EVIDENCE THAT DEPLETION OF INTERNAL CALCIUM STORES SENSITIVE TO NORADRENALINE ELICITS A CONTRACTILE RESPONSE DEPENDENT ON EXTRACELLULAR CALCIUM IN RAT AORTA

1 Noradrenaline 1 muM 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 alpha-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 alpha-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.