Ryanodine-induced vasoconstriction of the gerbil spiral modiolar artery depends on the Ca(2+) sensitivity but not on Ca(2+) sparks or BK channels

BACKGROUND: In many vascular smooth muscle cells (SMCs), ryanodine receptor-mediated Ca(2+) sparks activate large-conductance Ca(2+)-activated K(+) (BK) channels leading to lowered SMC [Ca(2+)](i) and vasodilation. Here we investigated whether Ca(2+) sparks regulate SMC global [Ca(2+)](i) and diameter in the spiral modiolar artery (SMA) by activating BK channels. METHODS: SMAs were isolated from adult female gerbils, loaded with the Ca(2+)-sensitive flourescent dye fluo-4 and pressurized using a concentric double-pipette system. Ca(2+) signals and vascular diameter changes were recorded using a laser-scanning confocal imaging system. Effects of various pharmacological agents on Ca(2+) signals and vascular diameter were analyzed. RESULTS: Ca(2+) sparks and waves were observed in pressurized SMAs. Inhibition of Ca(2+) sparks with ryanodine increased global Ca(2+) and constricted SMA at 40 cmH(2)O but inhibition of Ca(2+) sparks with tetracaine or inhibition of BK channels with iberiotoxin at 40 cmH(2)O did not produce a similar effect. The ryanodine-induced vasoconstriction observed at 40 cmH(2)O was abolished at 60 cmH(2)O, consistent with a greater Ca(2+)-sensitivity of constriction at 40 cmH(2)O than at 60 cmH(2)O. When the Ca(2+)-sensitivity of the SMA was increased by prior application of 1 nM endothelin-1, ryanodine induced a robust vasoconstriction at 60 cmH(2)O. CONCLUSIONS: The results suggest that Ca(2+) sparks, while present, do not regulate vascular diameter in the SMA by activating BK channels and that the regulation of vascular diameter in the SMA is determined by the Ca(2+)-sensitivity of constriction.