Borgs M, Bollen M, Keppens S, Yap S H, Stalmans W, Vanstapel F
Biomedical NMR Unit, Department of Radiology, Faculty of Medicine, Katholieke Universiteit Leuven, Belgium.
Hepatology. 1996 Jun;23(6):1564-71. doi: 10.1002/hep.510230637.
We perfused livers from fed rats with a balanced salt solution containing 1 mmol/L glucose. Under these conditions a low steady rate of glycogenolysis was observed (approximately 1.7 micromol glucose equivalents/g/min; 20% of the maximal glycogenolytic activity). Nitric oxide (NO) transiently stimulated hepatic glucose production. A maximal response (on average doubling basal glucose output) was observed with 34 micromol/L NO. The same concentration of nitrite (NO2-) was ineffective. Half-maximal effects were seen at 8 to 10 micromol/L NO, irrespective of the flow direction (portocaval or retrograde). This glycogenolytic response to NO corresponded to a partial activation of phosphorylase. The NO effect was not additive to maximal stimulation of glycogenolysis (7.7 +/- 0.2 micromol hexose equivalents/g/min; n = 4) by 100 micromol/L dibutyryl cyclic adenosine monophosphate (Bt2cAMP). The requirement for activation of phosphorylase was also evidenced by the ineffectiveness of NO in phosphorylase-kinase-deficient livers of gsd/gsd rats. The NO effect was blocked by co-administration of cyclooxygenase inhibitors (50 micromol/L ibuprofen, 50 micromol/L indomethacin, or 2 mmol/L aspirin), suggesting a mediatory role of prostanoids from nonparenchymal cells. This conclusion was confirmed by the fact that NO did not activate phosphorylase in isolated hepatocytes. Moreover, NO was no longer glycogenolytic in livers perfused with Ca2+-free medium, in agreement with the known mediatory role of Ca2+ in prostanoid-mediated responses. Surprisingly, in Ca2+-free medium NO inhibited the basal glucose production. This coincided with an increased elution of cyclic guanosine monophosphate (cGMP). Inhibition of glycogenolysis by NO under these conditions was blocked by 1 mmol/L theophylline, suggestive for involvement of cGMP-stimulated cAMP phosphodiesterase. However, we could not confirm that an increase in cGMP resulted in a drop in cAMP. In conclusion, NO recruits opposing mechanisms with respect to modulation of basal hepatic glycogenolysis. In the presence of Ca2+, activation of phosphorylase with stimulation of glycogenolysis dominates. Cyclooxygenase inhibitors abolish this effect. Activation by NO of the cyclooxygenase in nonparenchymal cells is a distinct possibility. In the absence of Ca2+, inhibition of basal glycogenolysis becomes observable. It remains to be established whether this results from cGMP-mediated stimulation of hydrolysis of cAMP.
我们用含有1 mmol/L葡萄糖的平衡盐溶液灌注喂食大鼠的肝脏。在这些条件下，观察到糖原分解的稳定低速率（约1.7微摩尔葡萄糖当量/克/分钟；最大糖原分解活性的20%）。一氧化氮（NO）短暂刺激肝脏葡萄糖生成。在34微摩尔/升NO时观察到最大反应（平均使基础葡萄糖输出量加倍）。相同浓度的亚硝酸盐（NO2-）无效。无论血流方向如何（门静脉-腔静脉或逆行），在8至10微摩尔/升NO时出现半数最大效应。这种对NO的糖原分解反应对应于磷酸化酶的部分激活。NO的作用与100微摩尔/升二丁酰环磷酸腺苷（Bt2cAMP）对糖原分解的最大刺激作用（7.7±0.2微摩尔己糖当量/克/分钟；n = 4）无相加性。在gsd/gsd大鼠的磷酸化酶激酶缺陷肝脏中，NO无效也证明了激活磷酸化酶的必要性。NO的作用被同时给予环氧化酶抑制剂（50微摩尔/升布洛芬、50微摩尔/升吲哚美辛或2毫摩尔/升阿司匹林）所阻断，提示非实质细胞中前列腺素的介导作用。这一结论得到以下事实的证实：NO在分离的肝细胞中不激活磷酸化酶。此外，在无钙培养基灌注的肝脏中，NO不再具有糖原分解作用，这与Ca2+在前列腺素介导反应中的已知介导作用一致。令人惊讶的是，在无钙培养基中，NO抑制基础葡萄糖生成。这与环磷酸鸟苷（cGMP）洗脱增加相吻合。在这些条件下，NO对糖原分解的抑制作用被1毫摩尔/升茶碱阻断，提示cGMP刺激的cAMP磷酸二酯酶参与其中。然而，我们无法证实cGMP增加会导致cAMP下降。总之，在调节基础肝脏糖原分解方面，NO引发了相反的机制。在有Ca2+存在时，磷酸化酶的激活与糖原分解的刺激占主导。环氧化酶抑制剂消除这种作用。NO激活非实质细胞中环氧化酶是一种明显的可能性。在无Ca2+时，基础糖原分解的抑制变得明显。这种情况是否由cGMP介导的cAMP水解刺激所致仍有待确定。
