A study of releasable Ca fractions in smooth muscle cells of the rabbit aorta.

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Abstract
The distribution of Ca in the cellular compartment of smooth muscle cells of the rabbit aorta was studied by analyzing the effect of norepinephrine, caffeine and DNP [2,4-dinitrophenol] on 45Ca exchange and on the pattern of tension development. These 3 substances increased the release of 45Ca from the tissue, but DNP acted more slowly than norepinephrine or caffeine. The effect of norepinephrine and caffeine on tension development occurred almost immediately, while that of DNP appeared only after a delay of 5 min. Study of the effect of these substances on the Ca efflux showed that norepinephrine and caffeine probably acted on the same Ca compartment, while DNP seemed to act on a different compartment with a slower exchange rate. The difference between these 2 pools were further demonstrated by studying Ca release after loading the tissues with tracer in either K-rich solution or in a solution with reduced Ca concentration. The K depolarization resulted in an excessive loading of the cells with 45Ca. Exposing these cells during the efflux procedure to a solution containing DNP caused a much larger release of 45Ca than that observed after a loading procedure in normal solution. The release of 45Ca elicited in such tissues by norepinephrine or caffeine disappeared. This diasppearance was due to the prolonged increase of the Ca exchangeability induced by K depolarization. During initial exposure to PSS [physiological solutions] the increased exchangeability caused an accelerated loss of tracer from the tissue compartment on which norepinephrine and caffeine acted, while the DNP sensitive compartment was not affected. Norepinephrine and caffeine acted on the same Ca pool close to the membrane and DNP acted mainly on the mitochondria.