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Stroke volume (SV) and cardiac output (CO) were measured in anesthetized larvae of Xenopus laevis from hatching (3 mg) to the end of metamorphosis (approximately 1 g). CO and SV were calculated from videotaped images of the intact beating heart. SV increased from 2.4 x 10(-3) microliters at 3 mg body mass to 7.6 microliters at 1 g. CO increased from 0.25 microliter/min at 3 mg to 623 microliters/min at 1 g. With use of CO, along with arterial pressures from another study [P.-C. L. Hou and W. W. Burggren. Am. J. Physiol. 269 (Regulatory Integrative Comp. Physiol. 38): R1120-R1125, 1995], peripheral resistance and cardiac work were also calculated. Resistance decreased rapidly from 701 peripheral resistance units (PRU, mmHg.s.mm-3) at 3 mg body mass to 79 PRU at 20 mg and gradually declined toward 0.9 PRU at 1 g. Cardiac work increased from 0.06 dyn.mm at 3 mg body mass to 1.27 dyn.mm at 20 mg and then climbed sharply to 717 dyn.mm at 1 g. The general pattern of change in hemodynamic variables (except heart rate) during larval development is similar in Xenopus laevis and chick embryos, suggesting a common pattern for hemodynamic development in vertebrate embryos/larvae.

Heart rate and blood pressure were measured in lightly anesthetized developing Xenopus laevis from hatching (body mass approximately 3 mg) to the end of metamorphosis (< or = 1 g). Blood pressures in the conus arteriosus, truncus arteriosus, and ventricle were measured by a servo-null micropressure system. Heart rate was determined from blood pressure recordings, and cardiac cycles were videotaped through a dissecting microscope. Heart rate varied from 50 to 150 beats/min and showed a negative correlation with body mass, with a slope less than predicted from allometric equations based on adult vertebrates. Mean truncus pressures showed a positive correlation with body mass, increasing from 4 mmHg in a 25-mg larva to 9 mmHg in a 1-g larva. The pressure waveform during ventricular systole was similar in all developmental stages examined, whereas those in conus and truncus varied with development. Conus pressures differed distinctly from truncus pressure during diastole in all larvae examined, suggesting the existence of functional valves between conus and truncus as early as stage 46 of the Nieuwkoop-Faber larval staging system. Although the developmental patterns of heart rate and blood pressure in X. laevis showed significant correlation with body mass, body mass explained less than one-half of the variation in these variables. Therefore developmental factors other than body mass, such as changes in heart mass and the addition of new resistance vessels, may influence heart rate and blood pressure during development in X. laevis.

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