Russo L M, Marano C W, Hagee M M, Laughlin K V, Guy A, Varimbi S, Mullin J M
Lankenau Medical Research Center, Wynnewood, PA 19096, USA.
Biochim Biophys Acta. 1995 May 24;1236(1):15-22. doi: 10.1016/0005-2736(95)00045-5.
In addition to the concentrative, Na(+)-dependent inositol transport system demonstrated in many cell types, carrier-mediated, Na(+)-independent inositol transport is also shown to exist in LLC-PK1 renal epithelia. Inhibition of inositol uptake in Na(+)-free saline by 0.1 mM phloretin, and self-inhibition by net concentrations of inositol exceeding 10 mM, demonstrate the carrier-mediation of the Na(+)-independent uptake and distinguish it from flux through anion channels. The Na(+)-dependent uptake exhibits higher affinity for inositol, as seen by the stronger self-inhibition at lower inositol concentrations in Na+ saline. Kinetic analyses indicate a Km of 178 microM and a Vmax of 2447 pmol/min per microgram DNA for the Na(+)-dependent system, whereas the lower affinity, lower capacity Na(+)-independent system manifests a Km of 5.2 mM and a Vmax of 249 pmol/min per microgram DNA. the Na(+)-independent uptake further differs from the Na(+)-dependent transport by the lack of inhibitory effect of 10 microM glucose, and the greater relative inhibition of phloretin compared to that of phlorizin. Both types of uptake appear to localize predominantly to the basal-lateral cell surface. The Na(+)-independent transport is bidirectional, functioning in efflux as well as influx of inositol.
除了在许多细胞类型中所证实的、依赖钠离子的肌醇浓缩转运系统外，在LLC-PK1肾上皮细胞中还存在载体介导的、不依赖钠离子的肌醇转运。0.1 mM根皮素对无钠盐溶液中肌醇摄取的抑制作用，以及净浓度超过10 mM的肌醇的自我抑制作用，证明了不依赖钠离子摄取的载体介导作用，并将其与通过阴离子通道的通量区分开来。如在含钠盐溶液中较低肌醇浓度下更强的自我抑制作用所示，依赖钠离子的摄取对肌醇表现出更高的亲和力。动力学分析表明，依赖钠离子的系统的Km为178 microM，Vmax为每微克DNA 2447 pmol/分钟，而亲和力较低、容量较小的不依赖钠离子的系统的Km为5.2 mM，Vmax为每微克DNA 249 pmol/分钟。不依赖钠离子的摄取与依赖钠离子的转运的进一步不同之处在于，10 microM葡萄糖没有抑制作用，且根皮素的相对抑制作用比根皮苷更大。两种摄取类型似乎主要定位于基底外侧细胞表面。不依赖钠离子的转运是双向的，在肌醇的外流和内流中均起作用。
