Clerici C, Soler P, Saumon G
INSERM U82, Faculté de médecine Xavier Bichat, Université Paris 7, France.
Biochim Biophys Acta. 1991 Mar 18;1063(1):27-35. doi: 10.1016/0005-2736(91)90349-d.
Inorganic phosphate, amino acids and sugars are of obvious importance in lung metabolism. We investigated sodium-coupled transports with these organic and inorganic substrates in type II alveolar epithelial cells from adult rat after one day in culture. Alveolar type II cells actively transported inorganic phosphate and alanine, a neutral amino acid, by sodium-dependent processes. Cellular uptakes of phosphate and alanine were decreased by about 80% by external sodium substitution, inhibited by ouabain (30 and 41%, respectively) and displayed saturable kinetics. Two sodium-phosphate cotransport systems were characterized: a high-affinity one (apparent Km = 18 microM) with a Vmax of 13.5 nmol/mg protein per 10 min and a low-affinity one (apparent Km = 126 microM) with a Vmax of 22.5 nmol/mg protein per 10 min. Alanine transport had an apparent Km of 87.9 microM and a Vmax of 43.5 nmol/mg protein per 10 min. By contrast, cultured alveolar type II cells did not express sodium-dependent hexose transport. Increasing time in culture decreased Vmax values of the two phosphate transport systems on day 4 while sodium-dependent alanine uptake was unchanged. This study demonstrated the existence of sodium-dependent phosphate and amino acid transports in alveolar type II cells similar to those documented in other epithelial cell types. These sodium-coupled transports provide a potent mechanism for phosphate and amino acid absorption and are likely to play a role in substrate availability for cellular metabolism and in regulating the composition of the alveolar subphase. The decrease in phosphate uptake with time in culture is parallel to decrease in surfactant synthesis reported in cultured alveolar type II cells, suggesting that phosphate availability for surfactant synthesis may be accomplished by a sodium-dependent phosphate uptake.
无机磷酸盐、氨基酸和糖类在肺代谢中具有明显的重要性。我们研究了培养一天后的成年大鼠II型肺泡上皮细胞中这些有机和无机底物的钠偶联转运。II型肺泡细胞通过钠依赖性过程主动转运无机磷酸盐和丙氨酸（一种中性氨基酸）。外部钠替代使磷酸盐和丙氨酸的细胞摄取量降低约80%，哇巴因抑制摄取（分别为30%和41%），并且呈现出饱和动力学。鉴定出两种钠-磷酸盐共转运系统：一种高亲和力系统（表观Km = 18 μM），每10分钟的Vmax为13.5 nmol/mg蛋白质，以及一种低亲和力系统（表观Km = 126 μM），每10分钟的Vmax为22.5 nmol/mg蛋白质。丙氨酸转运的表观Km为87.9 μM，每10分钟的Vmax为43.5 nmol/mg蛋白质。相比之下，培养的II型肺泡细胞不表达钠依赖性己糖转运。培养时间的延长使第4天的两种磷酸盐转运系统的Vmax值降低，而钠依赖性丙氨酸摄取未改变。本研究证明了II型肺泡细胞中存在钠依赖性磷酸盐和氨基酸转运，类似于其他上皮细胞类型中所记录的情况。这些钠偶联转运为磷酸盐和氨基酸吸收提供了一种有效机制，并且可能在细胞代谢的底物可用性以及调节肺泡亚相的组成中发挥作用。培养过程中磷酸盐摄取随时间的减少与培养的II型肺泡细胞中报道的表面活性剂合成减少平行，表明表面活性剂合成的磷酸盐可用性可能通过钠依赖性磷酸盐摄取来实现。
