Ryanodine receptor type 3 does not contribute to contractions in the mouse myometrium regardless of pregnancy.

Ryanodine receptor type 3 (RyR3) is expressed in myometrial smooth muscle cells (MSMCs). The short isoform of RyR3 is a dominant negative variant (DN-RyR3) and negatively regulates the functions of RyR2 and full-length (FL)-RyR3. DN-RyR3 has been suggested to function as a major RyR3 isoform in non-pregnant (NP) mouse MSMCs, and FL-RyR3 may also be upregulated during pregnancy (P). This increase in the FL-RyR3/DN-RyR3 ratio may contribute to the strong contractions by MSMCs for parturition. In the present study, spontaneous contractions by the myometrium in NP and P mice were highly susceptible to nifedipine but were not affected by ryanodine. Ca image analyses under a voltage clamp revealed that the influx of Ca through voltage-dependent Ca channels did not cause the release of Ca from the sarcoplasmic reticulum (SR). Cytosolic Ca concentrations ([Ca]) in MSMCs were not affected by caffeine. Despite the abundant expression of large conductance Ca-activated K channels in MSMCs, spontaneous transient outward currents were not observed in the resting state because of the substantive lack of Ca sparks. Quantitative PCR and Western blot analyses indicated that DN-RyR3 was strongly expressed in the NP myometrium, while the expression of FL-RyR3 and DN-RyR3 was markedly reduced in the P myometrium. The messenger RNA (mRNA) expression of RyR2 and RyR1 was negligible in the NP and P myometria. Moreover, RyR3 knockout mice may become pregnant and deliver normally. Thus, we concluded that none of the RyR subtypes, including RyR3, play a significant role in the regulation of [Ca] in or contractions by mouse MSMCs regardless of pregnancy.