Protection of dilator function of coronary arteries from homocysteine by tetramethylpyrazine: Role of ER stress in modulation of BK _Ca channels

Objectives: We recently reported the involvement of ER stress-mediated BK _Ca channel inhibition in homocysteine-induced coronary dilator dysfunction. In another study, we demonstrated that tetramethylpyrazine (TMP), an active ingredient of the Chinese herb Chuanxiong, possesses potent anti-ER stress capacity. The present study investigated whether TMP protects BK _Ca channels from homocysteine-induced inhibition and whether suppression of ER stress is a mechanism contributing to the protection. Furthermore, we explored the signaling transduction involved in TMP-conferred protection on BK _Ca channels. Methods: BK _Ca channel-mediated relaxation was studied in porcine small coronary arteries. Expressions of BK _Ca channel subunits, ER stress molecules, and E3 ubiquitin ligases, as well as BK _Ca ubiquitination were determined in porcine coronary arterial smooth muscle cells (PCASMCs). Whole-cell BK _Ca currents were recorded. Results: Exposure of PCASMCs to homocysteine or the chemical ER stressor tunicamycin increased the expression of ER stress molecules, which was significantly inhibited by TMP. Suppression of ER stress by TMP preserved the BK _Ca β1 protein level and restored the BK _Ca current in PCASMCs, concomitant with an improved BK _Ca -mediated dilatation in coronary arteries. TMP attenuated homocysteine-induced BK _Ca β1 protein ubiquitination, in which inhibition of ER stress-mediated FoxO3a activation and FoxO3a-dependent atrogin-1 and Murf-1 was involved. Conclusions: Reversal of BK _Ca channel inhibition via suppressing ER stress-mediated loss of β1 subunits contributes to the protective effect of TMP against homocysteine on coronary dilator function. Inhibition of FoxO3a-dependent ubiquitin ligases is involved in TMP-conferred normalization of BK _Ca β1 protein level. These results provide new mechanistic insights into the cardiovascular benefits of TMP.