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 a orta in the presence or in the absence of extracellular Ca2+ with depl etion of intracellular Ca2+ Stores. Thereafter, during incubation in t he presence of Ca2+, an increase in the resting tone was observed. Suc h a contractile response did not occur after exposure to caffeine or 5 -hydroxytryptamine. 2 This increase in tension was inhibited in a conc entration-dependent manner by alpha-adrenoceptor antagonists (prazosin , phentolamine and yohimbine), the non-specific relaxing compound, pap averine and by the Ca2+-entry blocker, nifedipine. Therefore, this con tractile process is related to depletion of Ca2+ stores sensitive to n oradrenaline and is linked to Ca2+ entry through voltage-operated Ca2 channels and alpha-adrenoceptors. 3 Phentolamine and yohimbine did no t 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-se nsitive Ca2+-stores.