Inhibition of Ca++ sparks by oxyhemoglobin in rabbit cerebral arteries

Oxyhemoglobin (HbO2) causes cerebral artery constriction and is one component of blood that likely contributes to the pathogenesis of cerebral vasospasm after aneurysm rupture. This study was designed to examine the acute effect of HbO2 on subcellular Ca(++) release events (Ca(++) sparks) in cerebral artery myocytes. Calcium sparks provide a tonic hyperpolarizing and relaxing influence to vascular smooth muscle by the activation of plasmalemmal large-conductance Ca(++)-activated K+ channels. Evidence is provided that HbO2 may contract cerebral vascular muscle in part by free radical-mediated inhibition of Ca(++) sparks.

Calcium sparks were visualized in intact pressurized rabbit cerebral arteries by using laser scanning confocal microscopy and a Ca(++) indicator dye. Calcium spark frequency was reduced by approximately 65% after a 15-minute application of HbO2 (10(-4) M). The HbO2-induced decrease in Ca(++) spark frequency was prevented by a combination of the free radical scavengers superoxide dismutase and catalase. Isometric force measurements were used to characterize the role of the vascular endothelium and smooth-muscle Ca(++) channels in HbO2-induced cerebral artery contraction. The HbO2-induced contractions were independent of the vascular endothelium, but were abolished by diltiazem, a blocker of L-type voltage-dependent Ca(++) channels (VDCCs). Ryanodine, a blocker of ryanodine-sensitive Ca(++) release channels located on the sarcoplasmic reticulum, also reduced HbO2-induced contractions by approximately 50%.

These results support the hypothesis that HbO2 may contract cerebral artery segments in part by inhibition of Ca(++) sparks, leading to decreased large-conductance Ca(++)-activated K+ channel activity, membrane potential depolarization, and enhanced Ca(++) entry through VDCCs.