Vascular control in larval Xenopus laevis: the role of endothelial-derived factors

We used novel digital contrasting methods to measure the effects of endothelin-1 (ET-1, 10(-6) to 10(-8) mol l(-1)), the nitric-oxide-releasing agent sodium nitroprusside (SNP, 10(-2) mol l(-1)) and N(omega)-nitro-L-arginine methyl ester hydrochloride (L-NAME, 10(-4) mol l(-1)), a non-selective inhibitor of nitric oxide synthase, on the diameter of tiny arteries and veins of the head in anaesthetized Xenopus laevis tadpoles (stage NF 50-53). Perfusion of the main artery and vein supplying the head with ET-1 caused an immediate, significant and dose-dependent vasoconstriction. While preincubation with L-NAME caused no significant change in vessel diameter, the constriction provoked by subsequent endothelin administration was significantly enhanced. Application of SNP caused a vasodilation that was significant only after preconstriction with ET-1 (10(-6) mol l(-1)). Our results provide strong evidence that the vasculature of developing Xenopus laevis tadpoles (NF stage 50-53) is influenced by endogenously released nitric oxide and endothelin. Vasoactive mediators released from vascular endothelial cells could be particularly important in vascular control in early embryos when the autonomic innervation is undifferentiated, poorly developed or even absent.