Changes in transcripts encoding calcium channel subunits of rat myometrium during pregnancy

Extracellular Ca2+ is normally required for myometrial cells to contract. Ca2+ enters muscle cells mainly through voltage-dependent Ca2+ channels (VDCCs) that open in response to action potentials. The synthesis of myometrial VDCCs may change during pregnancy to alter excitation-contraction coupling. We investigated the mRNA levels for the alpha 1- and beta-subunits of the L-type VDCC in rat myometrium to determine whether alterations are associated with term or preterm labor. RNA isolated from myometrial tissues was analyzed by reverse transcription-polymerase chain reaction (PCR) using specific primers designed according to the published sequences of the VDCC subunits. From pregnant rat myometrium, two distinct PCR products were obtained for the alpha 1-subunit: one of the expected size at 372 bp and a smaller at 339 bp. Sequence analysis of the larger product revealed a 99.5 or 88% sequence homology between rat myometrium and rat aorta or rabbit heart, respectively, and the smaller product had an identical sequence to a 33-bp deletion. The two alpha 1-products followed the same trend throughout pregnancy. VDCC alpha 1-mRNA levels increased gradually to 6.9-fold just before labor on day 22 but decreased during labor. However, the beta-subunit mRNA level increased sharply on day 22 and then also declined during labor. Progesterone treatment from day 19 to day 22 inhibited term delivery and prevented the significant increase in alpha 1-mRNA levels. In contrast, antiprogesterone (onapristone, ZK-98.299) treatment on day 17 caused a statistically significant increase in the alpha 1- and beta-VDCC subunit mRNA after 8 and 15 h, respectively, then a decrease during preterm labor at 24 h. We conclude that mRNA levels for the VDCC subunits increase before term and preterm labor but decline during periods when VDCCs are likely at their peaks. The increase in levels of mRNA for VDCC likely reflects changes in expression of VDCCs during periods of term and preterm labor that may facilitate uterine contractility required for this process. Progesterone withdrawal or blockade appears to be responsible for regulating levels of mRNA for VDCC in the myometrium in preparation for labor.