両生類の環境適応における水チャネル (AQP) と上皮性ナトリウムチャネル (ENaC)

抄録

Amphibians regulate the osmotic and ionic composition of their body fluids by water and ion transport across their skin. When dehydrated, desert toads press down the pelvic region of ventral skin on moist surfaces to achieve maximal water absorption. Aquaporins (AQP) are trans-membrane proteins permeable to water. We cloned three isoforms of AQP (AQP-h1, -h2, -h3) in tissues of tree frogs. AQP-h3 was abundantly expressed in the pelvic region of ventral skin, facilitating water absorption across the skin. Antibodies to AQP-h3 labeled the cell membrane of the granulosum cells in the skin. AQP-h3 was progressively expressed during metamorphosis of larval frogs. Expression of AQP-h3 was facilitated by and diuretic hormone, argine vasotocin. Collectively, AQP-h3 is a molecule to play a positive role in water absorption across the amphibian skin. In bullfrogs short circuit current across the skin was suppressed by amiloride, a blocker for epithelial Na⁺ channel (ENaC), showing that transport of sodium ions occurs through ENaC. Na⁺ transport increased in parallel with metamorphosis of the tadpole. In cultured skins of the tadpole Na⁺ transport was facilitated by aldosterone, but counteracted by prolactin. Such counteraction seemed to be removed by thyroid hormone (T₃) during metamorphosis to adapt a terrestrial environment.