The influence of low pH environments on ion regulation in the larval stages of the anuran amphibian, Rana clamitans

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Abstract
Epithelial ion regulation by larval Rana clamitans (developmental stages III to XX) and early stage juveniles (stage XXV) was examined in soft water (Ca2+ = 300 μequiv./L) at circumneutral pH and during exposure to low pH (nominally 4.0). In tadpoles and juveniles, the acute acid exposure initially caused substantial increases in transepithelial net ion losses and in net acid uptake, and a slight inhibition of active ion transport. In tadpoles, these disturbances had largely disappeared by 7 h of acid exposure but they persisted in early stage juveniles with no apparent sign of recovery. Prior acclimation of tadpoles to a sublethal pH (12 days at pH 5.0) did not reduce the ionoregulatory disturbance at pH 4.0 and the acclimated animals were less able to restore those losses at circumneutral pH compared with a control group. It is concluded that larval amphibians show many similarities with fish in ionoregulatory responses to acid exposure including an inability to adapt to low pH, but the magnitude and duration of disturbances are generally less. This suggests that larval amphibians may be more able to resist acid exposure than many acid-intolerant fish species such as the salmonids.