Respiration during chronic hypoxia and hyperoxia in larval and adult bullfrogs (Rana catesbeiana). I. Morphological responses of lungs, skin and gills.

Larval and adult bullfrogs, Rana catesbeiana (Shaw), were exposed to 28 days of normoxia (PO2 150 mmHg), hypoxia (PO2 70-80 mmHg) or hyperoxia (PO2 greater than 275 mmHg) at 20-23 degrees C, after which the following morphological measurements were made: (1) mass, thickness, capillary mesh density and blood-water barrier of the skin; (2) mass, volume, cava density and blood-gas barrier of lungs; and, for the larvae, (3) arch length, filament density and size, and blood-water barrier of the gills. Chronic hypoxia induced profound morphological changes in the gas exchange organs of larvae, but not of adults. In tadpoles, the skin became thinner, with a doubling of capillary mesh density and a having of the blood-water barrier. The gas diffusion barrier of the lungs remained unchanged, but the lung volume and density of the lung wall cava both increased significantly. The internal gills showed a marked enlargement upon hypoxic exposure, both in numbers of gill filaments and size of each filament. The blood-water barrier remained unchanged. Chronic hyperoxia, unlike chronic hypoxia, caused no significant changes in the morphology of the gas exchange organs of larvae. Chronic exposure to hypoxia or hyperoxia failed to produce any significant morphological changes in adult bullfrogs. These data indicate that the great morphological plasticity of larvae, culminating in metamorphosis, also extends to profound adjustments in the gas exchange organs when oxygen transfer becomes limited, a response lacking in adults.