Identification and quantitation of cAMP-dependent protein kinase R subunitisoforms in subcellular fractions of failing human myocardium

Isoforms of regulatory (R) subunits of cAMP-dependent protein kinase were i dentified immunochemically and quantified in soluble and washed particulate fractions of failing human left ventricular myocardium. The predominant is oforms in both fractions were RI alpha and RII alpha. Both isoforms were pr esent in comparable amounts in these fractions, although RII alpha subunits were somewhat more prevalent than RI alpha subunits in washed particulate fractions. The ratio of R subunits to catalytic (C) subunits was three-fold higher in soluble than in particulate fractions. Identical observations we re made in preparations from non-failing human left ventricular myocardium. Since RI and RII have different affinities for cAMP and may direct catalyt ic activity to different substrates, the presence of both subunits in both soluble and particulate fractions provides a mechanism whereby the compartm ent-selective changes in cAMP content that have been described in failing h uman myocardium may affect not only the level but also the profile of prote in phosphorylation in these compartments, The high R:C subunit ratio in sol uble fractions suggests that cytosolic kinase activity in human myocardium may be less sensitive to changes in cAMP content than membrane-bound kinase activity, and this may contribute to the different effects of increases in soluble and particulate cAMP content on intracellular Ca2+ transients and contraction and relaxation. (C) 1999 Academic Press.