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The distribution of Ca in the cellular compartment of smooth muscle cells of the rabbit aorta has been studied by analyzing the effect of norepinephrine, caffeine, and DNP on 45Ca exchange and on the pattern of tension development. These three substances increase the release of 45Ca from the tissue, but DNP acts more slowly than norepinephrine or caffeine. Also, the effect of norepinephrine and caffeine on tension development occurs almost immediately, while that of DNP appears only after a delay of 5 min. Study of the effect of these substances on the Ca efflux has shown that norepinephrine and caffeine act probably on the same Ca compartment, while DNP seems to act on a different compartment with a slower exchange rate. The difference between these two pools could be further demonstrated by studying Ca release after loading the tissues with tracer in either K-rich solution or in a solution with reduced …