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Purinergic and adrenergic Ca²⁺ transients during neurogenic contractions of rat mesenteric small arteries

Christine Lamont, Enrikas Vainorius and W. Gil Wier

Contraction of small arteries is regulated by the sympathetic nervous system, but the Ca²⁺ transients during neurally stimulated contraction of intact small arteries have not yet been recorded. We loaded rat mesenteric small arteries with the fluorescent Ca²⁺ indicator fluo-4 and mounted them in a myograph that permitted simultaneous (i) high-speed confocal imaging of fluorescence from individual smooth muscle cells, (ii) electrical stimulation of perivascular nerves, and (iii) recording of isometric tension. Sympathetic neuromuscular transmission was achieved by electrical field stimulation (EFS) (frequency, 10 Hz; pulse voltage, 40 V; pulse duration, 0.2 ms) in the presence of capsaicin and scopolamine (to inhibit 'sensory' and cholinergic nerves, respectively). During the first 20 s of EFS, force rose to a small peak and then declined. During this time, junctional Ca²⁺ transients (jCaTs) were present at relatively high frequency. We have previously attributed jCaTs to influx of Ca²⁺ through post-junctional P2X receptors activated by ATP. Propagating asynchronous Ca²⁺ waves, previously associated with bath-applied ₁-adrenoceptor agonists, were not initially present. During the next 2.5 min of EFS, force rose slowly, and asynchronous propagating Ca²⁺ waves appeared. The selective ₁-adrenoceptor antagonist prazosin abolished both the slowly developing contraction and the Ca²⁺ waves, but reduced the initial transient contraction by only ~25 %. During 3 min of EFS in prazosin, the frequency of jCaTs declined markedly; at sites at which at least one jCaT occurred, the average probability of a jCaT was 0.008 ± 0.002 pulse^-1 in the first 20 s and 0.0007 ± 0.0002 pulse^-1 in the last 20 s. We suggest that (i) ATP released from sympathetic varicosities activates the initial, transient, contraction and the activator Ca²⁺ is derived largely from jCaTs, and (ii) sympathetically released noradrenaline (NA) activates the later, major contraction through mechanisms involving ₁-adrenoceptors and which are associated with propagating Ca²⁺ waves.

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