Neufeld D S, Wright S H
Department of Physiology, College of Medicine, University of Arizona, Tucson 85724.
J Exp Biol. 1995 Feb;198(Pt 2):465-73. doi: 10.1242/jeb.198.2.465.
We found that the basolateral surface of the gill epithelium of the marine mussel Mytilus californianus possesses a carrier-mediated process capable of concentrating taurine within epithelial cells. We used retrograde perfusion of gill sections to demonstrate the kinetics, specificity and ion-dependence of taurine transport. [3H]taurine was concentrated relative to a space marker ([14C]mannitol); this accumulation was blocked by the inclusion of 10 mmol l-1 unlabeled taurine in the perfusate. The drop in [3H]taurine uptake at increasing concentrations of unlabeled taurine was fitted to Michaelis-Menten kinetics and indicated a basolateral process with a taurine concentration at which transport is half-maximal (Kt) of 35.3 mumol l-1 and a maximal flux (Jmax) of 0.35 mumol g-1 wet mass h-1. Taurine accumulation on the apical surface had a higher affinity (Kt = 9.5 mumol l-1) and a higher maximum rate of transport (Jmax = 1.23 mumol g-1 h-1). Basolateral transport was inhibited by inclusion in the perfusate of 1 mmol l-1 of another beta-amino acid (beta-alanine), but not by inclusion of alpha-alanine, glutamic acid or betaine. The dependence of basolateral taurine transport on Na+ (when replaced with N-methyl-D-glucamine) was sigmoidal with an apparent Hill coefficient of 2.3, indicating that more than one Na+ is necessary for the transport of each taurine molecule. Complete substitution of Cl- in bathing media reduced taurine accumulation by 90% and 70% on the apical and basolateral surfaces, respectively. Taurine accumulation on both surfaces was reduced by only 20% when Cl- was reduced from 496 to 73 mmol l-1, suggesting that taurine uptake is not significantly influenced by the changes in Cl- concentration accompanying the salinity fluctuations normally encountered by mussels. We estimate that the various Na+ and Cl- gradients naturally encountered by epithelial cells are capable of providing ample energy to maintain a high intracellular concentration of taurine. We suggest that the ability of epithelial cells to accumulate taurine across the basolateral surface from the hemolymph plays a significant role in the intracellular regulation of this important osmolyte and may effect osmolality-dependent changes in the intracellular concentration of taurine.
我们发现，加州贻贝鳃上皮细胞的基底外侧表面存在一种载体介导的过程，能够将牛磺酸浓缩在上皮细胞内。我们采用鳃切片逆行灌注法来证明牛磺酸转运的动力学、特异性和离子依赖性。相对于空间标记物（[14C]甘露醇），[3H]牛磺酸被浓缩；灌注液中加入10 mmol l-1未标记的牛磺酸可阻断这种积累。随着未标记牛磺酸浓度增加，[3H]牛磺酸摄取量的下降符合米氏动力学，表明基底外侧过程的牛磺酸浓度在转运达到半最大速率时（Kt）为35.3 μmol l-1，最大通量（Jmax）为0.35 μmol g-1湿质量 h-1。牛磺酸在顶端表面的积累具有更高的亲和力（Kt = 9.5 μmol l-1）和更高的最大转运速率（Jmax = 1.23 μmol g-1 h-1）。灌注液中加入1 mmol l-1另一种β-氨基酸（β-丙氨酸）可抑制基底外侧转运，但加入α-丙氨酸、谷氨酸或甜菜碱则无此作用。基底外侧牛磺酸转运对Na+（用N-甲基-D-葡萄糖胺替代时）的依赖性呈S形，表观希尔系数为2.3，表明每个牛磺酸分子的转运需要不止一个Na+。将浴液中的Cl-完全替代，顶端和基底外侧表面的牛磺酸积累分别减少90%和70%。当Cl-从496 mmol l-1降至73 mmol l-1时，两个表面的牛磺酸积累仅减少20%，这表明贻贝通常遇到的盐度波动伴随的Cl-浓度变化对牛磺酸摄取没有显著影响。我们估计上皮细胞自然遇到的各种Na+和Cl-梯度能够提供充足能量以维持细胞内高浓度的牛磺酸。我们认为上皮细胞从血淋巴中通过基底外侧表面积累牛磺酸的能力在这种重要渗透溶质的细胞内调节中起重要作用，并且可能影响牛磺酸细胞内浓度的渗透压依赖性变化。
