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Gestational Changes in Uterine L-Type Calcium Channel Function and Expression in Guinea Pig¹

^a Departments of Obstetrics/Gynecology, ^b Pediatrics, ^c Biochemistry, and ^d the Rammelcamp Center for Research, MetroHealth Campus, Case Western Reserve University, Cleveland, Ohio 44109 ^e Departments of Obstetrics/Gynecology, ^f Cell Biology, Neurobiology and Anatomy, and ^g The Cardiovascular Research Institute, Loyola University Medical Center, Maywood, Illinois 60153

ABSTRACT

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 _1c 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 _1c and ß₂ subunits but with the least detectable amounts of isradipine binding. The expressed _1c 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, _1c subunit mRNA expression, and isradipine binding at term gestation are consistent with a role for this ion channel in parturition.

FOOTNOTES

First decision: 2 March 2000.

¹ This study was funded in part by March of Dimes grants 6-1070 and 6-0247 (to P.L.C.) and 6-0649 and 6-0609 (to P.L.C. and A.S.C.) and by the NIH/NICHD HD-28433 (to P.L.C.). This work was presented in part in abstract form at The Society for Gynecologic Investigation 43rd annual meetings in Philadelphia, Pennsylvania.

² Correspondence: Patricia L. Collins, Department of Obstetrics and Gynecology, Division of Maternal Fetal Medicine, Loyola University Medical Center, 2160 South 1st Ave., Maywood, IL 60153. FAX: 708 216 5669; pcollin{at}luc.edu

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