Ryanodine-induced vasoconstriction of the gerbil spiral modiolar artery depends on the Ca2+ sensitivity but not on Ca2+ sparks or BK channels

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. 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. 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. 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.