Castillo G A, Orce G G
Department of Physiology, INSIBIO (UNT-CONICET), Tucumán, Argentina.
Comp Biochem Physiol A Physiol. 1997 Dec;118(4):1145-50. doi: 10.1016/s0300-9629(97)86799-0.
Maintenance of a hydrated integument is essential to the normal function of amphibian skin, and amphibians have developed mechanisms to minimize cutaneous dessication. The present work was conducted on skins of amphibians exhibiting a clear preference for either of two such mechanisms to study the influence of such mechanisms on the characteristics of epithelial transport. The response to norepinephrine (NE) was studied in isolated skins of a semiaquatic frog (Leptodactylus chaquensis), known to maintain indispensable skin moisture by secreting a superficial film of mucus via sympathetic stimulation of skin glands, and a terrestrial toad (Bufo arenarum), which replenishes a superficial film of fluid by drawing soil water upward by capillarity. In L. chaquensis skin, NE 5.0 x 10(-7) M, induced slow onset, sustained increases in short-circuit current (SCC) and transepithelial conductance, which were abolished by amiloride, a specific sodium transport inhibitor. At 1.2 x 10(-5) M, the response to NE exhibited a faster onset and a shorter time course. The SCC response also became insensitive to amiloride and could thus be induced by exposing the skin to NE in the presence of the inhibitor. The response was also greatly reduced in the absence of chloride, strongly suggesting a greater dependence on the glandular secretory response. In B. arenarum skin, the response to NE was far more sensitive to amiloride, regardless of the concentration of NE used. Induction of a response in the amiloride-blocked skin required a 10-fold higher concentration of NE, and the resulting effect was still considerably smaller than that observed in the skin of L. chaquensis after the same treatment. The number of mucous glands per unit area in B. arenarum skin was found to be around one-fifth of that observed in L. chaquensis, thus in part explaining the difference in the magnitude of the responses. The response of the skin of L. chaquensis to NE in the presence of sulfate was found to be consistent with the postulated involvement of frog skin glands in sulfate excretion. In contrast, this function was not evident in the skin of B. arenarum. The pattern of response of B. arenarum skin to all concentrations of NE tested closely resembles that seen after exposure to agents known to activate a cyclic AMP-dependent, high-permeability Cl pathway previously described by us in the skin of the toad. Our observations underscore the physiological differences existing in skins from different species, particularly regarding the relative importance of the glandular component of transport.
保持皮肤水润对于两栖动物皮肤的正常功能至关重要，两栖动物已进化出多种机制以尽量减少皮肤脱水。本研究以两栖动物的皮肤为对象，这些两栖动物对两种此类机制中的一种表现出明显偏好，旨在研究这些机制对上皮转运特性的影响。研究了去甲肾上腺素（NE）对半水生青蛙（Leptodactylus chaquensis）离体皮肤的反应，已知该青蛙通过交感神经刺激皮肤腺体分泌一层表面黏液来维持不可或缺的皮肤水分；还研究了陆栖蟾蜍（Bufo arenarum）的反应，它通过毛细作用向上吸收土壤水分来补充表面的一层液体。在Leptodactylus chaquensis皮肤中，5.0×10⁻⁷ M的NE诱导短路电流（SCC）和跨上皮电导缓慢开始并持续增加，而特异性钠转运抑制剂氨氯吡脒可消除这种增加。在1.2×10⁻⁵ M时，对NE的反应开始更快且时程更短。SCC反应对氨氯吡脒也变得不敏感，因此在抑制剂存在的情况下将皮肤暴露于NE可诱导该反应。在无氯离子的情况下，反应也大大降低，强烈表明对腺体分泌反应的依赖性更大。在Bufo arenarum皮肤中，无论使用何种浓度的NE，对NE的反应对氨氯吡脒都更为敏感。在氨氯吡脒阻断的皮肤中诱导反应需要高10倍的NE浓度，并且产生的效应仍比相同处理后在Leptodactylus chaquensis皮肤中观察到的效应小得多。发现Bufo arenarum皮肤中每单位面积的黏液腺数量约为Leptodactylus chaquensis中观察到的数量的五分之一，从而部分解释了反应幅度的差异。发现在有硫酸盐存在的情况下，Leptodactylus chaquensis皮肤对NE的反应与推测的青蛙皮肤腺体参与硫酸盐排泄一致。相比之下，这种功能在Bufo arenarum皮肤中不明显。Bufo arenarum皮肤对所有测试浓度的NE的反应模式与暴露于已知可激活我们先前在蟾蜍皮肤中描述的依赖环磷酸腺苷的高通透性氯通道的试剂后观察到的模式非常相似。我们的观察结果强调了不同物种皮肤中存在的生理差异，特别是关于转运的腺体成分的相对重要性。
