Background and purpose: The sarcoplasmic reticulum (SR), regulates the cytoplasmic Ca(2+) concentration ([Ca(2+)](cyto)) in vascular smooth muscle. Release from the SR is controlled by two intracellular receptor/channel complexes, the ryanodine receptor (RyR) and the inositol 1,4,5-trisphosphate receptor (IP(3)R). These receptors may be regulated by the accessory FK506-binding protein (FKBP) either directly, by binding to the channel, or indirectly via FKBP modulation of two targets, the phosphatase, calcineurin or the kinase, mammalian target of rapamycin (mTOR).
Experimental approach: Single portal vein myocytes were voltage-clamped in whole cell configuration and [Ca(2+)](cyto) measured using fluo-3. IP(3)Rs were activated by photolysis of caged IP(3) and RyRs activated by hydrostatic application of caffeine.
Key results: FK506 which displaces FKBP from each receptor (to inhibit calcineurin) increased the [Ca(2+)](cyto) rise evoked by activation of either RyR or IP(3)R. Rapamycin which displaces FKBP (to inhibit mTOR) also increased the amplitude of the caffeine-evoked, but reduced the IP(3)-evoked [Ca(2+)](cyto) rise. None of the phosphatase inhibitors, cypermethrin, okadaic acid or calcineurin inhibitory peptide, altered either caffeine- or IP(3)-evoked [Ca(2+)](cyto) release; calcineurin did not contribute to FK506-mediated potentiation of RyR- or IP(3)R-mediated Ca(2+) release. The mTOR inhibitor LY294002, like rapamycin, decreased IP(3)-evoked Ca(2+) release.
Conclusions and implications: Ca(2+) release in portal vein myocytes, via RyR, was modulated directly by FKBP binding to the channel; neither calcineurin nor mTOR contributed to this regulation. However, IP(3)R-mediated Ca(2+) release, while also modulated directly by FKBP may be additionally regulated by mTOR. Rapamycin inhibition of IP(3)-mediated Ca(2+) release may be explained by mTOR inhibition.