[Biphasic effects of caffeine on tension in isolated aorta of rat]

The dual effects of caffeine on the tension were observed in rat thoracic aorta. When the preparation was exposed to Tyrode's solution containing caffeine of 1, 5 and 10 mmol/L, the tension increased. In 1-2 min it slowly decreased to a level lower than the pre-caffeine tension. After norepinephrine (1 mumol/L) was added, caffeine produced a greater inhibitory effect and the contraction phase disappeared. The dual effects of caffeine were not affected by the vascular endothelium. In the Tyrode's solution containing NiCl2 2 mmol/L, the first addition of caffeine (10 mmol/L) or norepinephrine (0.1 mumol/L) caused a contraction. The one caused by norepinephrine was greater. A second addition of caffeine did not cause a contraction, but addition of norepinephrine at this time caused a small contraction. After the washout of norepinephrine which had caused a greater contraction, the addition of caffeine (10 mmol/L) did not cause a contraction. These results indicate that norepinephrine completely depletes caffeine-sensitive Ca2 stores but caffeine only partially depletes norepinephrine-sensitive Ca2+ stores.