Abstract
The following phenomena were observed when relative contraction and relaxation effects of ATP and creatine phosphate (CP) were studied in rabbit psoas muscle glycerinated fiber bundles containing native creatine phosphokinase (CPK) and ATPase activities: (1) nucleotide was absolutely necessary for contraction; (2) in the presence of a small amount of ADP (250 μM), physiological concentration of CP (10 mM) produced faster and stronger contraction and faster, more complete, relaxation than equimolar or higher concentrations of ATP; (3) if the nucleotide was in the form of ATP, the nucleotide K m for contraction was about 1.5 mM; (4) if the nucleotide was in the form of ADP, the nucleotide K m for contraction at physiological concentration of CP (10 mM) was 0.076 to 1.18 mM depending upon the order of addition of ADP and CP; (5) the apparent K m for CP for contraction was 2.67 mM independent of sequence of addition of ADP and CP.
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