Nitric Oxide does not suppress myogenic tone by activating ryanodine sensitive stores in rat middle cerebral artery

Abstract

Inhibition of NOS in cerebral arteries results in vasoconstriction and the development of vasomotion, indicating that basal NO synthesis suppresses myogenic tone. Recent studies in rat posterior cerebral artery suggest that NO acts via ryanodine-sensitive calcium stores, evoking Ca2+ sparks, which activate smooth muscle BKCa. We investigated whether a similar action of NO was responsible for the suppression of myogenic tone in the rat middle cerebral artery. Segments of the middle cerebral artery from male Wistar rats were mounted in a wire myograph. SMC membrane potential (Em) was recorded with sharp glass microelectrodes and changes in [Ca2+]i measured using a confocal microscope after loading the SMCs with Oregon Green. Asynchronous oscillations in smooth muscle [Ca2+]i were slightly reduced by nifedipine, but not affected further by the addition of L-NAME. In the additional presence of ryanodine (10 μM) the oscillations were abolished. In the absence of nifedipine, L-NAME caused a significant vasoconstriction (~ 75% max) and vasomotion. Both the BKCa blocker iberiotoxin and ryanodine (alone) mimicked the effect of L-NAME. Application of nifedipine decreased tension and markedly reduced L-NAME induced constriction. These results indicate that in rat middle cerebral arteries, ryanodine-sensitive Ca2+ stores are essential for maintaining the asynchronous oscillations in smooth muscle [Ca2+]i observed under resting conditions, NO does not appear to be essential for initiation of these oscillations. Therefore, the effect of basal release of NO to suppress myogenic tone by stimulating BKCa is not due to an action on ryanodine-sensitive stores but may reflect a direct action of NO on these KCa channels.