Behavioral, molecular and integrative mechanisms of amphibian osmoregulation

Amphibian water balance has been studied at many levels of biological order . Terrestrial species must react to environmental cues that relate to water availability while some arboreal species have cutaneous skin secretions th at can reduce evaporative water loss. The Indian tree frog, Polypedates mac ulatus, uses cutaneous secretions and wiping behavior to lower evaporation but also relies on moist microclimates to endure prolonged survival away fr om water. The related species, P. leucomystax, inhabits wetter forest habit ats. Preliminary studies with this species are unable to demonstrate the ex pression of wiping behavior, indicating that arid habitats may be a powerfu l selective force for this behavior. Laboratory experiments on rehydrating toads in the genus Bufo indicate that animals are able to detect changes in barometric pressure and humidity that might result in the availability of water under field situations. Experiments with Bufonid species and with spa defoot toads, Scaphiopus couchi, show that the peptide hormone, angiotensin II, stimulates cutaneous drinking in a similar manner seen for oral drinki ng by other vertebrate classes. Amphibian tissues have long been used as a model for the study of basic physiological principles of epithelial ion and water transport. Recent progress with tissue cultures has provided informa tion on the molecular structure of ion and water channels that can be appli ed to obtain a better understanding, at the molecular level, of ion and wat er balance strategies used by the wide variety of amphibian species. Terres trial amphibians are more tolerant of dehydration than are other vertebrate s and are able to store dilute urine in their urinary bladder. Toads appear to be able to detect the presence of water in their bladders in addition t o the availability of water in their environment. Dehydrated toads are able to rehydrate very rapidly by the coordination of behavioral and physiologi cal mechanisms to enhance cutaneous water absorption. The integration of be havior with cutaneous water gain, renal handling of ions and water and the role of the lymphatic system in overall water balance involves complex inte ractions between neural and hormonal factors. Experiments are summarized th at describe the contribution of individual factors however much more inform ation is needed before the nature of these interactions are fully understoo d. (C) 1999 Wiley-Liss, Inc.