In smooth muscle the spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) occurs at preferred locations called frequent discharge sites (FDSs) giving rise to localized intracellular Ca2+ transients (Ca2+ sparks). Laser scanning confocal microscopy of fluo-3-loaded single myocytes freshly isolated from small mesenteric arteries of guinea-pig was used to investigate the action of nitric oxide (NO) donors and noradrenaline on the position and activity of FDSs and on global intracellular Ca2+ concentration ([Ca2+]i). In 8% of cells ‘microsparks’, Ca2+ release events smaller in duration, spread and amplitude than Ca2+ sparks were observed. The location of the initiation point of Ca2+ sparks observed during line-scan imaging was found to ‘jitter’ by ± 0.41 μm. However, the general position of an FDS within the cell did not change; most FDSs were close (within 1.2 ± 0.1 μm) to the cell membrane and often multiple FDSs occurred in one confocal plane of the cell. In the resting state, NO donors S-nitroso-N-acetylpenicillamine (SNAP; 50 μm) and sodium nitroprusside (SNP; 100 μm) did not change the general position of FDSs and slightly depressed their activity, but did not affect the global [Ca2+]i significantly. Application of noradrenaline (1–10 μm) increased Ca2+ spark frequency at existing FDS(s) leading to a Ca2+ wave. The increase in FDS activity and in global [Ca2+]i produced by noradrenaline were inhibited by the presence of SNAP or SNP but not by 8-bromoguanosine cyclic 3′,5′-monophosphate (8-Br-cGMP; 100 μm). In the presence of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), inhibitor of soluble guanylate cyclase, SNAP and SNP still exerted their effects on the noradrenaline response. These results suggest that SNAP and SNP inhibit the noradrenaline-evoked rise in global [Ca2+]i by a cGMP-independent mechanism and that part of this effect is due to inhibition of the activity of FDSs; moreover, only the activity, but not the position, of FDSs is changed by either stimulant or inhibitory substances
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