Larval cardiorespiratory ontogeny and allometry in Xenopus laevis

Abstract

Very little is known about the early development of cardiorespiratory regulatory mechanisms in newly hatched amphibian larvae. We tested whether early cardiovascular responses to hypoxia reflect local flow regulation in tissues and whether regulation of ventilation would improve during larval development Cardiac output was calculated from heart rate and stroke volume, and buccal pumping rate was measured at 19 degree -21 degree C for Xenopus laevis larvae between Nieuwkoop and Faber stages 44 (just after hatching) and 57 (4-1,102 mg) denied access to air at a range of ambient aquatic PO-2 from normoxia (150-155 mmHg) to severe hypoxia (27-45 mmHg). Cardiac output decreased in severe hypoxia in stage 44-49.5 larvae, but not in stage 51-54 larvae, because heart rate decreased significantly in the early larvae, probably a direct effect of O-2 limitation on cardiac metabolism. Stroke volume did not change significantly in hypoxia in either early- or late-stage larvae. Thus there was no evidence of a tissue-mediated increase in cardiac output in hypoxia. Buccal pumping increased by about 50% over normoxic rates in moderate hypoxia in all larvae but sharply decreased in severe hypoxia, decreasing more in younger larvae than older. Younger larvae show significantly more variability in buccal pumping than older larvae, which suggests that regulatory mechanisms are not yet fully developed in early larvae. Cardiac output scales to body mass with a allometric coefficient of 1.15 +- 0.15 (95% confidence limits), significantly higher than literature values for O-2 uptake (0.83), implying that cardiovascular gas transport may be less important (compared to direct diffusion) in very small early-stage larvae than in larger, late-stage larvae.