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 singl e-channel isometric myograph designed to allow fluorescence measuremen ts concurrent with field stimulation of intramural nerves. Vessels wer e loaded with 6 mu M fura-2AM for 2 h and simultaneous recordings of n eurogenic 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 ryanodin e directly inhibited neuronal release of NA. 2 Arteries were activated by single intermittent field stimulation or continuously to excite in trinsic sympathetic nerves, or by cumulative addition of noradrenaline (1 nM-10 mu M) to the bathing solution. 3 Pre-incubation with ryanodi ne markedly inhibited the contraction and [Ca2+](i) release in respons e 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 stim ulation. However, stable maximal contraction and [Ca2+](i) in response to continuous field stimulation as well as maximal responses to exoge nous NA were unaffected. Release of[H-3]-NA in response to single inte rmittent 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 sma ll arteries primarily by releasing Ca2+ from a ryanodine-sensitive int racellular store. In contrast, the stable rise in tone and [Ca2+](i) r esulting from continuous nerve stimulation may largely depend on sourc es of Ca2+ other than the ryanodine-sensitive intracellular store.