Ryanodine receptors are uncoupled from contraction in rat vena cava

Ryanodine receptors (RyR) are Ca2+-sensitive ion channels in the sarcoplasmic reticulum (SR) membrane, and are important effectors of SR Ca2+ release and smooth muscle excitation-contraction coupling. While the relationship between RyR activation and contraction is well characterized in arteries, little is known about the role of RyR in excitation-contraction coupling in veins. We hypothesized that RyR are present and directly coupled to contraction in rat aorta (RA) and vena cava (RVC). RA and RVC expressed mRNA for all 3 RyR subtypes, and immunofluorescence showed RyR protein was present in RA and RVC smooth muscle cells. RA and RVC rings contracted when Ca2+ was re-introduced after stores depletion with thapsigargin (1 μM), indicating both tissues contained intracellular Ca2+ stores. To assess RyR function, contraction was then measured in RA and RVC exposed to the RyR activator caffeine (20 mM). In RA, caffeine caused contraction that was attenuated by the RyR antagonists ryanodine (10 μM) and tetracaine (100 μM). However, caffeine (20 mM) did not contract RVC. We next measured contraction and intracellular Ca2+ (Ca2+i) simultaneously in RA and RVC exposed to caffeine. While caffeine increased Ca2+i and contracted RA, it had no significant effect on Ca2+i or contraction in RVC. These data suggest that ryanodine receptors, while present in both RA and RVC, are inactive and uncoupled from Ca2+ release and contraction in RVC.