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.