Differential activation of ion channels by inositol 1,4,5-trisphosphate (IP3)- and ryanodine-sensitive calcium stores in rat basilar artery vasomotion

Spontaneous, rhythmical contractions, or vasomotion, can be recorded from cerebral vessels under both normal physiological and pathophysiological conditions. Using electrophysiology to study changes in membrane potential, the ratiometric calcium indicator Fura-2 AM to study changes in [Ca2+](i) in both the arterial wall and in individual smooth muscle cells (SMCs), and video microscopy to study changes in vessel diameter, we have investigated the cellular mechanisms underlying vasomotion in the juvenile rat basilar artery. During vasomotion, rhythmical oscillations in both membrane potential and [Ca2+](i) were found to precede rhythmical contractions. Nifedipine depolarized SMCs and abolished rhythmical contractions and depolarizations. [Ca2+] i oscillations in the arterial wall became reduced and irregular, while [Ca2+](i) oscillations in adjacent SMCs were no longer synchronized. BAPTA-AM, thapsigargin and U73122 hyperpolarized SMCs, relaxed the vessel, decreased basal calcium levels and abolished vasomotion. Chloride substitution abolished rhythmical activity, depolarized SMCs, increased basal calcium levels and constricted the vessel, while niflumic acid and DIDS abolished vasomotion. Ryanodine, charybdotoxin and TRAM-34, but not iberiotoxin, 4-aminopyridine or apamin, each depolarized SMCs and increased the frequency of rhythmical depolarizations and [Ca2+](i) oscillations. We conclude that vasomotion in the basilar artery depends on the release of intracellular calcium from IP3 (inositol 1,4,5,-trisphosphate) -sensitive stores which activates calcium-dependent chloride channels to depolarize SMCs. Depolarization in turn activates voltage-dependent calcium channels, synchronizing contractions of adjacent cells through influx of extracellular calcium. Subsequent calcium-induced calcium release from ryanodine-sensitive stores activates an intermediate conductance potassium channel, hyperpolarizing the SMCs and providing a negative feedback pathway for regeneration of the contractile cycle.