Action of ryanodine on neurogenic responses in rat isolated mesenteric small arteries

1 Rat mesenteric arteries (similar to 250 mu m) were set up in a single-channel isometric myograph designed to allow fluorescence measurements concurrent with field stimulation of intramural nerves. Vessels were loaded with 6 mu M fura-2AM for 2 h and simultaneous recordings of neurogenic contraction (force) and intracellular calcium [Ca2+](i) were obtained. In other experiments, arteries were loaded with 1 mu Ci ml(-1) [H-3]-noradrenaline (NA) for 30 min in order to measure release of [H-3]-NA in response to field stimulation to examine whether ryanodine directly inhibited neuronal release of NA. 2 Arteries were activated by single intermittent field stimulation or continuously to excite intrinsic sympathetic nerves, or by cumulative addition of noradrenaline (1 nM-10 mu M) to the bathing solution. 3 Pre-incubation with ryanodine markedly inhibited the contraction and [Ca2+](i) release in response to single-pulse nerve stimulation. Ryanodine also inhibited an early phasic component of the response to continuous field stimulation and reduced the rate of rise in force in response to continuous field stimulation. However, stable maximal contraction and [Ca2+](i) in response to continuous field stimulation as well as maximal responses to exogenous NA were unaffected. Release of[H-3]-NA in response to single intermittent field stimulation was not affected by ryanodine when compared to vehicle. 4 Our results suggest that brief intermittent activation of intramural sympathetic nerves increases [Ca2+](i) and contracts small arteries primarily by releasing Ca2+ from a ryanodine-sensitive intracellular store. In contrast, the stable rise in tone and [Ca2+](i) resulting from continuous nerve stimulation may largely depend on sources of Ca2+ other than the ryanodine-sensitive intracellular store.