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 [Ca]o. The K depolarization results in an excessive loading of the cells with 45Ca. Exposing these cells during the efflux procedure to a solution containing DNP causes a much larger release of 45Ca than that observed after a loading procedure in normal solution. In contrast, the release of 45Ca elicited in such tissues by norepinephrine or caffeine disappears. This disappearance is due to the prolonged increase of the Ca exchangeability induced by K depolarization. During initial exposure to PSS the increased exchangeability causes an accelerated loss of tracer from the tissue compartment on which norepinephrine and caffeine act, while the DNP sensitive compartment is not affected. It is suggested that noradrenaline and caffeine act on the same calcium pool close to the membrane and that DNP acts mainly on the mitochondria.

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