Ontogeny of heart rate regulation in the bullfrog, Rana catesbeiana.

Heart rate (fH) at 20-23 degrees C was recorded in six different developmental stages of the bullfrog, Rana catesbeiana (n = 104, body mass 40 mg to 90 g), at rest after normoxic acclimation, during acute changes in O2 availability, and after brief but intense activity. The effects of cholinergic blockade and combined cholinergic and beta-adrenergic blockade on the response to this experimental protocol were also assessed. Mild tonic vagal inhibition of fH was evident during larval development but disappeared after metamorphosis. There was no tonic sympathetic stimulation of fH at rest in any developmental stage. Intense activity produced a tachycardia in all developmental stages but newly hatched larvae. In adults, tachycardia during activity resulted from beta-adrenergic stimulation but in larvae may have resulted from the direct effects of increased venous return stretching the cardiac pacemaker. Neither acute hypoxia or hyperoxia affected fH in any developmental stage, with the single exception of a severe depression of fH occurring at a PO2 of 30 Torr in newly hatched larvae. These results indicate that, although the heart of the newly hatched larvae is essentially devoid of regulation, cholinergic and beta-adrenergic mechanisms for reflex regulation of fH appear early in larval development. These mechanisms, although most fully expressed in the adult bullfrog, are essentially intact before metamorphosis of the larva.