Development of the respiratory response to hypoxia in the isolated brainstem of the bullfrog Rana catesbeiana

Abstract

The aim of this study was to examine the effects of cellular hypoxia, and the contribution of anaerobic metabolism, on respiratory activity in bullfrogs at different stages of development. Respiratory-related neural activity was recorded from cranial nerve rootlets in isolated brainstem preparations from pre-metamorphic (Taylor-Kollros (T-K) stages VIII-XVI) and postmetamorphic tadpoles (T-K stages XXIV-XXV) and adults. Changes in fictive gill/lung activity in brainstems from pre-metamorphic tadpoles and lung activity in postmetamorphic tadpoles and adults were examined during superfusion with control (98% O(2)/2% CO(2)) or hypoxic (98% N(2)/2% CO(2)) artificial cerebrospinal fluid (aCSF). Iodoacetate (IAA; 100 micromol l(-1)) was used in conjunction with hypoxic aCSF to inhibit glycolysis. Gill burst frequency in pre-metamorphic brainstems did not change over a 3 h exposure to hypoxia and fictive lung burst frequency slowed significantly, but only after 3 h hypoxia. Blockade of glycolysis with IAA during hypoxia significantly reduced the time respiratory activity could be maintained in pre-metamorphic, but not in adult, brainstems. In brainstems from post-metamorphic tadpoles and adults, lung burst frequency became significantly more episodic within 5-15 min hypoxic exposure, but respiratory neural activity was subsequently abolished in every preparation. The cessation of fictive breathing was restored to control levels upon reoxygenation. Neither tadpole nor adult brainstems exhibited changes in neural bursts resembling 'gasping' that is observed in mammalian brainstems exposed to severe hypoxia. There was also a significant increase in the frequency of 'non-respiratory' bursts in hypoxic postmetamorphic and adult brainstems, but not in pre-metamorphic brainstems. These results indicate that pre-metamorphic tadpoles are capable of maintaining respiratory activity for 3 h or more during severe hypoxia and rely to a great extent upon anaerobic metabolism to maintain respiratory motor output. Upon metamorphosis, however, hypoxia results in significant changes in respiratory frequency and pattern, including increased lung burst episodes, non-ventilatory bursts and a reversible cessation of respiratory activity. Adults have little or no ability to maintain respiratory activity through glycolysis but, instead, stop respiratory activity until oxygen is available. This 'switch' in the respiratory response to hypoxia coincides morphologically with the loss of gills and obligate air-breathing in the postmetamorphic frog. We hypothesize that the cessation of respiratory activity in post-metamorphic tadpoles and adults is an adaptive, energy-saving response to low oxygen.