Noradrenaline, 1 microM, induced a sustained contractile response in rat isolated aorta in the presence and in the absence of extracellular Ca2+. After depleting the noradrenaline-sensitive intracellular Ca2+ stores, an increase in the basal tone of the aorta was observed during the incubation period in the presence of Ca2+ and in the absence of the agonist. We have tested the possible pathways through which Ca2+ enters the cell to refill the previously depleted Ca2+ pools, a process that is accompanied by an increase in tension. The magnitude of this increase does not depend on the presence of Mg2+ in the extracellular medium nor on the temperature, suggesting that it is mediated by an event that does not depend on intracellular energy or Ca2+, Mg(2+)-ATPase. It is inhibited in a concentration-dependent manner by an unspecific relaxing compound, caffeine, and an organic Ca2+ entry blocker, verapamil, but not by an inorganic Ca2+ entry blocker, lanthanum. Caffeine (10 mM) and verapamil (10(-5) M) completely inhibited the increase in the resting tone, but only verapamil abolished the refilling of the noradrenaline-sensitive Ca2+ pools, indicating that the extracellular Ca2+ enters the cell through voltage-operated Ca2+ channels. Caffeine inhibited the increase in the resting tone without blocking the refilling process of the stores at 37 degrees C, but at 25 degrees C a partial inhibition of the repletion of internal Ca2+ pools was observed. These results confirm previous work that showed a temperature-dependent activity of caffeine.