Alpers D H, Philpott G W
Gastroenterology. 1975 Oct;69(4):951-9.
Although cyclic adenosine 3':5'-monophosphate (cyclic AMP, cAMP) is known to suppress DNA synthesis is cultured cells and experimental tumors, its role in normal intact tissue has been little explored. This study helps to define the influence of modifiers of cyclic AMP levels on DNA synthesis in rabbit colonic mucosa maintained in short term organ culture system. Base line studies showed that incorporation of [3H]thymidine into DNA was linear for 24 hr and predominantly in mucosal cells, as shown by autoradiography. Colon from a normal fed rabbit showed a gradient of DNA synthesis, lowest in the cecum and increasing to a maximum, 3-fold greater, at the splenic flexure. This pattern was obliterated by fasting, at which time no formed stool remained in the colon, and all colon mucosa incorporated thymidine at the lower level of the right colon. Known modifiers of intracellular cAMP were found to depress colonic DNA synthesis. Theophylline inhibited DNA synthesis by 35% at 0.5 mM concentration and increased intracellular cAMP levels. This inhibition took 10 hr to be manifest and was at least partly reversible. It was by far the most active of the methylxanthines, consistent with its potency as a phosphodiesterase inhibitor. N6,02-dibutyryl cyclic AMP inhibited DNA synthesis at concentrations as low as 0.025 mM, whereas adenosine and sodium butyrate were ineffective up to 1.0 mM. 5'-AMP did inhibit DNA synthesis, but only at 0.1 mM or higher and did not elevate intracellular cAMP levels. Other modifiers of cAMP which were effective included prostaglandins E1, E2, and F2alpha (2 times 10(-6) M) and papaverine (1 muM). Thymidine uptake was not affected by any of these drugs. The intrinsic thymidine pool was estimated at 20 muM by isotope dilution, and was not altered by theophylline. DNA synthesis in rabbit colon can be suppressed by increased cAMP levels within the time period allowed by organ culture. Thus, these drugs that elevated cAMP levels did not seem to suppress DNA synthesis by decreasing intracellular thymidine concentrations.
尽管已知环磷腺苷（cAMP）可抑制培养细胞和实验性肿瘤中的DNA合成，但其在正常完整组织中的作用却鲜有研究。本研究有助于明确环磷腺苷水平调节剂对短期器官培养系统中兔结肠黏膜DNA合成的影响。基线研究表明，[3H]胸苷掺入DNA的过程在24小时内呈线性，且主要发生在黏膜细胞中，放射自显影显示了这一点。正常喂食兔子的结肠显示出DNA合成梯度，盲肠中最低，在脾曲处增加到最大值，是盲肠的3倍。禁食会消除这种模式，此时结肠中没有成形的粪便残留，所有结肠黏膜在右结肠较低水平掺入胸苷。已知的细胞内cAMP调节剂被发现可抑制结肠DNA合成。茶碱在0.5 mM浓度下可抑制DNA合成35%，并提高细胞内cAMP水平。这种抑制作用需要10小时才能显现，且至少部分是可逆的。它是迄今为止最活跃的甲基黄嘌呤，与其作为磷酸二酯酶抑制剂的效力一致。N6,02-二丁酰环磷腺苷在低至0.025 mM的浓度下即可抑制DNA合成，而腺苷和丁酸钠在高达1.0 mM时均无效。5'-AMP确实可抑制DNA合成，但仅在0.1 mM或更高浓度时有效，且不会提高细胞内cAMP水平。其他有效的cAMP调节剂包括前列腺素E1、E2和F2α（2×10(-6) M）以及罂粟碱（1 μM）。这些药物均未影响胸苷摄取。通过同位素稀释法估计内源性胸苷池为20 μM，且不受茶碱影响。在器官培养允许的时间段内，兔结肠中的DNA合成可被升高的cAMP水平所抑制。因此，这些升高cAMP水平的药物似乎并非通过降低细胞内胸苷浓度来抑制DNA合成。
