Grosell M, Genz J, Taylor J R, Perry S F, Gilmour K M
RSMAS, Division of Marine Biology and Fisheries, University of Miami, Miami, FL 33149, USA.
J Exp Biol. 2009 Jun;212(Pt 12):1940-8. doi: 10.1242/jeb.026856.
Pyloric caeca and anterior intestine epithelia from seawater-acclimated rainbow trout exhibit different electrophysiological parameters with lower transepithelial potential and higher epithelial conductance in the pyloric caeca than the anterior intestine. Both pyloric caeca and the anterior intestine secrete HCO(3)(-) at high rates in the absence of serosal HCO(3)(-)/CO(2), demonstrating that endogenous CO(2) is the principal source of HCO(3)(-) under resting control conditions. Apical, bafilomycin-sensitive, H(+) extrusion occurs in the anterior intestine and probably acts to control luminal osmotic pressure while enhancing apical anion exchange; both processes with implications for water absorption. Cytosolic carbonic anhydrase (CAc) activity facilitates CO(2) hydration to fuel apical anion exchange while membrane-associated, luminal CA activity probably facilitates the conversion of HCO(3)(-) to CO(2). The significance of membrane-bound, luminal CA may be in part to reduce HCO(3)(-) gradients across the apical membrane to further enhance anion exchange and thus Cl(-) absorption and to facilitate the substantial CaCO(3) precipitation occurring in the lumen of marine teleosts. In this way, membrane-bound, luminal CA thus promotes the absorption of osmolytes and reduction on luminal osmotic pressure, both of which will serve to enhance osmotic gradients to promote intestinal water absorption.
来自海水适应虹鳟鱼的幽门盲囊和前肠上皮表现出不同的电生理参数，幽门盲囊的跨上皮电位较低，上皮电导较高，高于前肠。在没有浆膜HCO(3)(-)/CO(2)的情况下，幽门盲囊和前肠都以高速率分泌HCO(3)(-)，这表明内源性CO(2)是静息控制条件下HCO(3)(-)的主要来源。顶端的、对巴弗洛霉素敏感的H(+)排出发生在前肠，可能起到控制管腔渗透压的作用，同时增强顶端阴离子交换；这两个过程都与水吸收有关。胞质碳酸酐酶(CAc)活性促进CO(2)水合以促进顶端阴离子交换，而膜相关的管腔CA活性可能促进HCO(3)(-)向CO(2)的转化。膜结合的管腔CA的意义可能部分在于降低顶端膜上的HCO(3)(-)梯度，以进一步增强阴离子交换，从而促进Cl(-)吸收，并促进硬骨鱼管腔中大量CaCO(3)沉淀的发生。通过这种方式，膜结合的管腔CA促进了渗透溶质的吸收和管腔渗透压的降低，这两者都将有助于增强渗透梯度以促进肠道水吸收。
