Molecular mechanism by which cyclic amp regulates myocardial contractility Page: 80 of 153
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influencing the phosphorylation of the 22,000 and 6,000 dalton
proteins or the specific site for cyclic AMP in the protein kinase
moiety is not 100% specific for cyclic AMP, and when cyclic 6MP is
present, it can partially substitute for cyclic AMP. Maximal
phosphorylation occurs when cyclic AMP and protein kinase are both
included in the incubation medium (Fig. 3). Phosphodiesterase
completely inhibits phosphorylation. This inhibition is
accomplished by phosphodiesterase breaking down cyclic
nucleotides. The fact that the level of phosphorylation in the
presence of phosphodiesterase is significantly lower than the basal
levels, gives credence to the idea that there is an adenyl cyclase
system in the sarcoplasmic reticulum. Phosphorylase kinase had
virtually no effect on the phosphorylation of either the 22,000 or
6,000 dalton proteins (Fig. 3). Schwartz et _al_. (1976) reported
that phosphorylase kinase was capable of both phosphorylating
phosphorylase a and stimulating calcium uptake in microsomes from
cardiac and skeletal muscles. Including phosphorylase kinase in
the incubation medium had no pronounced enhancement on the
phosphorylation of the 95,000-dalton protein, phosphorylase a.
After long period of incubation, the phosphoryl ation of
phosphorylase did increase but not nearly as dramatically as that
found in the 22,000 and 6,000 dalton proteins (Fig. 4). In
addition, in skeletal muscle microsomes neither the 22,000 or 6,000
dalton proteins appears phosphorylated; however, phosphoryl ase a is
phosphoryl ated (Data not shown). In cardiac sarcoplasmic
reticulum, only the Ca2++^g2+_^jpasej phosphorylase a, protein
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Bidlack, J. M. Molecular mechanism by which cyclic amp regulates myocardial contractility, report, January 1, 1979; United States. (https://digital.library.unt.edu/ark:/67531/metadc1085147/m1/80/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.