Evidence that depletion of internal calcium stores sensitive to noradrenaline elicits a contractile response dependent on extracellular calcium in rat aorta

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Abstract
1 Noradrenaline 1 μm induced a contractile response in rat isolated aorta in the presence or in the absence of extracellular Ca2+ with depletion of intracellular Ca2+ stores. Thereafter, during incubation in the presence of Ca2+, an increase in the resting tone was observed. Such a contractile response did not occur after exposure to caffeine or 5-hydroxytryptamine. 2 This increase in tension was inhibited in a concentration-dependent manner by α-adrenoceptor antagonists (prazosin, phentolamine and yohimbine), the non-specific relaxing compound, papaverine and by the Ca2+-entry blocker, nifedipine. Therefore, this contractile process is related to depletion of Ca2+ stores sensitive to noradrenaline and is linked to Ca2+ entry through voltage-operated Ca2+ channels and α-adrenoceptors. 3 Phentolamine and yohimbine did not block the Ca2+ refill pathway; prazosin and nifedipine inhibited the reuptake of Ca2+ by an internal store sensitive only to noradrenaline; papaverine inhibited the refilling of caffeine- and noradrenaline-sensitive Ca2+-stores.