Micromolar Ca2+ from sparks activates Ca2+-sensitive K+ channels in rat cerebral artery smooth muscle

The goal of the present study was to test the hypothesis that local Ca2+ release events (Ca2+ sparks) deliver high local Ca2+ concentration to activate nearby Ca(2+)sensitive K+ (BK) channels in the cell membrane of arterial smooth muscle cells. Ca2+ sparks and BK channels were examined in isolated myocytes from rat cerebral arteries with laser scanning confocal microscopy and patch-clamp techniques. BK channels had an apparent dissociation constant for Ca2+ of 19 muM and a Hill coefficient of 2.9 at -0 mV. At near-physiological intracellular Ca2+ concentration ([Ca2+](i); 100 nM) and membrane potential (-40 mV), the open probability of a single BK channel was low (1.2 x 10(-6)). A Ca2+ spark increased BK channel activity to 18. Assuming that 1-100% of the BK channels are activated by a single Ca2+ spark, BK channel activity increases 6 x 10(5)-fold to 6 x 10(3)-fold, which corresponds to similar to 30 muM to 4 muM spark Ca2+ concentration. 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'- tetraacetic acid acetoxymethyl ester caused the disappearance of all Ca2+ sparks while leaving the transient BK currents unchanged. Our results support the idea that Ca2+ spark sites are in close proximity to the BK channels and that local [Ca2+](i) reaches micromolar levels to activate BK channels.