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Capacitative Cation Entry in Human Myometrial Cells and Augmentation by hTrpC3 Overexpression¹

Department of Biochemistry and Molecular Biology³ Department of Pediatrics,⁴ University of Texas Medical School at Houston, Houston, Texas 77030

Transient receptor potential (Trp) channels have been implicated in mediating store- and receptor-activated Ca²⁺ influx. Different properties of this influx in various cell types may stem from the assembly of these Trp proteins into homo- or heterotetramers or association with other regulatory proteins. We examined the properties of endogenous capacitative Ca²⁺ entry in PHM1 immortalized human myometrial cells that express endogenous hTrpCs 1, 3, 4, 6, and 7 mRNA and in primary human myocytes. In PHM1 cells, activation of the oxytocin receptor or depletion of intracellular Ca²⁺ stores with the endoplasmic reticulum calcium pump-inhibitor thapsigargin induced capacitative Ca²⁺ entry, which was inhibited both by SKF 96365 and gadolinium (Gd³⁺). Whereas unstimulated cells did not exhibit Sr²⁺ entry, oxytocin and thapsigargin enhanced Sr²⁺ entry that was also inhibited by SKF 96365 and Gd³⁺. In contrast, Ba²⁺, a poor substrate for Ca²⁺ pumps, accumulated in these cells in the absence of the capacitative entry stimulus and also after oxytocin and thapsigargin treatment. Both types of entry were markedly decreased by SKF 96365 and Gd³⁺. The membrane-permeant derivative of diacylglycerol, 1-oleoyl-2-acetyl-sn-glycerol (OAG), elicited oscillatory increases in PHM1 intracellular Ca²⁺ that were dependent on extracellular Ca²⁺. These properties were also observed in primary human myocytes. Overexpression of hTrpC3 in PHM1 cells enhanced thapsigargin-, oxytocin-, and OAG-induced Ca²⁺ entry. These data are consistent with the expression of endogenous hTrpC activity in myometrium. Capacitative Ca²⁺ entry can potentially contribute to Ca²⁺ dynamics controlling uterine smooth muscle contractile activity.

¹ Supported in part by NIH-HD38970 and a Lalor Foundation Fellowship (M.Y.). S.G.S. and M.Y. contributed equally to this work.

² Correspondence: Barbara M. Sanborn, Department of Biomedical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523. FAX: 970 491 7569; barbara.sanborn{at}colostate.edu