BLOOD PRESSURES AND HEART-RATE DURING LARVAL DEVELOPMENT IN THE ANURAN AMPHIBIAN XENOPUS-LAEVIS

Heart rate and blood pressure were measured in lightly anesthetized de veloping Xenopus laevis from hatching (body mass similar to 3 mg) to t he end of metamorphosis (less than or equal to 1 g). Blood pressures i n the conus arteriosus, truncus arteriosus, and ventricle were measure d 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 a nd showed a negative correlation with body mass, with a slope less tha n predicted from allometric equations based on adult vertebrates. Mean truncus pressures showed a positive correlation with body mass, incre asing from 4 mmHg in a 25-mg larva to 9 mmHg in a 1-g larva. The press ure waveform during ventricular systole was similar in all development al stages examined, whereas those in conus and truncus varied with dev elopment. Conus pressures differed distinctly from truncus pressure du ring diastole in all larvae examined, suggesting the existence of func tional valves between conus and truncus as early as stage 46 of the Ni euwkoop-Faber larval staging system. Although the developmental patter ns of heart rate and blood pressure in X. laevis showed significant co rrelation with body mass, body mass explained less than one-half of th e 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.