Particulate cyclic 3′,5′-nucleotide phosphodiesterase and calmodulin of cardiac muscle

1. 1. Cyclic AMP and cyclic GMP phosphodiesterase and calmodulin were measured in purified subcellular fractions of cardiac muscle. 2. 2. Phosphodiesterase activity solubilized by sonication of the nuclear fraction yielded a major 6.6 S form which was calcium-sensitive and cyclic GMP-specific. 3. 3. Phosphodiesterase activity occurring in the nuclear fraction could be further enriched by subfractionation on sucrose density gradients in the presence of MgCl 2.