Mershon J L, Mikala G, Schwartz A
Department of Pharmacology and Cell Biophysics, University of Cincinnati Medical Center, Ohio 45267.
Biol Reprod. 1994 Nov;51(5):993-9. doi: 10.1095/biolreprod51.5.993.
The L-type voltage-dependent calcium channel (L-VDCC) is assumed to be a critical component of excitation-contraction coupling in smooth muscle. Using pregnant rat myometrium, we examined the hypothesis that parturition is associated with significant changes in the expression of the alpha 1 subunit of the L-VDCC at the mRNA or protein level. The binding of radiolabeled dihydropyridine, which correlates with the total number of calcium channels in the membrane, was increased by 14 days' gestation, in comparison to that in nonpregnant controls. The elevation in binding capacity persisted through labor and fell postpartum. Northern and RNA dot-blot analysis demonstrated the highest level of expression on Days 20 and 21, with a 3- to 10-fold decrease during parturition. We believe these studies are most consistent with a one-day lag time between mRNA and protein expression, and generally support a modest increase in L-VDCC expression in pregnancy and labor. Reverse transcriptase polymerase chain reaction was used to examine changes in isoform expression in Motif IV, a region of the alpha 1 subunit known to be alternatively spliced. These studies revealed the presence of multiple isoforms in rat myometrium, with a predominance of IVS3B. Interestingly, a marked increase in the ratio of S3B:S3A was noted at parturition. In summary, these data demonstrate that the number of L-type calcium channels, although increased in pregnancy, do not change prior to, or with the onset of, myometrial contraction. Intriguingly, mRNA expression was markedly decreased at parturition. The change in isoform expression during labor is of unknown, but potential, physiologic significance.
L型电压依赖性钙通道（L-VDCC）被认为是平滑肌兴奋-收缩偶联的关键组成部分。我们利用妊娠大鼠子宫肌层，检验了分娩与L-VDCCα1亚基在mRNA或蛋白质水平表达的显著变化相关这一假说。与未孕对照组相比，妊娠14天时，与膜中钙通道总数相关的放射性标记二氢吡啶的结合增加。结合能力的升高持续至分娩，并在产后下降。Northern印迹和RNA斑点印迹分析表明，在第20天和第21天表达水平最高，在分娩期间下降3至10倍。我们认为这些研究最符合mRNA和蛋白质表达之间一天的滞后时间，总体上支持妊娠和分娩期间L-VDCC表达适度增加。逆转录聚合酶链反应用于检测α1亚基已知存在可变剪接的基序IV中同工型表达的变化。这些研究揭示大鼠子宫肌层中存在多种同工型，以IVS3B为主。有趣的是，在分娩时观察到S3B:S3A的比例显著增加。总之，这些数据表明，L型钙通道的数量虽然在妊娠时增加，但在子宫肌层收缩之前或开始时并未改变。有趣的是，分娩时mRNA表达显著下降。分娩期间同工型表达的变化具有未知但潜在的生理意义。
