Development of cardiovascular responses to hypoxia in larvae of the frog Xenopus laevis

sponses (blood pressure, heart rate, stroke volume, cardiac output, and peripheral vascular resistance) to acute hypoxia (PO, = 70 mmHg) in developing larvae ofxenopus Zaevis from Nieuwkoop-Faber (NF) stage 45 and up to newly metamor-phosed froglets were investigated. The results revealed two distinct response patterns to acute hypoxia in “early ” (NF stages 45-48 and 49-51) and “late ” (NF stages 52-53,54-57, and 58-62) larval Xenopus. The early larvae responded to acute hypoxia with a significantly decreased stroke volume, cardiac output, and blood pressure. Peripheral resistance increased, whereas no change in heart rate occurred. In late larvae, stroke volume and blood pressure increased during acute hypoxia, but an offsetting bradycardia prevented major changes in cardiac output. We conclude that, up to stage 51 of development, hypoxia exerts a direct inhibitory effect on th