Pregnancy can influence both the resting membrane potential and the ion channel composition of the uterine myometrium. Calcium flux is essential for excitation-contraction coupling in pregnant uterus. The uterine L-type calcium channel is an important component in mediating calcium flux and is purported to play a role in parturition. This study was undertaken to characterize gestational changes in 1) the uterine contractile response to the L-type calcium channel agonist, Bay K 8644; 2) the mRNA expression of channel subunits by semiquantitative reverse transcriptase-polymerase chain reaction; and 3) estimate channel protein levels by measuring 3H-isradipine binding at the dihydropyridine binding site of the α1c subunit utilizing saturation binding methods. Sensitivity to Bay K 8644 increases beginning at 0.8 of gestation and persists through term. The change in sensitivity is coincident with an increased mRNA expression of the α1c and β2 subunits but with the least detectable amounts of isradipine binding. The expressed α1c transcript represents a novel structural variant with a 118-amino acid deletion in the III–IV linker and repeats IVS1–S3 of the protein sequence. The guinea pig uterine L-type calcium channel activity is highly regulated through gestation, but the regulation of mRNA expression may be different from regulation of protein levels, estimated by isradipine binding. The up-regulation of function, α1c subunit mRNA expression, and isradipine binding at term gestation are consistent with a role for this ion channel in parturition.
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