CHANGES IN TRANSCRIPTS ENCODING CALCIUM-CHANNEL SUBUNITS OF RAT MYOMETRIUM DURING PREGNANCY

Extracellular Ca2+ is normally required for myometrial cells to contra ct. Ca2+ enters muscle cells mainly through voltage-dependent Ca2+ cha nnels (VDCCs) that open in response to action potentials. The synthesi s 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 iso lated from myometrial tissues was analyzed by reverse transcription-po lymerase chain reaction (PCR) using specific primers designed accordin g to the published sequences of the VDCC subunits. From pregnant rat m yometrium, two distinct PCR products were obtained for the alpha(1)-su bunit: one of the expected size at 372 bp and a smaller at 339 bp. Seq uence analysis of the larger product revealed a 99.5 or 88% sequence h omology between rat myometrium and rat aorta or rabbit heart, respecti vely, and the smaller product had an identical sequence to a 33-bp del etion. The two alpha(1)-products followed the same trend throughout pr egnancy. VDCC alpha(1)-mRNA levels increased gradually to 6.9-fold jus t 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 l evels. In contrast, antiprogesterone (onapristone, ZK-98.299) treatmen t 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 d ecrease 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 l evels of mRNA for VDCC likely reflects changes in expression of VDCCs during periods of term and preterm labor that may facilitate uterine c ontractility required for this process. Progesterone withdrawal or blo ckade appears to be responsible for regulating levels of mRNA for VDCC in the myometrium in preparation for labor.