Changes in the expression of the L-type voltage-dependent calcium channel during pregnancy and parturition in the rat

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.