Decreased Ca2+ Spark Frequency and RyR2 Expression in Cerebral Arteries Following Subarachnoid Hemorrhage

Abstract

Ca2+ sparks represent local and transient increases in intracellular Ca2+ caused by the coordinated opening of ryanodine receptors (RyRs) located in the sarcoplasmic reticulum of muscle. Contrary to the contraction caused by global Ca2+ increases, Ca2+ sparks promote smooth muscle relaxation via activation of plasmalemmal large-conductance Ca2+-activated potassium (BK) channels leading to membrane potential hyperpolarization and decreased activity of voltage-dependent Ca2+ channels. Here, we examined whether impairment of this vasodilator pathway contributes to enhanced cerebral artery constriction associated with subarachnoid hemorrhage (SAH). Using a rabbit SAH model, Ca2+ sparks, single channel and whole-cell BK currents were measured in cerebral artery myocytes. Expression of BK channel alpha and beta-1 subunits and RyR2 was examined by RT-PCR. The effects of blockers for BK channels (paxilline, 1 μM) and RyRs (ryanodine, 10 μM) were examined on diameter of isolated cerebral arteries. Ca2+ spark frequency, but not amplitude, was decreased ∼50% following SAH. This decrease in Ca2+ spark frequency corresponded to a reduction in the number of functional Ca2+ spark sites and decreased RyR2 expression in myocytes from SAH animals. A similar reduction in the frequency of transient BK currents was observed following SAH, although the properties and expression of BK channels were similar between groups. Inhibition of this vasodilatory pathway by paxilline or ryanodine induced constriction of control arteries, which was greatly diminished following SAH. These data suggest decreased Ca2+ spark frequency in cerebral myocytes following SAH is due to decreased RyR2 expression and a reduction in functional spark sites. The resulting decrease in BK currents leads to an enhanced cerebral artery constriction that may contribute to the development of neurological deficits following SAH. (Supported by AHA 0725837T, 0725841T, NIH R01 HL078983, R01 HL44455, and the Totman Medical Research Trust).