Abstract
Physiological impacts of type 3 ryanodine receptors (RyR3) in smooth muscle (SM) tissues are not well understood, in spite of their wide expression. However, the short isoform of RyR3 is known to be a dominant negative variant (DN-RyR3), which may negatively regulate functions of both RyR2 and full length (FL)-RyR3 by forming hetero-tetramers. Here, functional roles of RyR3 in the regulation of Ca2+ signaling in mesenteric artery SM cells (MASMCs) were examined using RyR3 homozygous knockout mice (RyR3-/-). The predominant RyR3 subtype in MA from wild-type mice (RyR3+/+) was DN-RyR3, while RyR2 was the major RyR subtype. The Ca2+ sparks and spontaneous transient outward currents (STOCs) were enhanced in MASMCs from RyR3-/-. In the presence of BK channel inhibitor, paxilline, the pressure rises by BayK8644 in MA vascular beds of RyR3-/- were larger than in RyR3+/+. This indicates that the negative feedback effects of BK channel activity on intracellular Ca2+ signaling was enhanced in RyR3-/-. Thus, RyR3, and mainly DN-RyR3, presumably via a complex with RyR2 suppresses Ca2+ release and indirectly regulated membrane potential by reducing BK channel activity in MASMCs. In the series of studies, we unexpectedly found that RyR3-/- mice become pregnant and deliver normally, because the increase in the FL-RyR3/DN-RyR3 ratio in myometrial SMCs (MySMCs) of pregnant (P) mouse was suggested to contribute to the strong contractions for parturition. In non-pregnant (NP) mouse MySMCs, DN-RyR3 was suggested to function as a major RyR3 isoform and FL-RyR3 may also be up-regulated during P. Here, spontaneous contractions in the myometrium from NP- and P-mice were neither affected by ryanodine nor caffeine. STOCs were not observed because of the lack of Ca2+ sparks. DN-RyR3 protein was highly expressed in the NP-myometrium, while the expression of FL-RyR3 and DN-RyR3 was reduced in the P-myometrium. The mRNA expression of RyR2 and RyR1 was negligible in the NP- and P-myometria. Thus, none of the RyR subtypes, including RyR3, play a significant role for Ca2+ signaling in mouse MySMCs regardless of pregnancy. In conclusion, RyR3, particularly DN-RyR3, can affect to the tone regulation in various types of SMCs, but exceptionally not in MySMCs.
