1. Effects of noradrenaline (NAd) on changes in cellular Ca2+ concentration ([Ca2+]i) and tension were investigated, and these effects were compared with those evoked by 128 mm K+ or caffeine in intact smooth muscle strips or by inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) or caffeine in beta-escin-treated chemically skinned smooth muscle strips of the rabbit mesenteric artery. 2. In physiological solution containing 2.6 mm Ca2+, application of 128 mm K+ or 10 muM NAd produced a phasic, followed by a tonic increase in [Ca2+]i and tension. NAd (10 muM) produced a larger tonic tension than did 128 mm K+ but a smaller increase in [Ca2+]i. When the [Ca2+]i-tension relationship was observed in ionomycin- and 128 mm K+-treated muscle strips, 10 muM NAd shifted the relationship to the left and enhanced the maximum amplitude of contraction. These results suggest that NAd increases the sensitivity of contractile proteins to Ca2+ in smooth muscle of the rabbit mesenteric artery. 3. Noradrenaline (10 muM) or caffeine (10 muM), but not 128 mM K+, produced a phasic increase in both [Ca2+]i and tension in Ca2+-free solution containing 2 mm EGTA. When 10 mM caffeine had been applied in Ca2+-free solution, subsequent application of 10 mum NAd did not increase [Ca2+]i . By contrast, when 10 muM NAd had been applied in Ca2+-free solution, subsequent application of 10 mm caffeine still increased [Ca2+]i. Ryanodine (50 muM) abolished the increase in [Ca2+]i induced by 10 mm caffeine or 10 muM NAd in intact and in skinned smooth muscle strips. These results suggest that NAd releases Ca2+ from the ryanodine-sensitive Ca2+ storage sites. 4. Noradrenaline (10 muM) synthesized Ins(1,4,5)P3 in Ca2+-free solution in intact smooth muscle strips. Following application of 10 muM NAd, a relatively long time lag (around 1 s) was always observed before the initiation of the increase in [Ca2+]i whether in the presence or absence of Ca2+. The maximum rate of rise of [Ca2+]i induced by 10 mm caffeine was much larger than that induced by 10 muM NAd in Ca2+-containing or Ca2+-free solution (containing 2 mm EGTA). Both [Ca2+]i and tension reached their peak in a shorter time with caffeine (10 mm) than with 10 muM NAd. ln beta-escin-treated skinned smooth muscle strips, 20 muM Ins(1,4,5)P3, 10 muM caffeine or 10 muM NAd increased Ca2+ in Ca2+-free solution following brief application of 0.3 muM Ca2+. The extent of the increase in Ca2+ was in the rank order Ins(1,4,5)P3 > caffeine = NAd in identical conditions in terms of Ca2+ loading in the storage sites. The time required for [Ca2+]i to reach its peak was shorter for 20 muM Ins(1,4,5)P3 or 10 mm caffeine than for 10 muM NAd. These results suggest that NAd may require a certain time lag for alpha-receptor-phospholipase C coupling to synthesize Ins(1,4,5)P3 before it can release Ca2+. 5. The rate of decline of [Ca2+]i in the presence of 10 muM NAd or 10 mm caffeine depended on the concentration of [Ca2+]i whether in the presence or absence of extracellular Ca2+. The maximum rates of decline of [Ca2+]i were almost the same for these two agents, but the rate of decline of [Ca2+]i at lower [Ca2+]i (150-350 nm) was faster in the presence of NAd than in the presence of caffeine. NAd also enhanced the rate of decline of [Ca2+]i in the response to 1 muM ionomycin in Ca2+-free solution but to a lesser extent. The maximum rate of decline of [Ca2+]i in the presence of NAd with ionomycin was almost one-third that observed with NAd alone. These results suggest that, in smooth muscle cells, NAd promotes the process which restores [Ca2+]i to the resting level through activation of both the re-sequestration of Ca2+ and the extrusion of Ca2+ to the extracellular space. 6. It is concluded that, in smooth muscle of the rabbit mesenteric artery, NAd releases Ca2+ from the ryanodine-sensitive storage site, possibly through the action of Ins(1,4,5)P3. This agonist negatively controls the increased [Ca2+]i through activation of Ca2+ removal mechanisms. NAd also enhances the sensitivity of contractile proteins to Ca2+ and thus increases the tension at lower [Ca2+]i levels.
