Contribution of ryanodine receptor subtype 3 to Ca2+ responses in Ca2+-overloaded cultured rat portal vein myocytes

Using an antisense strategy, we have previously shown that in vascular myoc ytes, subtypes 1 and 2 of ryanodine receptors (RYRs) are required for Ca2release during Ca2+ sparks and global Ca2+ responses, evoked by activation of voltage-gated Ca2+ channels, whereas RYR subtype 3 (RYR3) has no contrib ution. Here, we investigated the effects of increased Ca2+ loading of the s arcoplasmic reticulum (SR) on the RYR-mediated Ca2+ responses and the role of the RYR3 by injecting antisense oligonucleotides targeting the RYR subty pes, RYR3 expression was demonstrated by immunodetection in both freshly di ssociated and cultured rat portal vein myocytes, Confocal Ca2+ measurements revealed that the number of cells showing spontaneous Ca2+ sparks was stro ngly increased by superfusing the vascular myocytes in 10 mM Ca2+-containin g solution. These Ca2+ sparks were blocked after inhibition of RYR1 or RYR2 by treatment with antisense oligolucleotides but not after inhibition of R YR3, In contrast, inhibition of RYR3 reduced the global Ca2+ responses indu ced by caffeine and phenylephrine, indicating that RYR3 participated togeth er with RYR1 and RYR2 to these Ca2+ responses in Ca2+-overloaded myocytes. Ca2+ transients evoked by photolysis of caged Ca2+ with increasing flash in tensities were also reduced after inhibition of RYR3 and revealed that the [Ca2+](i) sensitivity of RYR3 would be similar to that of RYR1 and RYR2. Ou r results show that, under conditions of increased SR Ca2+ loading, the RYR 3 becomes activable by caffeine and local increases in [Ca2+](i).