Babonis Leslie S, Womack Molly C, Evans David H
Department of Biology, University of Florida, Gainesville, Florida 32611, USA.
J Morphol. 2012 Jan;273(1):88-102. doi: 10.1002/jmor.11009. Epub 2011 Sep 20.
Among tetrapods, evidence for postrenal modification of the urine by the distal digestive tract (including the colon and cloaca) is highly variable. Birds and bladderless reptiles are of interest because the colon and cloaca represent the only sites from which water and ions can be reclaimed from the urine secreted by the kidney. For animals occupying desiccating environments (e.g., deserts and marine environments), postrenal modification of the urine may directly contribute to the maintenance of hypo-osmotic body fluids. We compared the morphology and distribution of key proteins in the colon, cloaca, and urogenital ducts of watersnakes from marine (Nerodia clarkii clarkii) and freshwater (Nerodia fasciata) habitats. Specifically, we examined the epithelia of each tissue for evidence of mucus production by examining the distribution of mucopolysaccharides, and for evidence of water/ion regulation by examining the distribution of Na(+) /K(+) -ATPase (NKA), Na(+) /K(+) /Cl(-) cotransporter (NKCC), and aquaporin 3 (AQP3). NKCC localized to the basolateral epithelium of the colon, urodeal sphincter, and proctodeum, consistent with a role in secretion of Na(+), Cl(-) , and K(+) from the tissue, but NKA was not detected in the colon or any compartment of the cloaca. Interestingly, NKA was detected in the basolateral epithelium of the ureters, suggesting the urothelium may play a role in active ion transport. AQP3 was detected in the ureters and coprodeal complex, consistent with a role in urinary and fecal dehydration or, potentially, in the production of the watery component of the mucus secreted by the coprodeal complex. Since no differences in general cloacal morphology, production of mucus, or the distribution of ion transporters/water channels were detected between the two species, cloacal osmoregulation may either be regulated by proteins not examined in this study or may not be responsible for the differential success of N. c. clarkii and N. fasciata in marine habitats.
在四足动物中，关于远端消化道（包括结肠和泄殖腔）对尿液进行肾后性修饰的证据差异很大。鸟类和无膀胱的爬行动物备受关注，因为结肠和泄殖腔是肾脏分泌的尿液中水分和离子能够被重吸收的唯一部位。对于处于干燥环境（如沙漠和海洋环境）中的动物而言，尿液的肾后性修饰可能直接有助于维持低渗性体液。我们比较了来自海洋栖息地（克拉克水蛇，Nerodia clarkii clarkii）和淡水栖息地（带纹水蛇，Nerodia fasciata）的水蛇的结肠、泄殖腔和泌尿生殖道中关键蛋白质的形态和分布。具体而言，我们通过检查黏多糖的分布来观察每个组织的上皮细胞中黏液产生的证据，并通过检查钠钾ATP酶（NKA）、钠钾氯共转运蛋白（NKCC）和水通道蛋白3（AQP3）的分布来观察水/离子调节的证据。NKCC定位于结肠、尿道括约肌和泄殖道的基底外侧上皮，这与该组织分泌钠、氯和钾的作用一致，但在结肠或泄殖腔的任何区域均未检测到NKA。有趣的是，在输尿管的基底外侧上皮中检测到了NKA，这表明尿路上皮可能在主动离子转运中发挥作用。在输尿管和粪道复合体中检测到了AQP3，这与在尿液和粪便脱水过程中发挥作用一致，或者可能与粪道复合体分泌的黏液的水性成分的产生有关。由于在这两个物种之间未检测到泄殖腔总体形态、黏液产生或离子转运蛋白/水通道分布的差异，泄殖腔的渗透调节可能要么由本研究未检测的蛋白质调节，要么与克拉克水蛇和带纹水蛇在海洋栖息地的不同生存成功率无关。
