Pregnant rat myometrial cells show heterogeneous ryanodine- and caffeine-sensitive calcium stores.

Intracellular Ca2+ release channels such as ryanodine receptors play crucial roles in the Ca2+-mediated signaling that triggers excitation-contraction coupling in muscles. Although the existence and the role of these channels are well characterized in skeletal and cardiac muscles, their existence in smooth muscles, and more particularly in the myometrium, is very controversial. We have now clearly demonstrated the expression of ryanodine receptor Ca2+ release channels in rat myometrial smooth muscle, and for the first time, intracellular Ca2+ concentration experiments with indo 1 on single myometrial cells have revealed the existence of a functional ryanodine- and caffeine-sensitive Ca2+ release mechanism in 30% of rat myometrial cells. RT-PCR and RNase protection assay on whole myometrial smooth muscle demonstrate the existence of all three ryr mRNAs in the myometrium: ryr3 mRNA is the predominant subtype, with much lower levels of expression for ryr1 and ryr2 mRNAs, suggesting that the ryanodine Ca2+ release mechanism in rat myometrium is largely encoded by ryr3. Moreover, using intracellular Ca2+ concentration measurements and RNase protection assays, we have demonstrated that the expression, the percentage of cells responding to ryanodine, and the function of these channels are not modified during pregnancy.