Bauernschmitt H G, Kinne R K
Max-Planck-Institut für molekulare Physiologie, Dortmund, Germany.
Biochim Biophys Acta. 1993 Jul 25;1150(1):25-34. doi: 10.1016/0005-2736(93)90117-i.
In isolated inner medullary collecting duct (IMCD) cells requirements for the organic osmolyte glycerophosphorylcholine (GPC) vary with extracellular osmolality. To investigate mechanisms of osmotic adaptation GPC metabolism was studied under different osmotic conditions. In contrast to the GPC precursors choline and phosphatidylcholine (PC) cellular GPC was proportional to the osmolality. Hypotonic decrease in cellular GPC was mediated by fast initial release significantly exceeding the low hypertonic release. In long-term studies the total amount of GPC decreased significantly under hypotonic conditions but remained constant under hypertonic conditions resulting in a significant difference after 15 h. To investigate osmotic influences on GPC synthesis and GPC degradation studies with [methyl-3H]choline were performed. Pulse-chase experiments displayed no significant osmotic differences in PC synthesis or in PC degradation to GPC indicated by a similar specific activity of PC. This suggested that phospholipase A2 (PC degradation) was osmotically insensitive. A small and distinct metabolic PC pool may be responsible for high radioactive labeling of newly synthesized GPC which displayed a significantly higher specific activity under hypotonic conditions accompanied by a decrease in GPC amount. Therefore, a higher activity of glycerophosphorylcholine:choline phosphodiesterase (GPC:choline phosphodiesterase) (GPC degradation) under hypotonic conditions is proposed. Similar conclusions can be drawn from using phosphatidyl[methyl-3H]choline. As further evidence for osmotic regulation of GPC:choline phosphodiesterase the specific activity of choline displayed a significant hypotonic increase with chase time which may be equivalent to increased GPC degradation. Therefore, the in vitro experiments suggest that cellular GPC is regulated by an osmosensitive GPC:choline phosphodiesterase. Such a regulation also seems to be present during long-term in vivo experiments. No evidence was found for a genetic adaptation of GPC:choline phosphodiesterase in vivo.
在分离的髓质内层集合管（IMCD）细胞中，有机渗透溶质甘油磷酸胆碱（GPC）的需求随细胞外渗透压而变化。为了研究渗透适应机制，在不同的渗透条件下对GPC代谢进行了研究。与GPC前体胆碱和磷脂酰胆碱（PC）不同，细胞内GPC与渗透压成正比。细胞内GPC的低渗降低是由快速的初始释放介导的，该释放显著超过低渗时的释放。在长期研究中，低渗条件下GPC的总量显著下降，但高渗条件下保持恒定，15小时后导致显著差异。为了研究渗透压对GPC合成和GPC降解的影响，进行了用[甲基-3H]胆碱的研究。脉冲追踪实验显示，PC合成或PC降解为GPC时，渗透压没有显著差异，PC的比活性相似。这表明磷脂酶A2（PC降解）对渗透压不敏感。一个小的且独特的代谢性PC池可能是新合成的GPC高放射性标记的原因，在低渗条件下新合成的GPC比活性显著更高，同时GPC量减少。因此，提出在低渗条件下甘油磷酸胆碱：胆碱磷酸二酯酶（GPC：胆碱磷酸二酯酶）（GPC降解）具有更高的活性。使用磷脂酰[甲基-3H]胆碱也可得出类似结论。作为GPC：胆碱磷酸二酯酶受渗透压调节的进一步证据，胆碱的比活性在追踪过程中随时间显示出显著的低渗增加，这可能等同于GPC降解增加。因此，体外实验表明细胞内GPC受渗透压敏感的GPC：胆碱磷酸二酯酶调节。这种调节在长期体内实验中似乎也存在。未发现体内GPC：胆碱磷酸二酯酶有遗传适应性的证据。
