area of arcade loops. ANG-II caused a 2-fold increase in the number of arcades versus 0.9% NaCl. Addition of ASA or losartan caused a further enhancement of arcade formation expressed in the number and branch-node ratio. CGP 42112A had no significant effect on arcade formation. From these data we hypothesize that ANG-II stimulates the process of capillary upgrading to arterioles by stimulation of arteriolar smooth muscle cell growth. Prostaglandins normally counteract this effect. After blockade of prostaglandin action, the ANG-II-induced arterialization is enhanced, resulting in pronounced arcade formation. The actions of losartan may be related to its inhibitory effects on prostaglandins rather than angiotensin receptor antagonism.