Cartledge Victoria A, Withers Philip C, Bradshaw S Don
Zoology, School of Animal Biology, MO92, University of Western Australia, Crawley, Western Australia 6009, Australia.
Physiol Biochem Zool. 2008 Jan-Feb;81(1):43-53. doi: 10.1086/523856. Epub 2007 Nov 16.
It is well established that forming a cocoon, for frog species capable of doing so, markedly reduces evaporative water loss; however, the capacity of cocooned frogs to maintain hydration during extended estivation is not well understood. The combined effects of long-term estivation and water loss were examined in the cocoon-forming species Cyclorana platycephala by assessing the hydration state of the frogs throughout a 15-mo estivation period. Frogs lost mass throughout the 15-mo period to a maximum of 36%+/-6.5% of their initial standard mass. Plasma osmolality reached maximal levels by the ninth month of estivation at 487 mOsm kg(-1) and then remained stable to the fifteenth month of estivation. Urine osmolality continued to increase to the fifteenth month of estivation, at which point plasma and urine concentrations were isosmotic. The use of bladder water to counter losses from circulation was indicated by the relatively slow rate of increase in plasma osmolality with mass loss and the progressive increase in urine osmolality. For estivating frogs, evidence was found for a possible threshold relationship between plasma osmolality and plasma arginine vasotocin (AVT) concentration. After estivation, plasma AVT concentrations decreased markedly after 15-mo estivators were placed in water for 2 h, suggesting that high levels of AVT may not be integral to rapid rehydration in this species.
对于能够结茧的蛙类而言，形成茧能显著减少水分蒸发损失，这一点已得到充分证实；然而，人们对结茧的蛙类在长时间夏眠期间维持水分平衡的能力了解并不充分。通过评估茧形成物种宽头姬蛙在长达15个月的夏眠期内的水分状态，研究了长期夏眠和水分流失的综合影响。在整个15个月的期间内，蛙类体重持续下降，最多降至其初始标准体重的36%±6.5%。血浆渗透压在夏眠的第九个月达到最高水平，为487 mOsm kg⁻¹，然后在夏眠的第十五个月保持稳定。尿渗透压持续增加至夏眠的第十五个月，此时血浆和尿液浓度呈等渗状态。血浆渗透压随体重减轻的相对缓慢增加以及尿渗透压的逐渐增加，表明蛙类利用膀胱水来抵消循环系统中的水分流失。对于夏眠的蛙类，发现血浆渗透压与血浆精氨酸血管加压素（AVT）浓度之间可能存在阈值关系。夏眠结束后，将经过15个月夏眠的蛙放入水中2小时后，血浆AVT浓度显著下降，这表明高水平的AVT可能并非该物种快速补水所必需的。
