The distribution of Ca45 in frog (Rana pipiens) sartorius muscle, after 4 hours' exposure to Ringer's solution containing radiocalcium, has been analyzed by observing the kinetics of escape of the radioisotope into a non-radioactive Ringer's solution with calcium present or absent and by assuming that the tendon of Achilles is a satisfactory model of the extent of the uptake and release of Ca45 by the interstitial connective tissue (c.t.). In a Ringer's solution containing 1 mM/liter calcium, the exchangeable calcium distribution in micromoles per gram wet weight is as follows: (a) Aqueous phase of c.t. space: 0.16; (b) bound to c.t.: 0.16; (c) bound to surface of fibers: 0.13, of which 0.03 is displaced only by self-exchange, whereas the rest, as in c.t., can be displaced by other ions; and (d) in myoplasm: 0.33. The kinetics of Ca45 exit suggests that in infinite time of exposure to Ca45 the myoplasmic component would rise to 0.85. In the muscles, the half-time of the quickly emerging Ca45 averages about 3 minutes, whereas the time constant of the slowly released component is about 500 minutes. In the tendons the percentage rate of escape falls exponentially, the half-time of emergence being about 10 minutes.

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